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The star-formation activity in the Universe is known to have peaked roughly 8-11 billion years ago, with about half of today's stars born in this period, the epoch of cosmic noon. Today, the star-formation activity is lower by more than an order of magnitude; the causes of this decline have been unknown for more than two decades. Addressing this question requires one to understand the evolution of the neutral atomic hydrogen in galaxies, the primary fuel for star formation. Unfortunately, the weakness of the hyperfine 21cm line, the main tracer of the atomic gas content of galaxies, has meant that we know little about the atomic gas mass of high-redshift galaxies and its evolution. ``Stacking'' of the 21cm emission signals from a large sample of galaxies, observed simultaneously with a radio interferometer, can allow one to determine the atomic gas properties of the galaxy population. In this talk, I will describe results from our 21cm stacking experiments, using the Giant Metrewave Radio Telescope to measure the atomic gas properties of galaxies during the era of cosmic noon, and the implications for star-formation activity in the Universe over the last 9 billion years.
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