The HI Mass Function of Star-forming Galaxies at z≈1

We present the first estimate of the HI mass function (HIMF) of star-forming galaxies at z≈1, obtained by combining our measurement of the scaling relation between HI mass (MHI) and B-band luminosity (MB) of star-forming galaxies with literature estimates of the B-band luminosity function at z≈1. We determined the MHI-MB relation by using the GMRT-CATz1 survey of the DEEP2 fields to measure the average HI mass of blue galaxies at $z=0.74-1.45$ in three separate MB subsamples. This was done by separately stacking the HI 21 cm emission signals of the galaxies in each subsample to detect, at (3.5-4.4)σ significance, the average HI 21 cm emission of each subsample. We find that the MHI-MB relation at z≈1 is consistent with that at z≈0. We combine our estimate of the MHI-MB relation at z≈1 with the B-band luminosity function at z≈1 to determine the HIMF at z≈1. We find that the number density of galaxies with MHI>10¹⁰ M_ (higher than the knee of the local HIMF) at z≈1 is a factor of ≈4-5 higher than that at z≈0, for a wide range of assumed scatters in the MHI-MB relation. We rule out the hypothesis that the number density of galaxies with MHI>10¹⁰ M_ remains unchanged between z ≈ 1 and z≈0 at 99.7% confidence. This is the first statistically significant evidence for evolution in the HIMF of galaxies from the epoch of cosmic noon.