Somak Raychaudhury
Research Interests
Galaxy evolution in groups of galaxies
Groups of galaxies are
believed to contain the bulk of the matter the Universe. They are the
characteristic structures formed at the present epoch through hierarchical
merging,
and thus an understanding of the formation of galaxies and of the large-scale
structure of the Universe requires an understanding of galaxy groups. The low
velocity dispersion of galaxies in groups encourages tidal interaction, and
results in an enhanced occurrence of strong encounters, accretion and mergers.
Hence, not only are groups the most common environment for galaxies, but they
also provide an environment which is conducive to a rapid change in galaxy
properties. I'm involved in
The Galaxy Evolution Multi-wavelength Study (GEMS), with a sample of 60 x-ray selected groups of galaxies. Together with my PhD student Trevor Miles, and Trevor Ponman and Duncan Forbes, I have analysed analysed the optical and near-IR observations. The first two papers [1,2] have produced very interesting results. We find that in the poorer groups, where the velocity dispersion is low and the resulting conditions amenable to tidal interaction and merger, the luminosity function of galaxies is bimodal, showing evidence of the depletion of intermediate-luminosity galaxies in such environments. This is a key ingredient in an emerging picture of the evolution of galaxies and its connection to their local environment.
Comparison with numerical simulations: collaboration with Frazer Pearce (Nottingham) and the Millennium simulations of the Virgo Consortium.
Birmingham-Carnegie XMM-IMACS project (XI): Valuable as the GEMS study will be, it is based on a non-statistical sample, for which ROSAT data happen to be available. To progress further, we (Trevor Ponman, Jesper Rasmussen and I) have teamed up with John Mulchaey (Carnegie Observatories, USA), for a much more detailed study of a sample of 25 groups selected from the 2dF galaxy redshift survey, using a well-defined overdensity criterion [5]. We are studying this sample by means of optical spectroscopy (using the multi-aperture IMACS instrument on the 8m Magellan telescope in Chile), combined with x-ray observations, for which we have been awarded substantial XMM-Newton time. We plan to study all galaxies down to small dwarfs in these groups, and look at the velocity structure and star-formation activity of the galaxies from their spectra, and how these relate to the hot baryonic intergalactic medium. Photometry is almost complete, and spectroscopy has been performed in three observing runs. Togther with Chandreyee Sengupta (RRI, India), we have observed one of these groups at 21cm with the Giant Metrewave radio-telescope (GMRT) in India, and are scheduled to observe another early next year.
Fossil groups: GMRT observations (with Nazirah Jetha and Habib Khosroshahi) and spectroscopic studies with Habib Khosroshahi (more coming).
My Publications in this field
Full List of publications