The HI Mass Function of Star-forming Galaxies at z 0.35

The neutral atomic hydrogen (HI) mass function (HIMF) describes the distribution of the HI content of galaxies at any epoch; its evolution provides an important probe of models of galaxy formation and evolution. Here, we report Giant Metrewave Radio Telescope HI 21 cm spectroscopy of blue star-forming galaxies at z ≈ 0.20-0.42 in the Extended Groth Strip, which has allowed us to determine the scaling relation between the average HI mass (M _HI ) and the absolute B-band magnitude (M _B ) of such galaxies at z ≈ 0.35, by stacking the HI 21 cm emission signals of galaxy subsamples in different M _B ranges. We combine this M _HI - M _B scaling relation (with a scatter assumed to be equal to that in the local universe) with the known B-band luminosity function of star-forming galaxies at these redshifts to determine the HIMF at z ≈ 0.35. We show that the use of the correct scatter in the M _HI - M _B scaling relation is critical for an accurate estimate of the HIMF. We find that the HIMF has evolved significantly from z ≈ 0.35 to z ≈ 0, i.e., over the last 4 Gyr, especially at the high-mass end. High-mass galaxies, with M _HI ≳ 10¹⁰ M _⊙, are a factor of ≈3.4 less prevalent at z ≈ 0.35 than at z ≈ 0. Conversely, there are more low-mass galaxies, with M _HI ≈ 10⁹ M _⊙, at z ≈ 0.35 than in the local universe. While our results may be affected by cosmic variance, we find that massive star-forming galaxies have acquired a significant amount of HI through merger events or accretion from the circumgalactic medium over the past 4 Gyr.