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Adaptive optics imaging and optical spectroscopy of a multiple merger in a luminous infrared galaxy
Stockholm University, Faculty of Science, Department of Astronomy.
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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).

Place, publisher, year, edition, pages
2008. Vol. 384, no 3, 886-906 p.
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:su:diva-18289DOI: doi:10.1111/j.1365-2966.2007.12703.xISI: 000253760300004OAI: diva2:184812
Available from: 2009-01-25 Created: 2009-01-25 Last updated: 2012-01-11Bibliographically 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.
starburst galaxy-extragalactic star clusters-dwarf merger
National Category
Astronomy, Astrophysics and Cosmology
Research subject
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)
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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