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

Abstract We present the first estimate, based on direct H i 21 cm observations, of the H i mass function (H i MF) of star-forming galaxies at z ≈ 1, obtained by combining our measurement of the scaling relation between H i mass ( M H i ) and B -band luminosity ( M B ) of star-forming galaxies with a literature estimate of the B -band luminosity function at z ≈ 1. We determined the M H i – M B relation by using the GMRT-CAT z 1 survey of the DEEP2 fields to measure the average H i mass of blue galaxies at z = 0.74–1.45 in three separate M B subsamples. This was done by separately stacking the H i 21 cm emission signals of the galaxies in each subsample to detect, at (3.5–4.4) σ significance, the average H i 21 cm emission of each subsample. We find that the M H i – M B relation at z ≈ 1 is consistent with that at z ≈ 0. We combine our estimate of the M H i – M B relation at z ≈ 1 with the B -band luminosity function at z ≈ 1 to determine the H i MF at z ≈ 1. We find that the number density of galaxies with M H i > 10 10 M ⊙ (higher than the knee of the local H i 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 M H i – M B relation. We rule out the hypothesis that the number density of galaxies with M H i > 10 10 M ⊙ remains unchanged between z ≈ 1 and z ≈ 0 at ≳99.7% confidence. This is the first statistically significant evidence for evolution in the H i MF of galaxies from the epoch of cosmic noon.