Dr Sugata Kaviraj

Visiting Researcher

s.kaviraj@ null imperial.ac.uk

Fax: +44 (0)20 759 47772
Room , Level
Imperial College London, Astrophysics, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK

My general interests are in the formation and evolution of galaxies. My past work covers a diverse range of topics, including early-type galaxies, extra-galactic globular clusters, luminous infrared galaxies, tidal dwarf galaxies, galaxy mergers and the impact of supernova and AGN feedback on star formation. See below for a brief summary of recent research highlights. 

 

Click these links for my CV and PUBLICATION LIST.

 

A significant fraction of my current work involves the HST's new Wide Field Camera 3 (WFC3). I am heavily involved in the WFC3 Early-Release Science (ERS) programme (designed to demonstrate WFC3's capabilities), which includes case studies of nearby galaxies and a prototype survey of the high-redshift (z>1) Universe in WFC3's near-infrared filters. I am PI of a new HST/WFC3 programme which will study young stars, globular clusters and the star formation law in elliptical galaxies in unprecedented detail and explore the scope of 2D studies using future telescopes such as the E-ELT. I lead several science themes (mergers, dusty galaxies, high-redshift spheroids) within the Galaxy Zoo project, which has used 300,000+ volunteers from the general public to visually classify galaxies in the entire SDSS and existing HST archives (1.3 million galaxies). I am currently involved in incorporating new and forthcoming WFC3 datasets (e.g. the ERS and CANDELS) into Galaxy Zoo for morphological studies at z>1.   

 

selected research highlights

 

EARLY-TYPE GALAXIES

Some of my recent work has used rest-frame ultra-violet (UV; 1200-3000 Å) data to reveal widespread star formation in early-type galaxies (traditionally thought to be old, passively-evolving systems), driven by minor mergers between early-types and small, gas-rich satellites. These studies are among the first in the recent survey era to explicitly demonstrate the link between star formation in massive galaxies at late epochs (z<1) and minor mergers. See: 

Kaviraj et al. 2007, ApJS, 173, 619

Kaviraj et al. 2008, MNRAS, 388, 67

Kaviraj et al. 2009, MNRAS, 394, 1713

Kaviraj et al. 2011, MNRAS, 411, 2148

 

While they significantly influence star formation in massive galaxies at late epochs, minor mergers remain poorly understood. A promising route to dissect this process and quantify the associated star formation on a galaxy-by-galaxy basis is via high-resolution spatially-resolved analyses of individual objects, using e.g. the WFC3. See:

Crockett et al. 2011, ApJ, 727, 115

Kaviraj et al. 2011, MNRAS submitted (arXiv:1107.5042)

 

These pilot studies will be extended to a representative sample of early-type galaxies using a new Cycle 19 HST WFC3 observing programme (PI: Kaviraj) to study young stars, globular clusters and the star formation law in unprecedented detail in local early-type galaxies. See the abstract for the HST proposal here

 

 

 

EXTRA-GALACTIC GLOBULAR CLUSTERS

The combination of UV and optical photometry breaks the 'age-metallicity degeneracy', that has plagued traditional optical studies, making it difficult to simultaneously estimate ages/metallicities of stellar populations using optical photometry alone. I have developed methods that use UV/optical colours alone to estimate ages/metallicities of globular clusters (GCs) to similar precision as spectroscopic methods (which are more expensive in telescope time). These photometric methods are widely applicable to GC studies using new and forthcoming instrumentation such as HST’s new Wide Field Camera 3 and the extremely large telescopes. See: 

Kaviraj et al. 2007, MNRAS, 381, L74

Kaviraj et al. 2011, MNRAS submitted (arXiv:1107.5042)

 

 

 

SUPER-HELIUM-RICH STELLAR POPULATIONS IN EXTRA-GALACTIC SYSTEMS

A study of M87 globular clusters is the first to have detected plausible signatures of super-Helium-rich subcomponents in extragalactic stellar populations, akin to recent findings in Galactic GCs such as Omega Cen, NGC 6388 and NGC 6441. Instead of being anomalous to a few Galactic globulars, this study suggests that He-enrichment may be a widespread phenomenon in the local Universe, possibly contributing to (or perhaps even driving!) other well-known phenomena such as the UV upturn in giant elliptical galaxies. See: 

Kaviraj et al. 2007, MNRAS, 377, 987

 

 

 

IMPACT OF SUPERNOVA AND AGN FEEDBACK ON STAR FORMATION

Energetic feedback from supernovae and AGN are fundamental features of modern galaxy formation models. While they are implemented via well-motivated recipes in these models, empirical constraints on these feedback processes are highly desirable.

 

Kaviraj et al. 2007, MNRAS, 382, 960 used post-starburst (E+A) galaxies to show, from a purely observational point of view, that the principal quenching mechanisms that truncate starbursts in the local Universe are AGN and supernovae, above and below a mass threshold of 10^10 MSun respectively. This supports the general framework of feedback employed in modern models. This study provides empirical constraints on the quenching efficiency as a function of galaxy mass, as a guide to improved prescriptions of feedback in the models.

 

While theoretical work has made a compelling case for AGN feedback truncating star formation in massive galaxies, the presence of low-level star formation in massive early-types suggests that the truncation is not comprehensive and that details of the feedback may have to be altered. Kaviraj et al. 2011, MNRAS, 315, 4798 has developed a simple, empirically-driven model for AGN feedback in nearby early-type galaxies, which leverages recent observational work to describe the fraction of AGN luminosity that couples to the gas reservoir and the timescale over which this coupling takes place.  

 

 

 

STAR FORMATION AND AGN ACTIVITY IN NEARBY LIRGs

Kaviraj 2009, MNRAS, 394, 1167 has performed the largest quantitative study of the star formation histories (SFHs) of nearby luminous infrared galaxies (LIRGs) and the interplay between AGN activity and star formation in these systems. The results indicate that AGN in these strongly star-forming systems typically appear 0.5-0.7 Gyrs after the onset of star formation and that there is no measurable evidence of AGN feedback in the LIRG phase. 

 

 

 

GALAXY MERGERS IN THE LOCAL UNIVERSE

Galaxy mergers are a fundamental part of the hierarchical evolution of the galaxy population, driving star formation, feeding black holes and altering the morphological mix of the Universe. However, since mergers are rare, a statistical study of galaxy merging in the nearby Universe has been difficult. With PhD student Darg, I have constructed the largest catalogue of nearby galaxy mergers to date, using visual morphological classifications of the entire SDSS DR6 using the Galaxy Zoo project. Galaxy Zoo is the largest citizen-science astronomy project, which has used 300,000+ members of the general public to visually classify the entire SDSS and existing HST archives. This work includes the first observational study of multi-mergers. See:

Darg et al. 2010, MNRAS, 401, 1043

Darg et al. 2010, MNRAS, 401, 1552

Darg et al. 2011, MNRAS, 416, 1745

 

 

 

TIDAL DWARF GALAXIES

Up to a third of the pre-encounter material in merging galaxies is tidally ejected during the interaction into the tidal tails and bridges that form around the remnant. In gas-rich mergers this debris typically hosts self-bound objects with masses typical of dwarf galaxies. Kaviraj et al. 2011, MNRAS in press (arXiv:1108.4410) has performed the first statistical study of these 'tidal dwarf' galaxies in the nearby Universe. In addition to a detailed analysis of the nearby tidal dwarf population, an important result is that, contrary to suggestions in the recent literature, only a few percent of dwarfs in present-day clusters have a tidal origin. It is unlikely that the entire local dwarf galaxy census is a result of merger activity over cosmic time, which, if true, would have significant implications for our understanding of the standard LCDM structure formation paradigm.