Research

Main Research Areas: Galactic Dynamics: Vertical Structure of Disk Galaxies and their Dark Matter Halos; Pattern Speeds of Bars and Spiral Arms; Disk Stability; Galaxy Formation and Evolution Research Description: The primary focus of my research is galactic dynamics with emphases on understanding (1) the shape and density profiles of dark matter halos of nearby spiral galaxies (2) the role of the dark matter halos in regulating disk structure and dynamics and (3) the effect of gas dynamics on disk vertical structure through theoretical modeling and numerical calculations. The shape and density profiles of dark matter halos of nearby spiral galaxies: Galaxies form because of cooling of baryons and subsequent star formation at the centers of dark matter potential wells. The shape and density profiles of the dark matter halos host a wealth of information on galaxy formation and evolution processes like the back-effect of the baryonic disk and the cosmological merger history of the galaxy. The shape of the halo also encodes clues about the nature of the constituent particles of dark matter and therefore also acts as an astrophysical probe of fundamental physics. Neutral hydrogen (HI) extends farther beyond the stellar (optical) disk of the galaxy and therefore serves as a useful diagnostic tracer of the underlying potential of the dark matter halo which alone regulates the disk dynamics in the outer galaxy. The observed HI rotation curve constrains the radial derivative of the potential while the HI vertical scale height constrains the vertical derivative of the same. In other words, the rotational curve fixes the total mass of the halo whereas the scale height uniquely determines the flattening of the same. We applied the joint constraints of the HI rotation curve and HI scale height data on our 3-component galactic disk model of gravitationally-coupled stars, atomic hydrogen (HI) and molecular hydrogen (H_2) gas in the force field of the dark matter halo, to determine the dark matter density profiles of three nearby spiral galaxies: the dark matter dominated superthin low surface brightness (LSB) galaxy UGC 7321, our nearest large spiral galaxy Andromeda (M31) and our own Galaxy. The main results are as follows: 1. We have obtained a spherical halo for the superthin LSB galaxy UGC 7321, an oblate halo for Andromeda (M31) and a halo which is progressively more prolate in our Galaxy (Milky Way). The discovery of the radial variation of dark matter halo shape within the Galactic disk is the very first of its kind to be reported in the literature. 2. We have obtained a compact dark matter halo (i.e., the core of the halo is comparable to the scale length of the exponential stellar disk) for UGC 7321. However, for the the two large spirals, Andromeda and the Milky Way, we obtain non-compact halos (i.e., the core of the halo is 3-4 times larger than the disk scale length). (Banerjee & Jog 2008, Banerjee et al. 2010, Banerjee & Jog 2011) The role of the dark matter halos in regulating disk structure and dynamics: We have also investigated the role of the dark matter halo in regulating the disk structure and dynamics in the dark matter rich galaxies like the low surface brightness (LSB) and dwarf irregular (dIrr) galaxies. Using our 3-component galactic disk model of gravitationally-coupled stars, atomic hydrogen (HI) and molecular hydrogen (H_2) gas in the force field of the dark matter halo, we have shown that the dense and compact dark matter halo drives the stellar disk superthin in UGC 7321 (Banerjee & Jog 2013). Using the Tremaine-Weinberg Method, we determined the pattern speed of the purely gaseous bar in the dwarf irregular galaxy UGC 3741 and found the bar to be slow. This is consistent with bar models in which dynamical friction results in a slow bar in dark-matter-dominated galaxies (Banerjee et al. 2013). The effect of gas dynamics on disk vertical structure: One of the direct applications of our 3-component galactic model lies in explaining the long-standing puzzle of the steep vertical stellar density profiles of the disk galaxies near the midplane. Over the past two decades, observations have revealed that the vertical density distribution of stars in galaxies near the midplane is substantially steeper than the sech^2 function that is expected from a self-gravitating layer of stars under isothermal approximation. However, the physical origin for this has not been explained so far. We have demonstrated that the inclusion of the self-gravity of the gas in the dynamical model of the Galaxy solves the problem. Being a low dispersion component, the gas resides closer to the mid-plane compared to the stars, and forms a thin, compact layer above it, thereby strongly governing the local disk dynamics (Banerjee & Jog 2007).