Distribution of Cold ($\lesssim 300$K) Atomic Gas in Galaxies: Results from the GBT HI Absorption Survey Probing the Inner Halos ($\rho<20$kpc) of Low-z Galaxies

Abstract

We present the Green Bank Telescope absorption survey of cold atomic hydrogen ($\lesssim 300$K) in the inner halo of low-redshift galaxies. The survey aims to characterize the cold gas distribution and to address where condensation - the process where ionized gas accreted by galaxies condenses into cold gas within the disks of galaxies - occurs. Our sample consists of 16 galaxy-quasar pairs with impact parameters of $\le$ 20kpc. We detected an HI absorber associated with J0958+3222 (NGC 3067) and HI emission from six galaxies. We also found two \ion{Ca}{2} absorption system in the archival SDSS data associated with galaxies J0958+3222 and J1228+3706, although the sample was not selected based on the presence of metals in absorption. Our detection rate of HI absorbers with optical depths of $\ge 0.06$ is $\sim$7\%. We also find that cold HI phase ($\lesssim$300K) is 44($\pm$18)\% of the total atomic gas in the sightline probing J0958+3222. We find no correlation between the peak optical depth and impact parameter or stellar and HI radii normalized impact parameters, $\rho/\rm R_{90}$ and $\rho/\rm R_{HI}$. We conclude that the process of condensation of inflowing gas into cold ($\lesssim$ 300K) HI occurs at the $\rho << 20$kpc. However, the warmer phase of neutral gas (T $\sim$ 1000K) can exists out to much larger distances as seen in emission maps. Therefore, the process of condensation of warm to cold HI is likely occurring in stages from ionized to warm HI in the inner halo and then to cold HI very close to the galaxy disk.