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Super Star Clusters in Blue Compact Galaxies: Evidence for a near-infrared flux excess and properties of the starburst phase
Stockholm University, Faculty of Science, Department of Astronomy.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Luminous Blue compact galaxies (BCGs) are metal-poor actively star-forming systems, characterised by bright ultraviolet and blue luminosities. Hubble Space Telescope high-resolution data have revealed that the luminous star-forming knots in these galaxies are composed of hundreds of young massive star clusters. In this work we present a systematic study of the star cluster populations in BCGs with important implications for the formation history of their host systems. The studied galaxies show recently increased star formation rates and a high fraction of massive clusters, probably as a result of minor/major merger events. The age distributions have a peak of cluster formation at only 3 - 4 Myr, 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 μm. 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). The origin of the NIR excess, which still remains unexplained, challenges our understanding of the cluster formation process under extreme conditions.

The results achieved in this work have produced important insights into the cluster formation process in BCGs. We suggest that the BCG environment has most likely favoured the compression and collapse of giant molecular clouds into compact massive star clusters. The cluster formation efficiency (i.e., the fraction of star formation happening in star clusters) in BCGs is higher than the reported 8 - 10 %, for quiescent spirals and local star-forming galaxies. Luminous BCGs have a cluster formation efficiency comparable to luminous infrared galaxies and spiral starburst nuclei (the averaged value is  about 30 %), suggesting an important role of the merger event in the cluster formation.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University , 2011. , 56 p.
Keyword [en]
starburst galaxy-extragalactic star clusters-dwarf merger
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-52681ISBN: 978-91-7447-215-8 (print)OAI: oai:DiVA.org:su-52681DiVA: diva2:388561
Public defence
2011-02-18, sal FB53, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Accepted. Paper 5: Manuscript. Paper 6: Manuscript.Available from: 2012-01-11 Created: 2011-01-17 Last updated: 2012-01-11Bibliographically approved
List of papers
1. Adaptive optics imaging and optical spectroscopy of a multiple merger in a luminous infrared galaxy
Open this publication in new window or tab >>Adaptive optics imaging and optical spectroscopy of a multiple merger in a luminous infrared galaxy
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2008 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 384, no 3, 886-906 p.Article in journal (Refereed) Published
Abstract [en]

We present near-infrared (NIR) adaptive optics imaging obtained with VLT/NACO and optical spectroscopy from the Southern African Large Telescope of a luminous IR galaxy (LIRG) IRAS 19115-2124. These data are combined with archival Hubble Space Telescope imaging and Spitzer imaging and spectroscopy, allowing us to study this disturbed interacting/merging galaxy, dubbed the Bird, in extraordinary detail. In particular, the data reveal a triple system where the LIRG phenomenon is dominated by the smallest of the components.

One nucleus is a regular barred spiral with significant rotation, while another is highly disturbed with a surface brightness distribution intermediate to that of disc and bulge systems, and hints of remaining arm/bar structure. We derive dynamical masses in the range 3-7 × 1010Msolarfor both. The third component appears to be a 1-2 × 1010Msolar mass irregular galaxy. The total system exhibits HII galaxy-like optical line ratios and strengths, and no evidence for active galactic nucleus (AGN) activity is found from optical or mid-IR data. The star formation rate is estimated to be ~190Msolaryr-1. We also report a search for supernovae from NIR images separated by five months and search for superstar cluster candidates. We detect outflowing gas from the Bird mostly in the range 100-300 km s-1 using NaI D absorption features. Overall, the Bird shows kinematic, dynamical and emission-line properties typical for cool ultraluminous IR galaxies (ULIRGs). However, the interesting features setting it apart for future studies are its triple merger nature, and the location of its star formation peak - the strongest star formation, as revealed by Spitzer imaging, does not come from the two major K-band nuclei, but from the third irregular component. This is in contrast to the conventional view that the (U)LIRG phases are powered by infalling gas to the major nuclei of the merging spiral galaxies. Aided by simulations, we discuss scenarios where the irregular component is on its first high-speed encounter with the more massive components.

Based on observations made with ESO Telescopes at the Paranal Observatory under programme 073.D-0406A, and with the Southern African Large Telescope (SALT).

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-18289 (URN)doi:10.1111/j.1365-2966.2007.12703.x (DOI)000253760300004 ()
Available from: 2009-01-25 Created: 2009-01-25 Last updated: 2017-12-13Bibliographically approved
2. Super star clusters in Haro 11: properties of a very young starburst and evidence for a near-infrared flux excess
Open this publication in new window or tab >>Super star clusters in Haro 11: properties of a very young starburst and evidence for a near-infrared flux excess
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2010 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, MNRAS, Vol. 407, no 2, 870-890 p.Article in journal (Refereed) Published
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.

Keyword
galaxies: irregular, galaxies: starburst, galaxies: star clusters: individual: Haro 11
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-52740 (URN)10.1111/j.1365-2966.2010.16983.x (DOI)
Available from: 2011-01-18 Created: 2011-01-18 Last updated: 2017-12-11Bibliographically approved
3. On the Origin of the Red Excess in Very Young Super Star Clusters: The Case of SBS 0335-052E
Open this publication in new window or tab >>On the Origin of the Red Excess in Very Young Super Star Clusters: The Case of SBS 0335-052E
2010 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 725, no 2, 1620-1628 p.Article in journal (Refereed) Published
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.

Keyword
galaxies: dwarf, galaxies: individual: SBS 0335-052E, galaxies: starburst, galaxies: star clusters: general
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-52670 (URN)10.1088/0004-637X/725/2/1620 (DOI)
Available from: 2011-01-17 Created: 2011-01-17 Last updated: 2017-12-11Bibliographically approved
4. The Massive Star Clusters in the Dwarf Merger ESO 185-IG13: is the Red Excess Ubiquitous in Starbursts?
Open this publication in new window or tab >>The Massive Star Clusters in the Dwarf Merger ESO 185-IG13: is the Red Excess Ubiquitous in Starbursts?
2011 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 414, no 3, 1793-1812 p.Article in journal (Other academic) Published
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.

Keyword
galaxies: irregular, galaxies: starburst, galaxies: star clusters: individual: ESO 185-IG13
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-52669 (URN)10.1111/j.1365-2966.2011.18262.x (DOI)000292977600001 ()
Available from: 2011-01-17 Created: 2011-01-17 Last updated: 2017-12-11Bibliographically approved
5. Star cluster formation and evolution in Mrk 930: properties of a metal-poor starburst
Open this publication in new window or tab >>Star cluster formation and evolution in Mrk 930: properties of a metal-poor starburst
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2011 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 415, no 3, 2388-2406 p.Article in journal (Refereed) Published
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.

Keyword
galaxies: irregular, galaxies: starburst, galaxies: star clusters: general, galaxies: star formation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-69268 (URN)10.1111/j.1365-2966.2011.18867.x (DOI)000294173900033 ()
Note
authorCount :7Available from: 2012-01-11 Created: 2012-01-11 Last updated: 2017-12-08Bibliographically approved
6. Probing Cluster Formation under Extreme Conditions: Super Star Clusters in Blue Compact Galaxies
Open this publication in new window or tab >>Probing Cluster Formation under Extreme Conditions: Super Star Clusters in Blue Compact Galaxies
(English)Manuscript (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 %).

Identifiers
urn:nbn:se:su:diva-52677 (URN)
Available from: 2011-01-17 Created: 2011-01-17 Last updated: 2012-01-11Bibliographically approved

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