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Star cluster formation and evolution in Mrk 930: properties of a metal-poor starburst
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
Stockholm University, Faculty of Science, Department of Astronomy.
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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.

Place, publisher, year, edition, pages
2011. Vol. 415, no 3, 2388-2406 p.
Keyword [en]
galaxies: irregular, galaxies: starburst, galaxies: star clusters: general, galaxies: star formation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-69268DOI: 10.1111/j.1365-2966.2011.18867.xISI: 000294173900033OAI: oai:DiVA.org:su-69268DiVA: diva2:475728
Note
authorCount :7Available from: 2012-01-11 Created: 2012-01-11 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Super Star Clusters in Blue Compact Galaxies: Evidence for a near-infrared flux excess and properties of the starburst phase
Open this publication in new window or tab >>Super Star Clusters in Blue Compact Galaxies: Evidence for a near-infrared flux excess and properties of the starburst phase
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
starburst galaxy-extragalactic star clusters-dwarf merger
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-52681 (URN)978-91-7447-215-8 (ISBN)
Public defence
2011-02-18, sal FB53, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
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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

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Adamo, AngelaÖstlin, GöranZackrisson, ErikMicheva, Genoveva
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