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

We present the first estimate, based on direct Hi 21 cm observations, 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 a literature estimate 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 > 1010M⊙ (higher than the knee of the local Hi mass function) 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 > 1010M⊙ 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.