The Cold Molecular Medium around Distant Massive Galaxies: Light up the Darkness!

The evolution of galaxies is tightly linked to processes that occur in the circum-galactic medium (CGM). Unfortunately, most of the baryons in the CGM are too faint to be easily detected. At high-z, we view glimpses of dark baryonic halos through quasar absorption lines, or cooling-radiation emitted as Ly-alpha. However, a direct connection with the stellar growth of massive galaxies has long remained missing, because we had yet to identify the ultimate reservoir of halo gas that can fuel the formation of stars and galaxies, namely the cold molecular gas (~10-100 K). I will present sensitive low-surface-brightness CO and [CI] observations of widespread molecular gas across giant Ly-alpha halos at high redshifts. These results implicate that massive galaxies forming within these halos grow from accreted and recycled gas in the CGM. I will explain the technical challenges involved in lighting up these hitherto hidden reservoirs of cold molecular gas. This leads to the question of how much molecular gas in the Universe we are still missing… I will show how low-surface-brightness observations with future radio interferometers, with particular emphasis on the Next- Generation Very Large Array, promise to revolutionize our view of the molecular Universe.