One of the outstanding questions in galactic cosmology is the detailed process by which galaxies acquire gas from the surrounding intergalactic medium, and how this process varies with galaxy mass and redshift. The rate at which gas is being accreted, and the fraction of hot and cold gas within the dark matter halos of galaxies are key factors in developing a clearer picture of disk and star formation. Numerical simulations that incorporate the complex baryonic physics of gas accretion are just beginning to probe these processes, and techniques for observing cold gas in galactic halos are essential.

New observations by KICP scientists, led by faculty member Hsiao-Wen Chen, graduate student Jean-Rene Gauthier and former KICP fellow Jeremy Tinker, have detected the presence of cold gas in the dark matter halos around luminous red galaxies, which are typically ten to one hundred times more massive than the Milky Way. Their data provide critical input for understanding the growth of these large systems, and an important complement to recent observations of cold gas around smaller galaxies.

One example of the close quasar-LRG pairs found in the Sloan Digital Sky Survey. In this image, the projected separation between the two objects is about 40 kpc. This quasar sightline passes through the gaseous halo of the luminous red galaxy, and a strong MgII absorber is found in the spectrum of the quasar at the redshift location of the LRG.

The cold gas in a dark matter halo, if present, is expected to imprint specific absorption features in the spectrum of a background quasar. The KICP group utilized MgII (2796, 2803 A) absorption doublets as a probe of such cold gas around luminous red galaxies that are at least ten times more massive than the Milky Way. Using analysis techniques developed in earlier work (Tinker & Chen 2008) they find that the two-point cross-correlation function of luminous red galaxies and MgII absorbers displays a strong signal extending to projected distances below 300 kpc, well within the virial radii of these luminous galaxies. This strong cross-correlation amplitude at galaxy-absorber pair separations much smaller than the expected halo size strongly suggests the presence of cold gas in the massive dark matter halos (M ~ 10¹³ M_sun) hosting these galaxies.

Pockets of cold (10⁴ K) gas in the halos of massive galaxies could be present via two distinct physical mechanisms: the classical thermal instability argument and a cold stream inflow. In the thermal instability scenario, cold clouds could form within a hot (10⁶ K) halo and maintain a pressure equilibrium with the surrounding hotter medium. This leads to the formation of many pockets of cold gas that eventually fall to the center of the halo and fuel star formation in the galactic disc. The total baryonic mass expected to be contained in these pockets in such scenarios is, however, not yet known.

On the other hand, recent state-of-the art cosmological simulations have predicted that cold streams from the intergalactic medium may penetrate a hot halo of mass comparable to the Milky Way and accrete to fuel star formation in the galactic disc. In simulations of more massive halos, however, such cold streams are not found. A quantitative understanding of the cold gas content around massive galaxies will shed light on the dominant accretion mechanism and reveal how the process varies with galaxy mass. The new observations by Chen et al. confirm the presence of cold gas in the dark matter halos of massive galaxies. Future work based on the techniques outlined in this paper will help to further characterize the amount and distribution of this cold gas – providing constraints on the baryonic content of dark matter halos.

Shown on the right side of the panel is the absorption profile of the MgII doublet along with other associated ions. The detection of such absorber indicates the presence of cold, ~ 10⁴ K gas, inside the halo of the massive, red galaxy.

Note: The QSO spectrum was obtained by the SDSS. The confirming spectrum of the red galaxy was obtained on the 3.5 m telescope at Apache Point Observatory.

References:

- The Clustering of MgII Absorption Systems at z=0.5 and Detection of Cold Gas in Massive Halos, Jean-Rene Gauthier, Hsiao-Wen Chen, Jeremy L. Tinker

- The Baryon Content of Dark Matter Halos: Empirical Constraints from MgII Absorbers, Hsiao-Wen Chen, Jeremy L. Tinker

- On the Halo Occupation of Dark Baryons, Jeremy L. Tinker, Hsiao-Wen Chen