SHOBITA SATYAPAL

NGC 4038/4039

HST IMAGE

For the past several years, my research has focused on the study of IR-luminous galaxies, the centers of which host massive starbursts and/or accreting suppermassive black holes (active galactic nuclei; AGN).  Most of these objects are found in interacting systems like the one shown here of the Antennae (NGC 4038/39).

Probing the energetics behind starburst galaxies and AGN has been a major thrust of modern astrophysics.  These galaxies manifest themselves via tremendous radiation in the infrared regime, with nuclear IR luminosities that exceed that from the entire galaxy from all other wavelengths combined.  In addition to being the dominant population of extragalactic objects in the local universe at these luminosities, IR luminous galaxies appear to be linked to a number of scientific research areas of fundamental importance, including globular cluster formation, the formation of elliptical galaxies, and the formation of quasars. Now being discovered at high redshifts, with infrared luminosities several thousand times greater than the luminosity of our Galaxy, the role of IR luminous galaxies in the genesis, characteristics, and evolution of galaxies in general is clear.  In addition, the Cosmic Background Explorer (COBE) team has recently shown that the cosmic infrared background radiation has an integrated output in the far-infrared in excess of the entire integrated optical emission from the Hubble Deep Field.  These findings reveal that infrared and submillimeter astrophysics will open up a new frontier for the exploration of the distant Universe. 

Some of the fundamental questions about these objects that I am interested in are:

• What is the energy source that powers the observed IR luminosity?

• What is the physical state of the ionized and neutral gas?

• What are the properties of the starbursts in these galaxies?

• What is the age/evolutionary status and initial mass function of the starburst stellar population?

• How do these properties change with redshift?

[FeII] image of M82

[FeII]  image of  NGC 1068

Starburst galaxies are galaxies that exhibit signs of vigorous star formation, with star formation rates several tens to at times hundreds of times greater than that seen in our Milky Way.  In some cases, massive stellar clusters in these galaxies have luminosities 100 million times the luminosity of the Sun in a dimension of a few parsecs.  The Milky Way is not forming stars in this way now, so study of these clusters may reveal a distinct mode of star formation different from the processes we are familiar with in our own neighborhood.  Indeed, these starburst clusters are the most dense and intense star-forming environments known, and may be analogs of typical objects in the early epochs of galaxy formation.  My research has centered on understanding the physical processes taking place in the centers of these galaxies with the ultimate goal of understanding their role in the evolution of galaxies.  Some of the questions that I have been interested in are: what are the properties of the stars formed in starbursts? Are they different from the distribution of stars seen in our Galaxy?  How are starbursts triggered and how are they connected to active galactic nuclei? 

Below are some of our publications:

High Spatial Resolution Fabry-Perot Imaging of M82: Near-Infrared Recombination Line Observations, ApJ, 1995

The Intrinsic Properties of the Stellar Clusters in the M82 Starburst Complex: Propagating Star Formation?, ApJ, 1997

Infrared Fabry-Perot Imaging of M82 [Fe II] Emission. II. Tracing Extragalactic Supernova Remnants, ApJ 1997

Probing the Dust-enshrouded Regions of the Interacting Galaxy System ARP 299:A Near-Infrared Study, ApJ, 1997

ISO LWS Spectroscopy of M82: A Unified Evolutionary Model, ApJ, 1999

ISO-LWS spectroscopy of Centaurus A: extended star formation, A&A, 2000

Chandra Images of LINERs

With the recent discovery that virtually all local galaxies harbor massive nuclear black holes, there is now convincing evidence that active galactic nuclei (AGN) and normal galaxies in our local Universe are fundamentally connected.  However, the nature of this connection and the detailed evolutionary history connecting these objects is unknown.  Low Ionization Nuclear Emission Line Regions (LINERs), defined by their narrow optical emission lines of low ionizatation uncharacteristic of photoionization by normal stars (Heckman et al. 1980), may constitute a vital piece of this puzzle.  These galaxies are the dominant population of “active” galaxies in our local Universe.  In fact, nearly half of all galaxies in the nearby Universe are classified as LINERs.  In addition, the low mass accretion rates inferred for many accretion-powered LINERs, may suggest that these objects capture the population of AGN just before accretion onto the black hole “turns off”.   As a consequence, establishing the number of accretion-powered LINERs, their luminosities, accretion rates, and the relationship of these quantities to the properties of the parent galaxy will provide critical insight into some of the most fundamental questions in extragalactic research today.

In an effort to determine the fraction of LINERs hosting AGN and to characterize their accretion properties, we are undertaking a Chandra snapshot survey of the largely unexplored population of nearby IR-bright LINERs to search for hard nuclear point sources indicative of an AGN.  In a parallel effort, we are undertaking a mid-IR spectroscopic investigation.  These observations are being used to determine the accretion properties and understand their relationship to the surrounding star formation.

Here are some of our recent publications:

A joint mid-infrared spectroscopic and X-ray imaging investigation of LINER galaxies, A&A 2004

A Chandra Snapshot Survey of IR-Bright LINERs: A Possible Link Between Accretion and Star Formation?, ApJ, 2004, accepted

Near-IR Cryogenic Fabry-Perots