How does Gas Get into Galaxies?

Abstract

Most stars reside within disk galaxies. We are far from understanding just why this should be. The conventional picture of baryon accretion invokes shock processes during cold dark matter (CDM) collapse followed by a cooling phase in the hot medium. Some have even suggested that we must search for evidence of the “missing link” – disk galaxies with relic hot halos – as proof of the conventional paradigm. But on the scale of individual disk galaxies, accretion through the hot phase cannot be the dominant process. So how do baryons find their way into disk galaxies? We review recent advances from multiwavelength observations. Interestingly, a diffuse hot halo can assist in the formation of disks by slowing down and disrupting the orbitting gas. A complete baryon inventory of disk galaxies may require much deeper observations of all gas phases at sub-virial temperatures, and only then can we properly account for the required accretion rate consistent with the stellar record.