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Unveiling the nature of blue compact galaxies
Stockholm University, Faculty of Science, Department of Astronomy.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Blue compact galaxies (BCGs) are gas-rich star-forming low redshift galaxies with low metallicities. In some cases the relative strength of the starburst can be so high that it completely dominates the light output of the galaxy, an obstacle which has been countered by deeper optical imaging data and observations in the near infra-red (NIR) regime. This has revealed an older population referred to as the "host". In an effort to study the hosts of BCGs we have analyzed new and extremely deep UBVRIHKs imaging data for 46 high and low luminosity BCGs. For several BCGs the data reveal previously undetected extended low surface brightness components beyond the μB~26 mag arcsec-2 isophote. These are predominantly the luminous BCGs in the sample, and they show tails, plumes, optical bridges between companion galaxies, and other signs of merging or strong tidal interactions. The low luminosity BCGs, on the other hand, are well represented by an exponential disk profile down to the reliability limit of the data at a surface brightness level of μB~28 mag arcsec-2.

The burst and host populations are examined separately. The integrated colors of both are compared to predictions from spectral evolutionary models, giving an indication of their respective ages and metallicities. Our analysis suggests that for the luminous BCGs a strong contribution by nebular emission is present almost down to the Holmberg radius, invalidating the host structural parameters obtained from brighter isophotes. Possible evolutionary links to quiescent galaxies like dEs, dIs, and LSBGs are explored by examining the structural parameters derived from two radial ranges typically assumed to be dominated by the underlying host galaxy. In this parameter space the luminous BCGs in our sample deviate from their low luminosity counterparts and from BCG data in the literature. They are instead consistent with the structural properties of giant low surface brightness galaxies with central surface brightnesses μB≥23 mag arcsec-2. We further examine the asymmetry and concentration parameters for the sample and study the correlation between the minimum asymmetry distribution in the optical and NIR vs morphological class, concentration and integrated colors to identify mergers/tidally interacting galaxies. A shift in the asymmetry distribution occurs for low luminosity BCGs from the optical to the NIR. In contrast, we find that the flocculent asymmetry component (due to star formation) completely dominates the composite asymmetry of high luminosity BCGs. We introduce an alternative asymmetry measure which successfully traces the dynamical asymmetry component (due to merging/tidal interaction) of the host.  

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University , 2012. , 45 p.
Keyword [en]
blue compact galaxies, starbursts, host galaxy, stellar populations, surface photometry, optical and NIR broadband
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-75765ISBN: 978-91-7447-525-8 (print)OAI: oai:DiVA.org:su-75765DiVA: diva2:523872
Public defence
2012-06-01, sal FB 55, 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: Manuscript. Paper 5: Manuscript.

Available from: 2012-05-11 Created: 2012-04-26 Last updated: 2013-11-19Bibliographically approved
List of papers
1. The faint outskirts of the blue compact galaxy Haro 11: is there a red excess?
Open this publication in new window or tab >>The faint outskirts of the blue compact galaxy Haro 11: is there a red excess?
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2010 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 405, no 2, 1203-1211 p.Article in journal (Refereed) Published
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.

Keyword
stellar content, dwarf, photometry, haloes, Haro 11
National Category
Astronomy, Astrophysics and Cosmology Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-50111 (URN)10.1111/j.1365-2966.2010.16525.x (DOI)000278677800035 ()
Note
authorCount :5Available from: 2010-12-29 Created: 2010-12-21 Last updated: 2017-12-11Bibliographically approved
2. How the extinction of extragalactic background light affects surface photometry of galaxies, groups and clusters
Open this publication in new window or tab >>How the extinction of extragalactic background light affects surface photometry of galaxies, groups and clusters
2009 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 397, no 4, 2057-2069 p.Article in journal (Refereed) Published
Abstract [en]

The faint regions of galaxies, groups and clusters hold important clues about how these objects formed, and surface photometry at optical and near-infrared wavelengths represents a powerful tool for studying such structures. Here, we identify a hitherto unrecognized problem with this technique, related to how the night sky flux is typically measured and subtracted from astronomical images. While most of the sky flux comes from regions between the observer and the target object, a small fraction - the extragalactic background light (EBL) - comes from behind. We argue that since this part of the sky flux can be subjected to extinction by dust present in the galaxy/group/cluster studied, standard reduction procedures may lead to a systematic oversubtraction of the EBL. Even very small amounts of extinction can lead to spurious features in radial surface brightness profiles and colour maps of extended objects. We assess the likely impact of this effect on a number of topics in extragalactic astronomy where very deep surface photometry is currently attempted, including studies of stellar haloes, starburst host galaxies, disc truncations and diffuse intragroup/intracluster light. We argue that EBL extinction may provide at least a partial explanation for the anomalously red colours reported for the haloes of disc galaxies and for the hosts of local starburst galaxies. EBL extinction effects also mimic truncations in discs with unusually high dust opacities, but are unlikely to be the cause of such features in general. Failure to account for EBL extinction can also give rise to a non-negligible underestimate of intragroup and intracluster light at the faintest surface brightness levels currently probed. Finally, we discuss how EBL extinction effects may be exploited to provide an independent constraint on the surface brightness of the EBL, using a combination of surface photometry and direct star counts.

Keyword
dust, extinction, galaxies: clusters: general, galaxies: haloes, galaxies: photometry, galaxies: spirals, diffuse radiation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-36180 (URN)10.1111/j.1365-2966.2009.15063.x (DOI)000268958400032 ()
Available from: 2010-01-22 Created: 2010-01-22 Last updated: 2017-12-12Bibliographically approved
3. Unlocking the secrets of stellar haloes using combined star counts and surface photometry
Open this publication in new window or tab >>Unlocking the secrets of stellar haloes using combined star counts and surface photometry
2012 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 421, no 1, 190-201 p.Article in journal (Refereed) Published
Abstract [en]

The stellar haloes of galaxies can currently be studied either throughobservations of resolved halo stars or through surface photometry.Curiously, the two methods appear to give conflicting results, asa number of surface photometry measurements have revealed integratedcolours that are too red to be reconciled with the halo propertiesinferred from the study of resolved stars. Several explanations forthis anomaly have been proposed - including dust photoluminescence,extinction of extragalactic background light and a bottom-heavy stellarinitial mass function. A decisive test is, however, still lacking.Here, we explain how observations of the halo of a nearby galaxy,involving a combination of both surface photometry and bright starcounts, can be used to distinguish between the proposed explanations.We derive the observational requirements for this endeavour and findthat star counts in filters VI and surface photometry in filtersVIJ appear to be the optimal strategy. Since the required halo starcounts are already available for many nearby galaxies, the most challengingpart of this test is likely to be the optical surface photometry,which requires several nights of exposure time on a 4-8 m telescope,and the near-infrared surface photometry, which is most readily carriedout using the upcoming James Webb Space Telescope.

Keyword
dust, extinction, galaxies: haloes, galaxies: photometry, diffuse radiation
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-75815 (URN)10.1111/j.1365-2966.2011.20290.x (DOI)000302693600046 ()
Available from: 2012-05-09 Created: 2012-04-27 Last updated: 2017-12-07Bibliographically approved
4. Deep multiband surface photometry on a sample of 24 blue compact galaxies - I
Open this publication in new window or tab >>Deep multiband surface photometry on a sample of 24 blue compact galaxies - I
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2013 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 431, no 1, 102-144 p.Article in journal (Refereed) Published
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.

Keyword
galaxies: fundamental parameters, galaxies: irregular, galaxies: photometry, galaxies: starburst
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-90774 (URN)10.1093/mnras/stt146 (DOI)000318343600034 ()
Funder
Swedish Research Council, 2011-5349Knut and Alice Wallenberg Foundation
Note

AuthorCount:8;

Available from: 2013-06-14 Created: 2013-06-11 Last updated: 2017-12-06Bibliographically approved
5. Deep multiband surface photometry on a sample of 24 blue compact galaxies II. A volume-limited sample of 21 emission line galaxies
Open this publication in new window or tab >>Deep multiband surface photometry on a sample of 24 blue compact galaxies II. A volume-limited sample of 21 emission line galaxies
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2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 556, A10- p.Article in journal (Refereed) Published
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.

Keyword
galaxies: starburst, galaxies: dwarf, galaxies: irregular, galaxies: photometry
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-94596 (URN)10.1051/0004-6361/201220485 (DOI)000323893500010 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council, 2011-5349
Note

AuthorCount:9;

Available from: 2013-10-07 Created: 2013-10-07 Last updated: 2017-12-06Bibliographically approved

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Citation style
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  • ieee
  • modern-language-association-8th-edition
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  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
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  • Other locale
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Output format
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