The 21 cm line of atomic hydrogen (HI) provides a powerful probe of the cool-to-warm interstellar gas in galaxies. Virtually all star-forming galaxies contain a cool neutral component of their interstellar medium representing the fuel reservoir from which stars may form in the future. In this talk, I will highlight the differences between galaxy populations traced by starlight and by radio HI line surveys, explore what HI reveals about the astrophysical processes at play within galaxies, and discuss the importance of HI-selected samples to the understanding of apparent conflicts between observation and theory on the abundance of low mass dark matter halos. I will conclude with an overview of the ongoing and planned major HI line surveys that will explore the cosmic evolution of HI in galaxies and probe the cosmic “Dark Ages”. I will briefly introduce a complementary survey of the evolution of ionized carbon using the Fred Young Submillimeter Telescope under construction by the CCAT collaboration in Chile.
Recording of the Rittenhouse Lecture: