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Super star clusters in Haro 11: properties of a very young starburst and evidence for a near-infrared flux excess
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
Observatoire Astronomique de l'Université de Genève.
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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.

Place, publisher, year, edition, pages
2010. Vol. 407, no 2, 870-890 p.
Keyword [en]
galaxies: irregular, galaxies: starburst, galaxies: star clusters: individual: Haro 11
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-52740DOI: 10.1111/j.1365-2966.2010.16983.xOAI: oai:DiVA.org:su-52740DiVA: diva2:388787
Available from: 2011-01-18 Created: 2011-01-18 Last updated: 2017-12-11Bibliographically 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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