The Circumgalactic Medium

The CircumGalactic Medium (CGM), is losely defined as a region spanning a few hundred kiloparsecs from a galaxy that contains an extended reservoir of diffuse gas. Our recent understanding of galaxy evolution has dramatically changed with new understanding of how the circumgalactic medium influences star-formation in galaxies. The CGM is a major reservoir of baryons in a galaxy and is actively regulating galaxy evolution.

I have been actively studying the CGM using both observations and simulations. Observationally, I have been extensively using both the Hubble Space Telescope and ground based large telescopes such as Magellan, VLT, Keck, Gemini to understand how the CGM impacts galaxy evolution.

Some of my recent work on this field are listed below.

The formation and physical origin of highly ionized cooling gas :

We present a physically clear cooling flow theory that explains the origin of warm diffuse gas seen primarily as highly ionized absorption line systems in the spectra of background sources. We predict the observed column densities of several highly ionized transitions such as O VI, O VII, Ne VIII, N V, and Mg X; and present a unified comparison of the model predictions with absorption lines seen in the Milky Way disk, Milky Way halo, starburst galaxies, the circumgalactic medium and the intergalactic medium at low and high redshifts. We show that diffuse gas seen in such diverse environments can be simultaneously explained by a simple model of radiatively cooling gas. We show that most of such absorption line systems are consistent with being collisionally ionized, and estimate the maximum likelihood temperature of the gas in each observation. This model satisfactorily explains why O VI is regularly observed around star-forming low-z L* galaxies, and why N V is rarely seen around the same galaxies. We predict that the typical O VI column densities seen around these galaxies would be an order of magnitude higher than the associated N V column densities. We further present some consequences of this model in quantifying the dynamics of the cooling gas around galaxies and predict the shock velocities associated with such flows. Useful formulae for both observers and simulators are presented. For more information see Bordoloi et al 2017.

COS-Dwarfs survey: The carbon reservoir around sub-L* galaxies :

We report new observations of circumgalactic gas from the COS-Dwarfs survey, a systematic investigation of the gaseous halos around 43 low-mass z < 0.1 galaxies using background QSOs observed with the Cosmic Origins Spectrograph. From the projected 1D and 2D distribution of CIV absorption, we find that CIV is detected out to ~ 100 kpc (corresponding roughly to ~ 0.5 Rvir) of the host galaxies. The CIV absorption strength falls off radially as a power law and beyond ~ 0.5 Rvir, no CIV absorption is detected above our sensitivity limit of ~ 50-100 mA. We find a tentative correlation between detected CIV absorption strength and star formation, paralleling the strong correlation seen in highly ionized oxygen for L ~ L* galaxies by the COS-Halos survey. The data imply a large carbon reservoir in the CGM of these galaxies, corresponding to a minimum carbon mass > 1.2 Million solar masses, out to 110 kpc. This mass is comparable to the carbon mass in the ISM and exceeds the carbon mass currently in the stars of these galaxies. The CIV absorption seen around these sub-L* galaxies can account for almost two-thirds of all W > 100 mA CIV absorption detected at low z. Comparing the CIV covering fraction with hydrodynamical simulations, we find that an energy-driven wind model is consistent with the observations whereas a wind model of constant velocity fails to reproduce the CGM or the galaxy properties. For details see Bordoloi et al. 2014.

Galaxies probing galaxies: The radial and azimuthal profiles of Mg II gas around galaxies: :

We present the first 2D absorption profile of cool circumgalactic gas using backgroud galaxy spectra to probe the CGM of foreground galaxies. We map the radial and azimuthal distribution of Mg II gas within ∼ 200 kpc (physical) of approximately 4000 galaxies at redshifts 0.5 < z < 0.9 using co-added spectra of more than 5000 background galaxies at z > 1. We investigate the variation of Mg II rest frame equivalent width as a function of the radial impact parameter for different subsets of foreground galaxies selected in terms of their rest-frame colors and masses. Blue galaxies have a significantly higher average Mg II equivalent width at close galacto-centric radii as compared to the red galaxies. Amongst the blue galaxies, there is a correlation between Mg II equivalent width and galactic stellar mass of the host galaxy. We also find that the distribution of Mg II absorption around group galaxies is more extended than that for non-group galaxies, and that groups as a whole have more extended radial profiles than individual galaxies. Interestingly, these effects can be satisfactorily modelled by a simple superposition of the absorption profiles of individual member galaxies, assuming that these are the same as those of non-group galaxies, suggesting that the group environment may not significantly enhance or diminish the Mg II absorption of individual galaxies. We show that there is a strong azimuthal dependence of the Mg II absorption within 50 kpc of inclined disk-dominated galaxies, indicating the presence of a strongly bipolar outflow aligned along the disk rotation axis. For more information see Bordoloi et al 2011.

Modeling the distribution of MgII absorbsers around galaxies using background galaxies and quasars :

We present joint constraints on the distribution of MgII absorption around high redshift galaxies obtained by combining two orthogonal probes, the integrated MgII absorption seen in stacked background galaxy spectra and the distribution of parent galaxies of individual strong MgII systems as seen in the spectra of background quasars. We present a suite of models that can be used to predict, for different two- and three-dimensional distributions, how the projected MgII absorption will depend on a galaxy's apparent inclination, the impact parameter and the azimuthal angle between the projected vector to the line of sight and the projected minor axis. In general, we find that variations in the absorption strength with azimuthal angles provide much stronger constraints on the intrinsic geometry of the MgII absorption than the dependence on the inclination of the galaxies. In addition to the clear azimuthal dependence in the integrated MgII absorption that we reported earlier, we show that strong equivalent width MgII absorbers (Wr (2796) >= 0.3 Å) are also asymmetrically distributed in azimuth around their host galaxies: 72% of the absorbers in Kacprzak et al., and 100% of the close-in absorbers within 35 kpc of the center of their host galaxies, are located within 50° of the host galaxy's projected semi minor axis. It is shown that either composite models consisting of a simple bipolar component plus a spherical or disk component, or a single highly softened bipolar distribution, can well represent the azimuthal dependencies observed in both the stacked spectrum and quasar absorption-line data sets within 40 kpc. Simultaneously fitting both data sets, we find that in the composite model the bipolar cone has an opening angle of ~100° (i.e., confined to within 50° of the disk axis) and contains about two-thirds of the total MgII absorption in the system. The single softened cone model has an exponential fall off with azimuthal angle with an exponential scale length in opening angle of about 45°. We conclude that the distribution of MgII gas at low impact parameters is not the same as that found at high impact parameters. At larger impact parameters beyond 40 kpc, there is evidence for a much more symmetric distribution, significantly different from that closer in to the galaxies. For more details see Bordoloi et al 2014.