Atomic Hydrogen in Galaxies at z~1

Neutral Atomic Hydrogen (HI) is the dominant component in the interstellar medium of star-forming galaxies and is the primary fuel for star- formation.  Understanding galaxy evolution thus critically requires measurements of the HI mass of galaxies at different cosmological epochs. Unfortunately, the main tracer of the HI mass of galaxies, the 21 cm line, is very weak, and difficult to detect at cosmological distances. We have used the upgraded Giant Metrewave Radio Telescope (GMRT) to carry out the first measurement of the HI mass of galaxies at a redshift of one. In this talk, I will describe our survey, the stacking analysis used to carry out the detection, and how our measurement of the HI mass of galaxies at z~1 provides a plausible explanation to the long-standing puzzle of why the star-formation activity of the Universe declined rapidly after its peak at z~1-3. We find that, in the absence of fresh accretion of HI from the circum-galactic medium, these high-galaxies can sustain their star-formation rates (SFR) for only ~1-2 Gyr. This is similar to the timescale in which the SFR density of the Universe is observed to decline. Further, I will describe initial results from an even deeper GMRT survey of HI in galaxies at z~1. We find tantalizing evidence that supports our hypothesis that the decline in the SFR of galaxies from z~1 to z~0 is due to insufficient accretion of HI into galaxies. Finally, I will talk about our immediate plans which involve: (a) carrying out a detailed characterization of HI in galaxies at z~1, and (b) pushing HI 21 cm emission studies out to z~2-3 with an upcoming GMRT survey. These observations will provide an understanding of the build-up of HI in galaxies and its connection to the star formation history over the past 11 billion years.