Somak Raychaudhury
Research Interests

X-ray and optical observations of normal galaxies

Early-type galaxies (ellipticals and lenticulars) dominate the population of galaxies in most clusters and groups. In the hierarchical picture of structure formation, these galaxies are assembled by the merger of smaller galaxies. However, observational evidence in support of this picture remains sketchy, and even state-of-the-art simulations are unable to represent the rich complexity of galactic structure, or to model the crucial effects of "feedback", whereby young stars and supernova explosions reheat gas, and hence prevent an unrealistically large fraction of the baryons from cooling out to form stars.

• The optical identification of X-ray point sources in nearby early-type galaxies: A program of identifying point sources with their optical counterparts from deep-ish Chandra observations of early-type galaxies, undertaken in collaboration with Christine Jones, Bill Forman and Ralph Kraft (CfA). The low-mass X-ray binary population of NGC 4636 (Figure right) in Virgo seems to lie mostly in the redder globular clusters. Is this trend seen in galaxies with different histories, like Cen A or NGC 1399? Why are there almost no X-ray point sources in NGC 5102, a smaller member of the Cen A group?
  
• The Local Early-type Galaxies Survey (LEGS): I am involved in a detailed study of the nearest two hundred early-type galaxies, for which we will obtain optical imaging and spectroscopy. X-ray observations will reveal properties of their hot interstellar medium, and stellar population modelling which can be used to reconstruct their star formation history. We have initial results on a few individual galaxies [25], but this project is in growing mode, involving all three of our postdoctoral fellows and various collaborators elsewhere (USA, India, Chile).
  

• Fossil groups: GMRT observations (with Nazirah Jetha and Habib Khosroshahi) and spectroscopic studies with Habib Khosroshahi (more coming) of isolated giant elliptical galaxies with group-scale X-ray emission, thought to the remnant of the rapid merging of a group of galaxies.
  

   

My Publications in this field           

1. A Chandra Observation of the Nearby Lenticular Galaxy NGC 5102: Where are the X-ray Binaries?
   Kraft R. P., Nolan L. A., Ponman T. J. , Jones C. and Raychaudhury S., 2005, ApJ, 625, 785-795
2. X-Ray Observations of the edge-on star-forming galaxy NGC 891 and its supernova
   SN1986J
   Temple Rowan F., Raychaudhury Somak and Stevens Ian R., 2005, MNRAS, 362, 581-591 
3. Chandra Observations of the X-Ray Point Source Population in NGC 4636
   Posson-Brown Jennifer, Jones Christine, Forman William, Donnelly R. Hank, Raychaudhury Somak, Murray Stephen, 2005, in "Six Years of Science with Chandra", Cambridge MA, 2-4 November 2005

Full List of publications