The Indian Institute of Astrophysics traces its origins to an observatory established in 1786 at Madras, which, from 1792, began to operate formally at its Nungambakkam premises as the Madras Observatory. In 1899, the observatory moved to Kodaikanal. On 1 April 1971, the Kodaikanal Observatory became an autonomous society under the name Indian Institute of Astrophysics. To commemorate this occasion, the institute organises the annual Foundation Day Lecture delivered by distinguished speakers.
The weakness of the hyperfine HI 21cm line, the main tracer of the HI content of galaxies, has meant that we know little about the atomic gas mass of high-redshift galaxies and its evolution. Indeed, the evolution of the HI content of galaxies is one of the main open questions in galaxy evolution today. Stacking of the HI 21cm emission signals from a large sample of galaxies, observed simultaneously with a radio interferometer, can allow one to determine the HI properties of the galaxy population. In this talk, I will describe results from the Cold-HI AT z>~1 (CATz) HI 21cm surveys that we have been carrying out with the Giant Metrewave Radio Telescope, aiming to detect HI 21cm emission from star-forming galaxies in the redshift range z~1-3.0, i.e. the period of cosmic noon and beyond, to characterize the HI properties, and measure the gas accretion rate, the HI mass function, and the cosmological gas mass density, of star-forming galaxies over the last 11 billion years.