Astronomy Colloquium

  Understanding how galaxies form and evolve over cosmic time is one of the central topics in observational astrophysics and the last decade has seen dramatic advances in our understanding of cosmic structure formation. The cosmic star formation rate density (the 'star formation history of the Universe'), and stellar mass build-up, have been quantified back to first light and cosmic reionization, within 1Gyr of the Big Bang. Studies of galaxy formation are now turning attention to the evolution of the cool gas, the fuel for star formation, in galaxies. Here, observations of the cool interstellar medium in distant galaxies, via molecular and atomic fine structure line emission, has gone from a curious look into a few extreme, rare objects, to a mainstream tool to study galaxy formation, out to the highest redshifts. I will highlight some of the recent results emerging from molecular gas studies of main sequence galaxies at the 'peak of galaxy assembly' at z~2 that form stars at much higher rates than in today's Universe. I will also present results based on a molecular line scan of the Hubble Deep Field North that provides first blind constraints on the evolution of the cosmic molecular gas density. I will close by highlighting how the Cosmic Microwave Background may hamper studies of the interstellar medium of the highest redshift, main sequence galaxies, even in the era of ALMA.