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  • 1. Adamo, A.
    et al.
    Smith, L. J.
    Gallagher, J. S.
    Bastian, N.
    Ryon, J.
    Westmoquette, M. S.
    Konstantopoulos, I. S.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Larsen, S. S.
    Silva-Villa, E.
    Charlton, J. C.
    Weisz, D. R.
    Revealing a ring-like cluster complex in a tidal tail of the starburst galaxy NGC 21462012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 426, no 2, p. 1185-1194Article in journal (Refereed)
    Abstract [en]

    We report the discovery of a ring-like cluster complex in the starburst galaxy NGC?2146. The Ruby Ring, so named due to its appearance, shows a clear ring-like distribution of star clusters around a central object. It is located in one of the tidal streams that surround the galaxy. NGC?2146 is part of the Snapshot Hubble U-band Cluster Survey (SHUCS). The WFC3/F336W data have added critical information to the available archival Hubble Space Telescope imaging set of NGC?2146, allowing us to determine ages, masses and extinctions of the clusters in the Ruby Ring. These properties have then been used to investigate the formation of this extraordinary system. We find evidence of a spatial and temporal correlation between the central cluster and the clusters in the ring. The latter are about 4?Myr younger than the central cluster, which has an age of 7?Myr. This result is supported by the Ha emission which is strongly coincident with the ring, and weaker at the position of the central cluster. From the derived total Ha luminosity of the system, we constrain the star formation rate density to be quite high (SSFR = 0.47?M??yr-1?kpc-2). The Ruby Ring is the product of an intense and localized burst of star formation, similar to the extended cluster complexes observed in M?51 and the Antennae, but more impressive because it is quite isolated. The central cluster contains only 5 per cent of the total stellar mass in the clusters that are determined within the complex. The ring-like morphology, the age spread and the mass ratio support a triggering formation scenario for this complex. We discuss the formation of the Ruby Ring in a collect and collapse framework. The predictions made by this model agree quite well with the estimated bubble radius and expansion velocity produced by the feedback from the central cluster, making the Ruby Ring an interesting case of triggered star formation.

  • 2.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Ostlin, Goeran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Probing Cluster Formation under Extreme Conditions: Super Star Clusters in Blue Compact GalaxiesManuscript (preprint) (Other academic)
    Abstract [en]

    The numerous and massive young star clusters in blue compact galaxies (BCG) are used to investigate the properties of their hosts. We test whether BCGs follow claimed relations between the cluster population and their hosts, such as the cluster specific luminosity in the U band, TL(U), and the star formation rate density ΣSFR; the V bandluminosity of the brightest youngest cluster, Mbrightest, and the mean star formation Vrate (SFR); the cluster formation efficiency versus the ΣSFR. We find that BCGs fairly well reproduce the relations, supporting a scenario where cluster formation and environmental properties of the host are correlated. They occupy, in all the diagrams, the regions of higher SFRs, suggesting the extreme nature of the starburst operating in these systems. We suggest that the BCG environment has most likely favoured the compression and collapse of the giant molecular clouds, enhancing the local star formation efficiency, so that massive clusters have been formed. The cluster formation efficiency (i.e., the fraction of star formation happening in star clusters) in BCGs is higher than the 8-10 % reported from quiescent spirals and dwarf starburst galaxies. BCGs have a cluster formation efficiency comparable to luminous IR galaxies and spiral starburst nuclei (the averaged value is ∼ 30 %).

  • 3.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hayes, M.
    Observatory of Geneva, Switzerland.
    On the Origin of the Red Excess in Very Young Super Star Clusters: The Case of SBS 0335-052E2010In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 725, no 2, p. 1620-1628Article in journal (Refereed)
    Abstract [en]

    The spectral energy distribution analysis of very young unresolved star clusters challenges our understanding of the cluster formation process. Studies of resolved massive clusters in the Milky Way and in the nearby Magellanic Clouds show us that the contribution from photoionized gas is very important during the first Myr of cluster evolution. We present our models which include both a self-consistent treatment of the photoionized gas and the stellar continuum and quantify the impact of such a nebular component on the total flux of young unresolved star clusters. A comparison with other available models is considered. The very young star clusters in the SBS 0335-052E dwarf starburst galaxy are used as a test for our models. Due to the low metallicity of the galactic medium our models predict a longer lasted nebular phase which contributes between 10% and 40% of the total near-infrared (NIR) fluxes at around 10 Myr. We thus propose a possible solution for the observed flux excess in the six bright super star clusters (SSCs) of SBS 0335-052E. Reines et al. showed that the observed cluster fluxes, in the red-optical and NIR range, sit irreconcilably above the stellar continuum models provided. We find that in the age range estimated from the Hα emission we can explain the red excess in all six SSCs as due to nebular emission, which at cluster ages around 10 Myr still affects the NIR wavebands substantially.

  • 4. Adamo, Angela
    et al.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bastian, N.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Livermore, R. C.
    Guaita, Lucia
    Stockholm University, Faculty of Science, Department of Astronomy.
    HIGH-RESOLUTION STUDY OF THE CLUSTER COMPLEXES IN A LENSED SPIRAL AT REDSHIFT 1.5: CONSTRAINTS ON THE BULGE FORMATION AND DISK EVOLUTION2013In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 766, no 2, p. 105-Article in journal (Refereed)
    Abstract [en]

    We analyze the clump population of the spiral galaxy Sp 1149 at redshift 1.5. Located behind the galaxy cluster MACS J1149.5+2223, Sp 1149 has been significantly magnified allowing us to study the galaxy on physical scales down to similar to 100 pc. The galaxy cluster frame is among the targets of the Cluster Lensing And Supernova survey with Hubble (CLASH), an ongoing Hubble Space Telescope (HST) Multi-Cycle Treasury program. We have used the publicly available multi-band imaging data set to reconstruct the spectral energy distributions of the clumps in Sp 1149, and derive, by means of stellar evolutionary models, their physical properties. We found that 40% of the clumps observed in Sp 1149 are older than 30 Myr and can be as old as 300 Myr. These are also the more massive (luminous) clumps in the galaxy. Among the complexes in the local reference sample, the star-forming knots in luminous blue compact galaxies could be considered progenitor analogs of these long-lived clumps. The remaining 60% of clumps have colors comparable to local cluster complexes, suggesting a similar young age. We observe that the Sp 1149 clumps follow the M proportional to R-2 relation similar to local cluster complexes, suggesting similar formation mechanisms although they may have different initial conditions (e.g., higher gas surface densities). We suggest that the galaxy is experiencing a slow decline in star formation rate and a likely transitional phase toward a more quiescent star formation mode. The older clumps have survived between 6 and 20 dynamical times and are all located at projected distances smaller than 4 kpc from the center. Their current location suggests migration toward the center and the possibility of being the building blocks of the bulge. On the other hand, the dynamical timescale of the younger clumps is significantly shorter, meaning that they are quite close to their birthplace. We show that the clumps of Sp 1149 may account for the expected metal-rich globular cluster population usually associated with the bulge and thick disk components of local spirals.

  • 5.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Probing cluster formation under extreme conditions: massive star clusters in blue compact galaxies2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 417, no 3, p. 1904-1912Article in journal (Refereed)
    Abstract [en]

    The numerous and massive young star clusters in blue compact galaxies (BCGs) are used to investigate the properties of their hosts. We test whether BCGs follow claimed relations between cluster populations and their hosts, such as the fraction of the total luminosity contributed by the clusters as function of the mean star formation rate (SFR) density, the V-band luminosity of the brightest youngest cluster as related to the mean host SFR and the cluster formation efficiency (i.e. the fraction of star formation happening in star clusters) versus the density of the SFR. We find that BCGs follow the trends, supporting a scenario where cluster formation and environmental properties of the host are correlated. They occupy, in all the diagrams, the regions of higher SFRs, as expected by the extreme nature of the starbursts operating in these systems. We find that the star clusters contribute almost to the 20 per cent of the UV luminosity of the hosts. We suggest that the BCG starburst environment has most likely favoured the compression and collapse of the giant molecular clouds, enhancing the local star formation efficiency, so that massive clusters have been formed. The estimated cluster formation efficiency supports this scenario. BCGs have a cluster formation efficiency comparable to luminous IR galaxies and spiral starburst nuclei (the averaged value is similar to 35 per cent) which is much higher than the 8-10 per cent reported for quiescent spirals and dwarf star-forming galaxies.

  • 6.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Probing the near-IR flux excess in young star clusters2011Conference paper (Refereed)
    Abstract [en]

    We report the results of a recent study of young star clusters (YSCs) in luminous blue compact galaxies (BCGs). The age distributions of the YSCs suggest that the starburst episode in Haro 11, ESO 185-IG13, and Mrk 930 started not more than 30-40 Myr ago. A peak of cluster formation only 3 - 4 Myr old is observed, unveiling a unique sample of clusters still partially embedded. A considerable fraction of clusters (30 - 50%), mainly younger than 10 Myr, shows an observed flux excess between 0.8 and 2.2 micron. This so-called near-infrared (NIR) excess is impossible to reproduce even with the most recent spectral synthesis models (that include a self-consistent treatment of the photoionized gas). We have used these YSCs to probe the very early evolution phase of star clusters. In all the three host galaxies, the analysis is limited to the optically brightest objects, i.e., systems that are only partially embedded by their natal cocoons (since deeply embedded clusters are probably too faint to be detected). We discuss possible explanations for this NIR excess, in the context of IR studies of both extragalactic young star clusters and resolved massive star forming regions in the Milky Way and in the nearby Magellanic Clouds.

  • 7.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hayes, Matthew
    The Extremely Young Star Cluster Population In Haro 112010In: Galaxy Wars: Star Formation and Stellar Populations in Interacting Galaxies / [ed] Beverly Smith, James Higdon, Sarah Higdon, and Nathan Bastian, San Francisco: Astronomical Society of the Pacific , 2010, p. 74-Conference paper (Refereed)
    Abstract [en]

    We have performed a deep multi-band photometric analysis of the star cluster population of Haro 11. This starburst galaxy (log L_FUV = 10.3 L_sun) is considered a nearby analogue of Lyman break galaxies (LBGs) at high redshift. The study of the numerous star clusters in the systems is an effective way to investigate the formation and evolution of the starburst phase. In fact, the SED fitting models have revealed a surprisingly young star cluster population, with ages between 0.5 and 40 Myr, and estimated masses between 10^3 and 10^7 solar masses. An independent age estimation has been done with the EW(Halpha) analysis of each cluster. This last analysis has confirmed the young ages of the clusters. We noticed that the clusters with ages between 1 and 10 Myr show a flux excess in H (NIC3/F160W) and/or I (WFPC2/F814W) bands with respect to the evolutionary models. Once more Haro 11 represents a challenge to our understanding.

  • 8.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hayes, Matthew
    The Massive Star Clusters in the Dwarf Merger ESO 185-IG13: is the Red Excess Ubiquitous in Starbursts?2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 414, no 3, p. 1793-1812Article in journal (Other academic)
    Abstract [en]

    We have investigated the starburst properties of the luminous blue compact galaxy ESO 185-IG13. The galaxy has been imaged with the high resolution cameras onboard to the Hubble Space Telescope. From the UV to the IR, the data reveal a system shaped by hundreds of young star clusters, and fine structures, like a tidal stream and a shell. The presence of numerous clusters and the perturbed morphology indicate that the galaxy has been involved in a recent merger event. Using previous simulations of shell formation in galaxy mergers we constrain potential progenitors of ESO 185-IG13. The analysis of the star cluster population is used to investigate the properties of the present starburst and to date the final merger event, which has produced hundreds of clusters younger than 100 Myr. We have found a peak of cluster formation only 3.5 Myr old. A large fraction of these clusters will not survive after 10-20 Myr, due to the "infant mortality" caused by gas expulsion. However, this sample of clusters represents an unique chance to investigate the youngest phases of cluster evolution. As already observed in the analog blue compact galaxy Haro 11, a fraction of young clusters are affected by a flux excess at wavelengths longer than 8000 \AA. Ages, masses, and extinctions of clusters with this NIR excess are estimated from UV and optical data. We discuss similarities and differences of the observed NIR excess in ESO 185-IG13 clusters with other cases in the literature. The cluster ages and masses are used to distinguish among the potential causes of the excess. We observe, as in Haro 11, that the use of the IR and the (commonly used) I band data results in overestimates of age and mass in clusters affected by the NIR excess. This has important implications for a number of related studies of star clusters.

  • 9.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Hayes, Matthew
    Tracing the star formation history of three Blue Compact galaxies through the analysis of their star clusters2009Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    We present preliminary results from a study of the compact star cluster populations in three local luminous blue compact galaxies: ESO 185-IG 013, ESO 350-IG 038 (a.k.a. Haro 11), and MRK 930. These systems show peculiar morphologies and the presence of hundreds of SCs that have been produced by the past, recent, and/or current starburst phases. We use a complete set of HST images ranging from the UV to IR for each galaxy. Deep images in V (WFPC2/f606w) and I (WFPC2/f814w) are used to capture most of the star cluster candidates up to the old ones (fainter) which have had, in the past, less possibility to be detected. The other bands are used in the SED fitting technique for constraining ages and masses. Our goals are to investigate the evolution of these three blue compact galaxies and the star cluster formation impact on their star formation history.

  • 10.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hayes, Matthew
    Observatoire Astronomique de l'Université de Genève.
    Cumming, Robert
    Stockholm University, Faculty of Science, Department of Astronomy.
    Micheva, Genoveva
    Stockholm University, Faculty of Science, Department of Astronomy.
    Super star clusters in Haro 11: properties of a very young starburst and evidence for a near-infrared flux excess2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, MNRAS, Vol. 407, no 2, p. 870-890Article in journal (Refereed)
    Abstract [en]

    We have used multiband imaging to investigate the nature of an extreme starburst environment in the nearby Lyman break galaxy analogue Haro 11 (ESO350-IG038) by means of its stellar cluster population. The central starburst region has been observed in eight different high-resolution Hubble Space Telescope (HST) wavebands, sampling the stellar and gas components from UV to near-infrared. Photometric imaging of the galaxy was also carried out at 2.16μm by NaCo AO instrument at the ESO Very Large Telescope. We constructed integrated spectral energy distributions (SEDs) for about 200 star clusters located in the active star-forming regions and compared them with single stellar population models (suitable for physical properties of very young cluster population) in order to derive ages, masses and extinctions of the star clusters. The cluster age distribution we recover confirms that the present starburst has lasted for 40Myr, and shows a peak of cluster formation only 3.5 Myr old. With such an extremely young cluster population, Haro 11 represents a unique opportunity to investigate the youngest phase of the cluster formation process and evolution in starburst systems. We looked for possible relations between cluster ages, extinctions and masses. Extinction tends to diminish as a function of the cluster age, but the spread is large and reaches the highest dispersion for clusters in partial embedded phases (<5Myr). A fraction of low-mass (below 104 Msolar), very young (1-3Myr) clusters is missing, either because they are embedded in the parental molecular cloud and heavily extinguished, or because of blending with neighbouring clusters. The range of the cluster masses is wide; we observe that more than 30 per cent of the clusters have masses above 105 Msolar, qualifying them as super star clusters. Almost half of the cluster sample is affected by flux excesses at wavelengths >8000Å which cannot be explained by simple stellar evolutionary models. Fitting SED models over all wavebands leads to systematic overestimates of cluster ages and incorrect masses for the stellar population supplying the light in these clusters. We show that the red excess affects also the HST F814W filter, which is typically used to constrain cluster physical properties. The clusters which show the red excess are younger than 40Myr we discuss possible physical explanations for the phenomenon. Finally, we estimate that Haro 11 has produced bound clusters at a rate almost a factor of 10 higher than the massive and regular spirals, like the Milky Way. The present cluster formation efficiency is ~38 per cent of the galactic star formation rate.

  • 11.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Papaderos, P.
    Bergvall, N.
    Rich, R. M.
    Micheva, Genoveva
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Star cluster formation and evolution in Mrk 930: properties of a metal-poor starburst2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 415, no 3, p. 2388-2406Article in journal (Refereed)
    Abstract [en]

    We present the analysis of the large population of star clusters in the blue compact galaxy (BCG) Mrk 930. The study has been conducted by means of a photometric analysis of multiband data obtained with the Hubble Space Telescope (HST). We have reconstructed the spectral energy distributions of the star clusters and estimated the age, mass and extinction for a representative sample. Similar to previous studies of star clusters in BCGs, we observe a very young cluster population with 70 per cent of the systems formed less than 10 Myr ago. In Mrk 930, the peak in the star cluster age distribution at 4 Myr is corroborated by the presence of Wolf-Rayet spectral features, and by the observed optical and infrared (IR) line ratios [OIII]/H beta and [Ne III]/[Ne II]. The recovered extinction in these very young clusters shows large variations, with a decrease at older ages. It is likely that our analysis is limited to the optically brightest objects (i.e. systems only partially embedded in their natal cocoons; the deeply embedded clusters being undetected). We map the extinction across the galaxy using low-resolution spectra and the H alpha-to-H beta ratio, as obtained from ground-based narrow band imaging. These results are compared with the extinction distribution recovered from the clusters. We find that the mean optical extinction derived in the starburst regions is close to the averaged value observed in the clusters [more than 80 per cent of the systems have E(B - V) <= 0.2mag], but locally, do not trace the more extinguished clusters. Previous HST studies of BCGs have revealed a population of young and extremely red super star clusters. We detect a considerable fraction of clusters affected by a red excess also in Mrk 930. The nature of the red excess, which turns up at near-IR wavelengths (I band and longwards), remains unknown. We compare the cluster formation history and the star formation history, the latter derived from the fit of spectral population synthesis models to the spectra. We find a general agreement between the two independently estimated quantities. Using the cluster properties, we perform a study of the host environmental properties. We find that the cluster formation efficiency (the fraction of star formation happening in clusters) is significantly higher, suggesting a key role of the environment for the formation of these massive objects.

  • 12. Bastian, N.
    et al.
    Adamo, Angela
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Gieles, M.
    Lamers, H. J. G. L. M.
    Larsen, S. S.
    Silva-Villa, E.
    Smith, L. J.
    Kotulla, R.
    Konstantopoulos, I. S.
    Trancho, G.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Evidence for environmentally dependent cluster disruption in M832011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 417, no 1, p. l6-L10Article in journal (Refereed)
    Abstract [en]

    Using multiwavelength imaging from the Wide Field Camera 3 on the Hubble Space Telescope we study the stellar cluster populations of two adjacent fields in the nearby face-on spiral galaxy, M83. The observations cover the galactic centre and reach out to similar to 6 kpc, thereby spanning a large range of environmental conditions, ideal for testing empirical laws of cluster disruption. The clusters are selected by visual inspection to be centrally concentrated, symmetric and resolved on the images. We find that a large fraction of objects detected by automated algorithms (e. g. SEXTRACTOR or DAOFIND) are not clusters, but rather are associations. These are likely to disperse into the field on time-scales of tens of Myr due to their lower stellar densities and not due to gas expulsion (i.e. they were never gravitationally bound). We split the sample into two discrete fields (inner and outer regions of the galaxy) and search for evidence of environmentally dependent cluster disruption. Colour-colour diagrams of the clusters, when compared to simple stellar population models, already indicate that a much larger fraction of the clusters in the outer field are older by tens of Myr than in the inner field. This impression is quantified by estimating each cluster's properties (age, mass and extinction) and comparing the age/mass distributions between the two fields. Our results are inconsistent with 'universal' age and mass distributions of clusters, and instead show that the ambient environment strongly affects the observed populations.

  • 13.
    Bastian, N.
    et al.
    Excellence Cluster Universe, D-85748 Garching, Germany.
    Adamo, Angela
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Gieles, M.
    Univ Cambridge, Inst Astron, Cambridge CB3 0HA, England .
    Silva-Villa, E.
    Univ Utrecht, Astron Inst, NL-3584 CC Utrecht, Netherlands.
    Lamers, H. J. G. L. M.
    Univ Utrecht, Astron Inst, NL-3584 CC Utrecht, Netherlands.
    Larsen, S. S.
    Univ Utrecht, Astron Inst, NL-3584 CC Utrecht, Netherlands .
    Smith, L. J.
    Space Telescope Sci Inst, Baltimore, MD 21218 USA .
    Konstantopoulos, I. S.
    Penn State Univ, Dept Astron & Astrophys, University Pk, PA 16802 USA .
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Stellar clusters in M83: formation, evolution, disruption and the influence of the environment2012In: Monthly Notices of the Royal Astronomical Society, ISSN 1365-2966, Vol. 419, no 3, p. 2606-2622Article in journal (Refereed)
    Abstract [en]

    We study the stellar cluster population in two adjacent fields in the nearby, face-on spiral galaxy M83 using multiwavelength Wide Field Camera 3/Hubble Space Telescope imaging. After automatic detection procedures, the clusters are selected through visual inspection to be centrally concentrated, symmetric, and resolved on the images, which allows us to differentiate between clusters and likely unbound associations. We compare our sample with previous studies and show that the differences between the catalogues are largely due to the inclusion of a large numbers of diffuse associations within previous catalogues as well as the inclusion of the central starburst region, where the completeness limit is significantly worse than in the surrounding regions. We derive the size distribution of the clusters, which is well described by a lognormal distribution with a peak at ˜2.5 pc, and find evidence for an expansion in the half-light radius of clusters with age. The luminosity function of the clusters is well approximated by a power law with an index of -2 over most of the observed range; however, a steepening is seen at MV=-9.3 and -8.8 in the inner and outer fields, respectively. Additionally, we show that the cluster population is inconsistent with a pure power-law mass distribution, but instead exhibits a truncation at the high-mass end. If described as a Schechter function, the characteristic mass is 1.6 × 105 and 0.5 × 105 M&sun; for the inner and outer fields, respectively, in agreement with previous estimates of other cluster populations in spiral galaxies. Comparing the predictions of the mass-independent disruption (MID) and mass-dependent disruption (MDD) scenarios with the observed distributions, we find that both models can accurately fit the data. However, for the MID case, the fraction of clusters destroyed (or mass lost) per decade in age is dependent on the environment; hence, the age and mass distributions of clusters are not universal. In the MDD case, the disruption time-scale scales with galactocentric distance (being longer in the outer regions of the galaxy) in agreement with analytic and numerical predictions. Finally, we discuss the implications of our results on other extragalactic surveys, focusing on the fraction of stars that form in clusters and the need (or lack thereof) for infant mortality.

  • 14. Bergvall, N.
    et al.
    Leitet, E.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Marquart, T.
    Lyman continuum leaking galaxies: search strategies and local candidates2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 554, article id A38Article in journal (Refereed)
    Abstract [en]

    Context. Star-forming dwarf galaxies may have played an important role in the reionization of the Universe, provided that some fraction of their ionizing radiation were able to escape into the intergalactic medium. Local galaxies exhibiting such Lyman-continuum (LyC) leakage could potentially shed light on the escape mechanisms involved, but only two low-redshift cases of LyC leakage have been identified so far. Here, we argue that this meager harvest may be caused by unsuitable selection criteria. Candidates for LyC leakage are normally selected by indicators of starburst activity, one of which is a high equivalent width in H alpha. Such a criterion will guarantee a high production of LyC photons but will also bias the selection in favour of a high column density in the neutral gas, effectively ruling out LyC escape. Aims. In this work we want to investigate whether the lack of local LyC emitters can be caused in part by biased selection criteria, and we present a novel method of selecting targets with high escape fractions. By applying these criteria, we assemble a sample of observation targets to study their basic properties. Methods. We introduce a new selection strategy here where the potential LyC leakers are selected by their blue colours and weak emission lines. The selection is based on data from the Sloan Digital Sky Survey (SDSS). We also take a closer look at the properties of 8 LyC leaking candidates at z similar to 0.03 which we have observed with ESO/NTT in broadband B and H alpha. Results. We find that 7 of the 8 target galaxies are involved in interaction with neighbours or show signs of mergers. In 7 cases the young stellar population is clearly displaced relative to the main body of these galaxies, often directly bordering the halo region. In about half of our targets the absorption spectra show young post-starburst signatures. Comparing the scale lengths in H alpha with those of the stellar continua shows that the scale lengths in H alpha typically are 30% smaller, which is characteristic of galaxies influenced by ram pressure stripping. We tentatively identify a few mechanisms that could improve the conditions for leakage: 1) the combined effects of ram pressure stripping with supernova winds from young stars formed in the front, 2) merger events that increase the star formation rate and displace stars from gas, 3) starbursts in the centres of post-starburst galaxies, whose previous activity has cleared channels for leakage into the intergalactic medium, and 4) a low dust content. Although our target galaxies are rare species in the local universe, we argue that related types could have played a major role in producing ionizing radiation at high redshifts.

  • 15. Bergvall, Nils
    et al.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Caldwell, Brady
    The red haloes of SDSS low surface brightness disc galaxies2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 405, no 4, p. 2697-2716Article in journal (Refereed)
    Abstract [en]

    The faint stellar haloes of galaxies contain key information about the oldest stars and the process of galaxy formation. A previous study of stacked SDSS images of disc galaxies has revealed a halo with an abnormally red r - i colour, seemingly inconsistent with our current understanding of the stellar populations inhabiting stellar haloes. Measurements of this type are, however, plagued by large uncertainties which calls for follow-up studies. Here, we investigate the statistical properties of the faint envelopes of low surface brightness disc galaxies to look for further support for a red excess. A total of 1510 nearly edge-on, bulgeless low surface brightness galaxies were selected from the SDSS Data Release 5, rescaled to the same apparent size, aligned and stacked. This procedure allows us to reach a surface brightness of mu(r) similar to 31 mag arcsec-2. After a careful assessment of instrumental light scattering effects in the stacked images, we derive median and average radial surface brightness and colour profiles in g, r and i. The sample is then divided into three subsamples according to g - r colour. All three samples exhibit a red colour excess in r - i in the thick disc/halo region. The halo colours of the full sample, g - r = 0.60 +/- 0.15 and r - i = 0.80 +/- 0.15, are found to be incompatible with the colours of any normal type of stellar population. The fact that no similar colour anomaly is seen at comparable surface brightness levels along the disc rules out a sky subtraction residual as the source of the extreme colours. A number of possible explanations for these abnormally red haloes are discussed. We find that two different scenarios - dust extinction of extragalactic background light and a stellar population with a very bottom-heavy initial mass function - appear to be broadly consistent with our observations and with similar red excesses reported in the haloes of other types of galaxies.

  • 16. de Grijs, Richard
    et al.
    Anders, Peter
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    The NGC 5253 star cluster system - I. Standard modelling and infrared-excess sources2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 431, no 3, p. 2917-2932Article in journal (Refereed)
    Abstract [en]

    Using high-resolution Hubble Space Telescope data, we re-examine the fundamental properties (ages, masses and extinction values) of the rich star cluster population in the dwarf starburst galaxy NGC 5253. The gain in resolution compared to previous studies is of order a factor of 2 in both spatial dimensions, while our accessible wavelength range transcends previous studies by incorporation of both near-ultraviolet and near-infrared (IR) passbands. We apply spectral synthesis treatments based on two different simple stellar population model suites to our set of medium-, broad-band and H alpha images to gain an improved physical understanding of the IR-excess flux found for a subset of young clusters (30 of 149). With the caveat that our models are based on fully sampled stellar mass functions, the NGC 5253 cluster population is dominated by a significant number of relatively low-mass (M-cl less than or similar to a few x 10(4) M-circle dot) objects with ages ranging from a few x 10(6) to a few x 10(7) yr, which is in excellent agreement with the starburst age of the host galaxy. The IR-excess clusters are almost all found in this young age range and have masses of up to a few x 10(4) M-circle dot. The IR excess in the relatively low-mass NGC 5253 clusters is most likely caused by a combination of stochastic sampling effects and colour variations due to the presence of either luminous red or pre-main-sequence stars. We also find a small number of intermediate-age (similar to 1 Gyr old), similar to 10(5) M-circle dot clusters, as well as up to a dozen massive, similar to 10 Gyr old globular clusters. Their presence supports the notion that NGC 5253 is a very active galaxy that has undergone multiple episodes of star cluster formation.

  • 17.
    Duval, Florent
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hayes, Matthew
    Stockholm University, Faculty of Science, Department of Astronomy.
    Melinder, Jens
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Adamo, Angela
    Stockholm University, Faculty of Science, Department of Astronomy.
    Guaita, L.
    Rivera-Thorsen, Thöger
    Stockholm University, Faculty of Science, Department of Astronomy.
    Verhamme, A.
    Orlitova, I.
    Schaerer, D.
    Herenz, E. C.
    Gruyters, P.
    Mansson, T.
    LARS VIII: Lyman alpha escape from the edge-on disk galaxy Mrk1486In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746Article in journal (Refereed)
  • 18. Finlator, Kristian
    et al.
    Thompson, Robert
    Huang, Shuiyao
    Dave, Romeel
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Oppenheimer, B. D.
    The reionization of carbon2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 447, no 3, p. 2526-2539Article in journal (Refereed)
    Abstract [en]

    Observations suggest that CII was more abundant than CIV in the intergalactic medium towards the end of the hydrogen reionization epoch (z similar to 6). This transition provides a unique opportunity to study the enrichment history of intergalactic gas and the growth of the ionizing ultraviolet background (UVB) at early times. We study how carbon absorption evolves from z = 10 to 5 using a cosmological hydrodynamic simulation that includes a self-consistent multifrequency UVB as well as a well-constrained model for galactic outflows to disperse metals. Our predicted UVB is within similar to 2-4 times of that from Haardt & Madau, which is fair agreement given the uncertainties. Nonetheless, we use a calibration in post-processing to account for Lyman a forest measurements while preserving the predicted spectral slope and inhomogeneity. The UVB fluctuates spatially in such a way that it always exceeds the volume average in regions where metals are found. This implies both that a spatially uniform UVB is a poor approximation and that metal absorption is not sensitive to the epoch when HII regions overlap globally even at column densities of 10(12) cm(-2). We find, consistent with observations, that the CII mass fraction drops to low redshift while CIV rises owing the combined effects of a growing UVB and continued addition of carbon in low-density regions. This is mimicked in absorption statistics, which broadly agree with observations at z = 6-3 while predicting that the absorber column density distributions rise steeply to the lowest observable columns. Our model reproduces the large observed scatter in the number of low-ionization absorbers per sightline, implying that the scatter does not indicate a partially neutral Universe at z similar to 6.

  • 19. Holopainen, Janne
    et al.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Knebe, Alexander
    Nurmi, Pasi
    Heinämäki, Pekka
    Flynn, Chris
    Gill, Stuart
    Riehm, Teresa
    Stockholm University, Faculty of Science, Department of Astronomy.
    An analytical model of surface mass densities of cold dark matter haloes - with an application to MACHO microlensing optical depths2008In: Monthly Notices of the Royal Astronomical Society, Vol. 283, s. 120Article in journal (Refereed)
  • 20. Holopainen, Janne
    et al.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Knebe, Alexander
    Nurmi, Pasi
    Heinämäki, Pekka
    Gill, Stuart
    Flynn, Chris
    Riehm, Teresa
    Stockholm University, Faculty of Science, Department of Astronomy.
    The True Surface Mass density of Cold Dark Matter Halos2008In: Dark Galaxies and Lost Baryons: IAU Symposium, Volume 244, 2008Conference paper (Other academic)
  • 21.
    Jensen, Hannes
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hayes, Matthew
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Iliev, I. T.
    Laursen, P.
    Mellema, Garrelt
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Studying reionization with the next generation of Ly alpha emitter surveys2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 444, no 3, p. 2114-2127Article in journal (Refereed)
    Abstract [en]

    We study the prospects for constraining the ionized fraction of the intergalactic medium (IGM) at z > 6 with the next generation of large Ly alpha emitter surveys. We make predictions for the upcoming Subaru Hyper Suprime-Cam (HSC) Ly alpha survey and a hypothetical spectroscopic survey performed with the JamesWebb Space Telescope (JWST). Considering various scenarios where the observed evolution of the Ly alpha luminosity function of Ly alpha emitters at z > 6 is explained partly by an increasingly neutral IGM and partly by intrinsic galaxy evolution, we show how clustering measurements will be able to distinguish between these scenarios. We find that the HSC survey should be able to detect the additional clustering induced by a neutral IGM if the global IGM neutral fraction is greater than similar to 20 per cent at z = 6.5. If measurements of the Ly alpha equivalent widths (EWs) are also available, neutral fractions as small as 10 per cent may be detectable by looking for correlation between the EW and the local number density of objects. In this case, if it should turn out that the IGM is significantly neutral at z = 6.5 and the intrinsic EW distribution is relatively narrow, the observed EWs can also be used to construct a map of the locations and approximate sizes of the largest ionized regions. For the JWST survey, the results appear a bit less optimistic. Since such surveys probe a large range of redshifts, the effects of the IGM will be mixed up with any intrinsic galaxy evolution that is present, making it difficult to disentangle the effects. However, we show that a survey with the JWST will have a possibility of observing a large group of galaxies at z similar to 7, which would be a strong indication of a partially neutral IGM.

  • 22. Konstantopoulos, I. S.
    et al.
    Smith, L. J.
    Adamo, A.
    Silva-Villa, E.
    Gallagher, J. S.
    Bastian, N.
    Ryon, J. E.
    Westmoquette, M. S.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Larsen, S. S.
    Weisz, D. R.
    Charlton, J. C.
    THE SNAPSHOT HUBBLE U-BAND CLUSTER SURVEY (SHUCS). I. SURVEY DESCRIPTION AND FIRST APPLICATION TO THE MIXED STAR CLUSTER POPULATION OF NGC 40412013In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 145, no 5, p. 137-Article in journal (Refereed)
    Abstract [en]

    We present the Snapshot Hubble U-band Cluster Survey (SHUCS), a project aimed at characterizing the star cluster populations of 10 nearby galaxies (d < 23 Mpc, half within approximate to 12 Mpc) through new F336W (U-band equivalent) imaging from Wide Field Camera 3, and archival BVI-equivalent data with the Hubble Space Telescope. Completing the UBVI baseline reduces the age-extinction degeneracy of optical colors, thus enabling the measurement of reliable ages and masses for the thousands of clusters covered by our survey. The sample consists chiefly of face-on spiral galaxies at low inclination, in various degrees of isolation (isolated, in group, merging), and includes two active galactic nucleus hosts. This first paper outlines the survey itself, the observational datasets, the analysis methods, and presents a proof-of-concept study of the large-scale properties and star cluster population of NGC 4041, a massive SAbc galaxy at a distance of approximate to 23 Mpc, and part of a small grouping of six giant members. We resolve two structural components with distinct stellar populations, a morphology more akin to merging and interacting systems. We also find strong evidence of a truncated, Schechter-type mass function, and a similarly segmented luminosity function. These results indicate that binning must erase much of the substructure present in the mass and luminosity functions, and might account for the conflicting reports on the intrinsic shape of these functions in the literature. We also note a tidal feature in the outskirts of the galaxy in Galaxy Evolution Explorer UV imaging, and follow it up with a comprehensive multi-wavelength study of NGC 4041 and its parent group. We deduce a minor merger as a likely cause of its segmented structure and the observed pattern of a radially decreasing star formation rate. We propose that combining the study of star cluster populations with broadband metrics is not only advantageous, but often easily achievable thorough archival datasets.

  • 23. Leitet, E.
    et al.
    Bergvall, N.
    Hayes, M.
    Linne, S.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Escape of Lyman continuum radiation from local galaxies: Detection of leakage from the young starburst Tol 1247-2322013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 553, article id A106Article in journal (Refereed)
    Abstract [en]

    Context. It has been suggested that the escape fraction of hydrogen ionizing photons (f(esc)) from galaxies is evolving with time, but the picture is far from clear. While evidence for significant escape fractions has been found at high redshifts in several studies, the picture looks different in the closer universe. The lack of local detections could in principle be a combined effect of an evolving escape fraction, the low number galaxies observed, the selection criteria of these targets, and technical problems associated with the instrument best adapted for low redshift targets, the Far Ultraviolet Spectroscopic Explorer (FUSE). Aims. In an attempt to tackle the last of these effects, we apply a new background routine developed for FUSE data to study galaxies from the archive in search of possible Lyman continuum (LyC) leakage. In the process, for the first time a stacked spectrum in the LyC has been produced for local galaxies. With this small sample, we also make a more tentative approach to looking for possible correlations between f(esc) and physical parameters, such as internal absorption E(B - V)(i), mass, H I mass, specific star formation rate (SSFR), metallicity, and Ly alpha emission. Methods. Eight star forming galaxies with redshifts z > 0.015 from the FUSE archive were re-examined. Also, a subsample of an additional four galaxies with lower redshifts were included, for which the escape fraction was estimated from residual flux in the low ionization interstellar C II lambda 1036 angstrom line. Results. Out of the eight galaxies, only one was found to have significant LyC leakage, Tol 1247-232 (S/N = 5.2). This is the second detection of a leaking galaxy in the local universe. From the first case, Haro 11, we derive an intrinsic Lyman break amplitude for starbursts at this young age of (f(1500) (angstrom)/f(900) (angstrom))(int) = 1.5(-0.5)(+0.6), which gives an absolute escape fraction for Tol 1247-232 of f(esc) = 2.4(-0.8)(+0.9) %. Tol 1247-232 exhibits an extremely blue far-UV slope reminiscent of high redshift LyC leaking galaxies, and although it does not classify as an AGN in the BPT diagram or by other available diagnostics, a minor AGN cannot be completely excluded. The stacked sample shows an excess in the LyC with f(esc) = 1.4(-0.5)(+0.6) %, but we note that there might be important selection biases involved, since the galaxies were originally handpicked for their star forming qualities. With the small sample, we suggest a possible trend toward higher f(esc) with lower mass and with enhanced SSFR. None of the galaxies with high values of E(B - V)(i) were found to show any sign of leakage.

  • 24. Lietzen, Heidi
    et al.
    Flynn, Chris
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Nurmi, Pasi
    Deciphering Quasar Millilensing How to Distinguish Between Dark and Luminous Lenses2008In: Dark Galaxies and Lost Baryons: IAU Symposium, Volume 244, 2008, p. 368-369Conference paper (Other academic)
  • 25.
    Micheva, Genoveva
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bergvall, Nils
    Department of Physics and Astronomy, Uppsala University.
    A Super-Deep Study of the Outskirts of Haro 112010In: Hunting for the dark: The hidden side of galaxy formation / [ed] Victor P. Debattista, Cristina C. Popescu, New York: American Institute of Physics , 2010, Vol. 1240, p. 295-296Conference paper (Other academic)
    Abstract [en]

    The low surface brightness host of Haro 11 has been observed to have anomalously red optical/NIR colors of V-K = 4.2+/-0.8 [1]. Predictions from current stellar evolutionary models for any normal stellar population over a wide range of metallicities are inconsistent with this color. We present the deepest V and K band observations to date of Haro 11 and derive a new V-K color for the host galaxy. Our new data suggest a far less extreme colour of V-K = 2.3+/-0.2, which is perfectly consistent with the expectations for an old host galaxy with the same metallicity as that derived from nebular emission lines in the star-forming center.

  • 26.
    Micheva, Genoveva
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bergvall, Nils
    Deep V and K band photometry of the host galaxy of Haro 112009Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    Previous studies of the host galaxy of Haro 11 have suggested an extreme red color of V-K=4.2+-0.8 which cannot be reconciled with any normal stellar population of reasonable metallicity. We present the deepest V and K band data to date of the blue compact galaxy (BCG) Haro 11 and derive a new V-K color for the host. Our new data suggest a much more modest value of V-K=2.3+-0.2, which is in the same range as V-K colors measured for several other BCGs. The new Haro 11 color is not abnormally red and can be attributed to an old metal-poor stellar population with a Salpeter initial mass function.

  • 27.
    Micheva, Genoveva
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bergvall, Nils
    Pursimo, Tapio
    The faint outskirts of the blue compact galaxy Haro 11: is there a red excess?2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 405, no 2, p. 1203-1211Article in journal (Refereed)
    Abstract [en]

    Previous studies of the low surface brightness host of the blue compact galaxy (BCG) Haro 11 have suggested an abnormally red colour of V - K = 4.2 +/- 0.8 for the host galaxy. This colour is inconsistent with any normal stellar population over a wide range of stellar metallicities (Z = 0.001-0.02). Similar though less extreme host colours have been measured for other BCGs and may be reconciled with population synthesis models, provided that the stellar metallicity of the host is higher than that of the ionized gas in the central starburst. We present the deepest V- and K-band observations to date of Haro 11 and derive a new V - K colour for the host galaxy. Our new data suggest a far less extreme colour of V - K = 2.3 +/- 0.2, which is perfectly consistent with the expectations for an old host galaxy with the same metallicty as that derived from nebular emission lines in the star-forming centre.

  • 28.
    Micheva, Genoveva
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bergvall, Nils
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Masegosa, Josefa
    Marquez, Isabel
    Marquart, Thomas
    Stockholm University, Faculty of Science, Department of Astronomy. Uppsala University.
    Durret, Florence
    Deep multiband surface photometry on a sample of 24 blue compact galaxies - I2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 431, no 1, p. 102-144Article in journal (Refereed)
    Abstract [en]

    We present deep optical and near-infrared (NIR) UBVRIHKs imaging data for 24 blue compact galaxies (BCGs). The individual exposure times are on average similar to 40 min in the optical (B) and similar to 90 min in the NIR, but on occasion up to similar to 5 h for a single target and filter, observed with 2.5, 3.5, 8.2-m telescopes. The sample contains luminous dwarf and intermediate-mass BCGs which are predominantly metal poor, although a few have near-solar metallicities. We have analysed isophotal and elliptical integration surface brightness and colour profiles, extremely deep (mu(B) less than or similar to 29 mag arcsec(-2)) contour maps and RGB images for each galaxy in the sample, and provide a morphological classification where such is missing. Separating the burst from the underlying host we find that regardless of the total luminosity the host galaxy has the properties of a low surface brightness (LSB) dwarf with M-B greater than or similar to -18. For a number of galaxies we discover a distinct LSB component dominant around and beyond the Holmberg radius. For the specific case of ESO 400-43A&B we detect an optical bridge between the two companion galaxies at the mu(V) similar to 28th mag arcsec(-2) isophotal level. Synthetic disc tests are performed to verify that we can trace such faint components with negligible errors down to mu(B) = 28 and mu(K) = 23 mag arcsec(-2). By examining the structural parameters (central surface brightness mu(0) and scalelength h(r)) derived from two radial ranges typically assumed to be dominated by the underlying host galaxy, we demonstrate the importance of sampling the host well away from the effects of the burst. We find that mu(0) and h(r) of the BCGs host deviate from those of dwarf ellipticals (dEs) and dwarf irregulars (dI) solely due to a strong burst contribution to the surface brightness profile almost down to the Holmberg radius. Structural parameters obtained from a fainter region, mu(B) = 26-28 mag arcsec(-2), are consistent with those of true LSB galaxies for the starbursting BCGs in our sample, and with dEs and dIs for the BCGs with less vigorous star formation.

  • 29.
    Micheva, Genoveva
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bergvall, N.
    Marquart, Thomas
    Stockholm University, Faculty of Science, Department of Astronomy.
    Masegosa, J.
    Marquez, I.
    Cumming, R. J.
    Durret, F.
    Deep multiband surface photometry on a sample of 24 blue compact galaxies II. A volume-limited sample of 21 emission line galaxies2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 556, p. A10-Article in journal (Refereed)
    Abstract [en]

    Aims. We present deep surface photometry of a volume-limited sample of 21 UM emission line galaxies in broadband optical UBVRI and near infrared (NIR) HKs filters. The sample comprises 19 blue compact galaxies (BCGs) and two spirals. Methods. We separated the burst and host populations for each galaxy and compared them to stellar evolutionary models with and without nebular emission contribution. We measured and analyzed the A(180) asymmetry in all filters, the concentration index C, the scale length, and the central surface brightness of the host galaxy. Results. A shift in the average A180 asymmetry is detected from optical to NIR. This shift seems correlated with the morphological class of the BCGs. Using the color-asymmetry relation, we identify five BCGs in the sample as mergers, which is confirmed by their morphological class. Though clearly separated from normal galaxies in the concentration-asymmetry parameter space, we find that it is not possible to distinguish luminous starbursting BCGs from the merely star forming low luminosity BCGs.

  • 30.
    Riehm, T.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, E.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Mortsell, E.
    Stockholm University, Faculty of Science, Department of Physics.
    Wiik, K.
    Detecting CDM substructure via gravitational millilensing2008In: Identification of dark matter 2008, 2008, p. 54-Conference paper (Refereed)
    Abstract [en]

    Not Available

  • 31.
    Riehm, Teresa
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Möller, Olle
    Mörtsell, Edvard
    Department of Physics.
    Wiik, Kaj
    Prospects for CDM sub-halo detection using high angular resolution observations2008In: The Universe Under the Microscope - Astrophysics at High Angular Resolution, Journal of Physics: Conference Series, Volume 131, 2008, p. 012045-Conference paper (Refereed)
  • 32.
    Riehm, Teresa
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Mörtsell, Edvard
    Stockholm University, Faculty of Science, Department of Physics.
    Wiik, K.
    Strong Lensing by Subhalos in the Dwarf-galaxy-mass Range. II. Detection Probabilities2009In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 700, no 2, p. 1552-1558Article in journal (Refereed)
    Abstract [en]

    The dark halo substructures predicted by current cold dark matter simulations may in principle be detectable through strong-lensing image-splitting of quasars on small angular scales (0.01 arcsec or below). Here, we estimate the overall probabilities for lensing by substructures in a host halo closely aligned to the line of sight to a background quasar. Under the assumption that the quasar can be approximated as a point source, the optical depth for strong gravitational lensing by subhalos typically turns out to be very small (τ < 0.01), contrary to previous claims. We therefore conclude that it is currently not feasible to use this strategy to put the simulation predictions for the dark matter subhalo population to the test. However, if one assumes the source to be spatially extended, as is the case for a quasar observed at radio wavelengths, there is a reasonable probability for witnessing substructure lensing effects even at rather large projected distances from the host galaxy, provided that the angular resolution is sufficient. While multiply imaged, radio-loud quasars would be the best targets for unambiguously detecting dark matter subhalos, even singly imaged radio quasars might be useful for setting upper limits on the abundance and central surface mass density of subhalos.

  • 33.
    Riehm, Teresa
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Wiik, Kaj
    Möller, Olle
    On the Probability for Sub-Halo Detection through Quasar Image Splitting2008In: Dark Galaxies and Lost Baryons: IAU Symposium, Volume 244, 2008, p. 376-377Conference paper (Other academic)
  • 34.
    Rydberg, Claes-Erik
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Lundqvist, Peter
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Scott, Pat
    Detection of isolated Population III stars with the James Webb Space Telescope2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 429, no 4, p. 3658-3664Article in journal (Refereed)
    Abstract [en]

    The first Population III stars are predicted to form in minihaloes at z approximate to 10-30. The James Webb Space Telescope (JWST), tentatively scheduled for launch in 2018, will probably be able to detect some of the first galaxies, but whether it will also be able to detect the first stars remains more doubtful. Here, we explore the prospects of detecting an isolated Population III star or a small cluster of Population III stars down to z = 2 in either lensed or unlensed fields. Our calculations are based on realistic stellar atmospheres and take into account the potential flux contribution from the surrounding HII region. We find that unlensed Population III stars are beyond the reach of JWST, and that even lensed Population III stars will be extremely difficult to detect. However, the main problem with the latter approach is not necessarily that the lensed stars are too faint, but that their surface number densities are too low. To detect even one 60 M-circle dot Population III star when pointing JWST through the galaxy cluster MACS J0717.5+3745, the lensing cluster with the largest Einstein radius detected so far, the cosmic star formation rate of Population III stars would need to be approximately an order of magnitude higher than predicted by the most optimistic current models.

  • 35.
    Rydberg, Claes-Erik
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Uppsala University, Sweden.
    Zitrin, Adi
    Guaita, Lucia
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Melinder, Jens
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Asadi, Saghar
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Gonzalez, Juan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ström, Tina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    A search for Population III galaxies in CLASH. I. Singly-imaged candidates at high redshift2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 804, no 1, article id 13Article in journal (Refereed)
    Abstract [en]

    Population III galaxies are predicted to exist at high redshifts and may be rendered sufficiently bright for detection with current telescopes when gravitationally lensed by a foreground galaxy cluster. Population III galaxies that exhibit strong Lyα emission should furthermore be identifiable from broadband photometry because of their unusual colors. Here, we report on a search for such objects at z > 6 in the imaging data from the Cluster Lensing And Supernova survey with Hubble (CLASH), covering 25 galaxy clusters in 16 filters. Our selection algorithm returns five singly-imaged candidates with Lyα-like color signatures, for which ground-based spectroscopy with current 8-10 m class telescopes should be able to test the predicted strength of the Lyα line. None of these five objects have been included in previous CLASH compilations of high-redshift galaxy candidates. However, when large grids of spectral synthesis models are applied to the study of these objects, we find that only two of these candidates are significantly better fitted by Population III models than by more mundane, low-metallicity stellar populations.

  • 36.
    Rydberg, Claes-Erik
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Zitrin, Adi
    Melinder, Jens
    Stockholm University, Faculty of Science, Department of Astronomy.
    Gonzalez, Juan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    A search for Population III galaxies in CLASH. II. A multiply-imaged candidate at z ~ 7.8 behind Abell 22612014Manuscript (preprint) (Other academic)
    Abstract [en]

    While the Lyα emission line has served as an important tool in the study of galaxies at z < 6.5, Lyα emitters (LAE) have proved to be elusive at higher redshifts. If galaxies exhibiting high Lyα equivalent widths exist at z > 6.5, such rare objects may potentially stand out in multiband imaging surveys because of their unusual colors. We have conducted a search for such objects in the Cluster Lensing And Supernova survey with Hubble (CLASH) survey, and here report on the discovery of three gravitationally lensed images of a single LAE candidate behind the Abell 2261 cluster. Model fits to the CLASH broadband photometry suggests very strong Lyα emission (rest-frame Lyα equivalent width EW(Lyα) > 200 Å, prior to any IGM correction) at a redshift of z ≈ 7.8.

  • 37.
    Rydberg, Claes-Erik
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Universität Heidelberg, Germany.
    Zitrin, Adi
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Uppsala University, Sweden.
    Melinder, Jens
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Whalen, Daniel J.
    Klessen, Ralf S.
    Gonzalez, Juan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Carollo, Daniela
    A multiply imaged z similar to 6.3 Lyman a emitter candidate behind Abell 22612017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 467, no 1, p. 768-777Article in journal (Refereed)
    Abstract [en]

    While the Lyman alpha (Ly alpha) emission line serves as an important tool in the study of galaxies at z less than or similar to 6, finding Ly a emitters (LAE) at significantly higher redshifts has been more challenging, probably because of the increasing neutrality of the intergalactic medium above z similar to 6. Galaxies with extremely high rest-frame Ly alpha equivalent widths, EW(Ly alpha) greater than or similar to 150 A degrees, at z > 6, are good candidates for Ly alpha follow-up observations, and can stand out in multiband imaging surveys because of their unusual colours. We have conducted a photometric search for such objects in the Cluster Lensing And Supernova survey with Hubble (CLASH), and report here the identification of three likely gravitationally lensed images of a single LAE candidate at z-6.3, behind the galaxy cluster Abell 2261 (z=0.225). In the process, we also measured with Keck/Multi-Object Spectrometer For Infra-Red Exploration the first spectroscopic redshift of a multiply imaged galaxy behind Abell 2261, at z=3.337. This allows us to calibrate the lensing model, which, in turn, is used to study the properties of the candidate LAE. Population III galaxy spectral energy distribution model fits to the CLASH broad-band photometry of the possible LAE provide a slightly better fit than Population I/II models. The best-fitting model suggests intrinsic EW(Ly alpha) approximate to 160 A degrees after absorption in the interstellar and intergalactic medium. Future spectroscopic observations will examine this prediction as well as shed more light on the morphology of this object, which indicates that it may be a merger of two smaller galaxies.

  • 38. Ryon, J. E.
    et al.
    Adamo, Angela
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bastian, N.
    Smith, L. J.
    Gallagher, J. S. , I I I
    Konstantopoulos, I. S.
    Larsen, S.
    Silva-Villa, E.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    THE SNAPSHOT HUBBLE U-BAND CLUSTER SURVEY (SHUCS). II. THE STAR CLUSTER POPULATION OF NGC 29972014In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 148, no 2, p. 33-Article in journal (Refereed)
    Abstract [en]

    We study the star cluster population of NGC 2997, a giant spiral galaxy located at 9.5 Mpc and targeted by the Snapshot Hubble U-band Cluster Survey (SHUCS). Combining our U-band imaging from SHUCS with archival BVI imaging from the Hubble Space Telescope, we select a high confidence sample of clusters in the circumnuclear ring and disk through a combination of automatic detection procedures and visual inspection. The cluster luminosity functions in all four filters can be approximated by power laws with indices of -1.7 to -2.3. Some deviations from pure power-law shape are observed, hinting at the presence of a high-mass truncation in the cluster mass function. However, upon inspection of the cluster mass function, we find it is consistent with a pure power law of index -2.2 +/- 0.2 despite a slight bend at similar to 2.5 x 10(4) M-circle dot. No statistically significant truncation is observed. From the cluster age distributions, we find a low rate of disruption(zeta similar to -0.1) in both the disk and circumnuclear ring. Finally, we estimate the cluster formation efficiency (Gamma) over the last 100 Myr in each region, finding 7% +/- 2% for the disk, 12% +/- 4% for the circumnuclear ring, and 10% +/- 3% for the entire UBVI footprint. This study highlights the need for wide-field UBVI coverage of galaxies to study cluster populations in detail, though a small sample of clusters can provide significant insight into the characteristics of the population.

  • 39. Silva-Villa, E.
    et al.
    Adamo, A.
    Bastian, N.
    Fouesneau, M.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    The age distribution of stellar clusters in M832014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 440, no 1, p. L116-L120Article in journal (Refereed)
    Abstract [en]

    In order to empirically determine the time-scale and environmental dependence of stellar cluster disruption, we have undertaken an analysis of the unprecedented multipointing (seven), multiwavelength (U, B, V, H alpha, and I) Hubble Space Telescope imaging survey of the nearby, face-on spiral galaxy M83. The images are used to locate stellar clusters and stellar associations throughout the galaxy. Estimation of cluster properties (age, mass, and extinction) was done through a comparison of their spectral energy distributions with simple stellar population models. We constructed the largest catalogue of stellar clusters and associations in this galaxy to-date, with similar to 1800 sources with masses above similar to 5000 M-circle dot and ages younger than similar to 300 Myr. In this Letter, we focus on the age distribution of the resulting clusters and associations. In particular, we explicitly test whether the age distributions are related with the ambient environment. Our results are in excellent agreement with previous studies of age distributions in the centre of the galaxy, which gives us confidence to expand out to search for similarities or differences in the other fields which sample different environments. We find that the age distribution of the clusters inside M83 varies strongly as a function of position within the galaxy, indicating a strong correlation with the galactic environment. If the age distributions are approximated as a power law of the form dN/dt alpha t(zeta), we find zeta values between 0 and -0.62 (zeta similar to -0.40 for the whole galaxy), in good agreement with previous results and theoretical predictions.

  • 40. Wiik, K.
    et al.
    Zackrisson, E.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Riehm, T.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Dark Matter Millilensing and VSOP-22009In: Approaching Micro-Arcsecond Resolution with VSOP-2 / [ed] Yoshiaki Hagiwara, Ed Fomalont, Masato Tsuboi, and Yasuhiro Murata, 2009, Vol. 402, p. 326-Conference paper (Other academic)
    Abstract [en]

    According to the cold dark matter scenario, a large number of dark subhalos should be located within the halo of each Milky-way sized galaxy. One promising possibility for detecting such subhalos is to try to observe their gravitational lensing effects on background sources. Dark matter subhalos in the 10^6 -- 1010 Mȯ mass range should cause strong gravitational lensing on the (sub)milliarcsecond scales, which can be observed only using space VLBI. We study the feasibility of a strong-lensing detection of dark subhalos by deriving the image separations expected for density profiles favoured by current simulations and comparing it to the angular resolution of both existing and upcoming observational facilities. We show that the detection of subhalos is likely much more difficult than suggested in previous studies, due to the smaller image separations predicted for subhalo density profiles that are more realistic than the singular isothermal sphere models often adopted.

  • 41.
    Zackrisson, E.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Flynn, C.
    Bergvall, N.
    Micheva, G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Östlin, G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Caldwell, B.
    Red halos and dark baryons2008In: Identification of dark matter 2008, 2008, p. 47-Conference paper (Refereed)
    Abstract [en]

    Not Available

  • 42.
    Zackrisson, E.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Micheva, G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bergvall, N.
    Ostlin, G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Do blue compact galaxies have red halos?2009Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    Red halos are faint, extended and extremely red structures that have been reported around various types of galaxies since the mid-1990s. The colours of these halos are too red to be reconciled with any hitherto known type of stellar population, and instead indicative of a very bottom-heavy stellar initial mass function (IMF). Due to the large mass-to-light ratios of such stellar halos, they could contribute substantially to the baryonic masses of galaxies while adding very little to their overall luminosities. The red halos of galaxies therefore constitute potential reservoirs for some of the baryons still missing from inventories in the low-redshift Universe. While most studies of red halos have focused on disk galaxies, a red excess has also been reported in the faint outskirts of blue compact galaxies (BCGs). A bottom-heavy IMF can explain the colours of these structures as well, but due to model degeneracies, stellar populations with standard IMFs and abnormally high metallicities have also been demonstrated to fit the data. Here, we show that due to recent developments in the field of spectral synthesis, the metallicities required in this alternative scenario may be less extreme than previously thought. This suggests that the red excess seen in the outskirts of BCGs may stem from a normal, intermediate-metallicity host galaxy rather than a red halo of the type seen around disk galaxies. The inferred host metallicity does, however, still require the host to be more metal-rich than the gas in the central starburst of BCGs, in contradiction with current simulations of how BCGs form.

  • 43.
    Zackrisson, E.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Riehm, T.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Gravitational lensing as a probe of cold dark matter subhalos2010In: Advances is astronomy, Vol. 2010Article, review/survey (Refereed)
    Abstract [en]

    In the cold dark matter scenario, dark matter halos are assembled hierarchically from smaller subunits. Some of these subunits are disrupted during the merging process, whereas others survive temporarily in the form of subhalos. A long-standing problem with this picture is that the number of subhalos predicted by simulations exceeds the number of luminous dwarf galaxies seen in the the vicinity of large galaxies like the Milky Way. Many of the subhalos must therefore have remained dark or very faint. If cold dark matter subhalos are as common as predicted, gravitational lensing may in principle offer a promising route to detection. In this review, we describe the many ways through which lensing by subhalos can manifest itself, and summarize the results from current efforts to constrain the properties of cold dark matter subhalos using such effects.

  • 44.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Mörk materia kan vara röd2008In: Forskning och Framsteg, ISSN 0015-7937, no 3Article in journal (Other (popular science, discussion, etc.))
  • 45.
    Zackrisson, Erik
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pointing the James Webb Space Telescope through lensing clusters - can the first stars and galaxies be detected?2010In: Cosmic Radiation Fields: Sources in the Early Universe / [ed] Martin Raue (chair), Tanja Kneiske, Dieter Horns, Dominik Elsaesser, and Peter Hauschildt, 2010, p. 22-Conference paper (Other academic)
    Abstract [en]

    Not Available

  • 46.
    Zackrisson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Asadi, Saghar
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Wiik, Kaj
    Jonsson, Jakob
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Scott, Pat
    Datta, Kanan K.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Friedrich, Martina M.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Jensen, Hannes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Johansson, Joel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Rydberg, Claes-Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sandberg, Andreas
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hunting for dark halo substructure using submilliarcsecond-scale observations of macrolensed radio jets2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 431, no 3, p. 2172-2183Article in journal (Refereed)
    Abstract [en]

    Dark halo substructure may reveal itself through secondary, small-scale gravitational lensing effects on light sources that are macrolensed by a foreground galaxy. Here, we explore the prospects of using Very Long Baseline Interferometry (VLBI) observations of multiply-imaged quasar jets to search for submilliarcsecond-scale image distortions produced by various forms of dark substructures in the 10(3)-10(8) M-circle dot mass range. We present lensing simulations relevant for the angular resolutions attainable with the existing European VLBI Network, the global VLBI array and an upcoming observing mode in which the Atacama Large Millimeter Array (ALMA) is connected to the global VLBI array. While observations of this type would not be sensitive to standard cold dark matter subhaloes, they can be used to detect the more compact forms of halo substructure predicted in alternative structure formation scenarios. By mapping approximately five strongly lensed systems, it should be possible to detect or robustly rule out primordial black holes in the 10(3)-10(6) M-circle dot mass range if they constitute greater than or similar to 1 per cent of the dark matter in these lenses. Ultracompact minihaloes are harder to detect using this technique, but 10(6)-10(8) M-circle dot ultracompact minihaloes could in principle be detected if they constitute greater than or similar to 10 per cent of the dark matter.

  • 47.
    Zackrisson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bergvall, Nils
    Flynn, Chris
    Caldwell, Brady
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Micheva, Genoveva
    Stockholm University, Faculty of Science, Department of Astronomy.
    The Red Halos of Disk Galaxies2008In: Formation and Evolution of Galaxy Disks: ASP Conference Series, Vol. 396, 2008, p. 229-230Conference paper (Other academic)
  • 48.
    Zackrisson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bergvall, Nils
    Flynn, Chris
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Micheva, Genoveva
    Stockholm University, Faculty of Science, Department of Astronomy.
    Caldwell, Brady
    Red Halos of Galaxies - Reservoirs of Baryonic Dark Matter?2008In: Dark Galaxies and Lost Baryons: IAU Symposium, Volume 244, 2008, p. 17-25Conference paper (Other (popular science, discussion, etc.))
  • 49.
    Zackrisson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bergvall, Nils
    Leitet, Elisabet
    The Impact of Nebular Emission on the Broadband Fluxes of High Redshift Galaxies2008In: Formation and Evolution of Galaxy Disks: ASP Conference Series, Vol. 396, 2008, p. 435-436Conference paper (Other academic)
  • 50.
    Zackrisson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bergvall, Nils
    Leitet, Elisabet
    The Impact of Nebular Emission on the Broadband Fluxes of High-Redshift Galaxies2008In: Astrophysical Journal, Vol. 676, p. 9Article in journal (Refereed)
12 1 - 50 of 67
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