A blind H i mass function from the Arecibo Ultra-Deep Survey (AUDS)

The Arecibo Ultra-Deep Survey (AUDS) combines the unique sensitivity of the telescope with the wide field of the Arecibo L-band Feed Array (ALFA) to directly detect 21 cm |H\, i| emission from galaxies at distances beyond the local Universe bounded by the lower frequency limit of ALFA (z = 0.16). AUDS has collected 700 h of integration time in two fields with a combined area of 1.35 deg2. In this paper, we present data from 60 per cent of the total survey, corresponding to a sensitivity level of 80 μJy. We discuss the data reduction, the search for galaxies, parametrization, optical identification and completeness. We detect 102 galaxies in the mass range of |log (M_ H\, I/M)-2log h=5.6 -10.3|⁠. We compute the |H\, i| mass function (HIMF) at the highest redshifts so far measured. A fit of a Schechter function results in α = − 1.37 ± 0.03, Φ* = (7.72 ± 1.4) × 10−3 h3 Mpc−3 and |log \,(M_ H\, i*/ M_ )=(9.75± 0.041)+2log h|⁠. Using the measured HIMF, we find a cosmic |H\, i| density of |Ω _ H\, i=(2.33± 0.07)× 10⁻⁴\, h⁻¹| for the sample (z = 0.065). We discuss further uncertainties arising from cosmic variance. Because of its depth, AUDS is the first survey that can determine parameters for the HIMF in independent redshift bins from a single homogeneous data set. The results indicate little evolution of the comoving mass function and |Ω _ H\, i| within this redshift range. We calculate a weighted average for |Ω _ H\, i| in the range 0 < z < 0.2, combining the results from AUDS as well as results from other 21 cm surveys and stacking, finding a best combined estimate of |Ω _ H\, i=( 2.63± 0.10)× 10⁻⁴\, h⁻¹|⁠.