2018MNRAS.477....2J

We present the most precise measurement of the z = 0 H I mass function (HIMF) to date based on the final catalogue of the ALFALFA (Arecibo Legacy Fast ALFA) blind H I survey of the nearby Universe. The Schechter function fit has a 'knee' mass log(M_*h²₇₀/M_☉) = 9.94 ±0.01 ±0.05, a low-mass slope parameter α = -1.25 ± 0.02 ± 0.1, and a normalization φ_* = (4.5 ±0.2 ±0.8) ×10^–3 h³₇₀ Mpc^–3dex^–1, with both random and systematic uncertainties as quoted. Together these give an estimate of the H I content of the z = 0 Universe as Ω_H I_ = (3.9 ±0.1 ±0.6) ×10^–4 h^–1₇₀ (corrected for H I self-absorption). Our analysis of the uncertainties indicates that the 'knee' mass is a cosmologically fair measurement of the z = 0 value, with its largest uncertainty originating from the absolute flux calibration, but that the low-mass slope is only representative of the local Universe. We also explore large-scale trends in α and M_* across the ALFALFA volume. Unlike with the 40 per cent sample, there is now sufficient coverage in both of the survey fields to make an independent determination of the HIMF in each. We find a large discrepancy in the low-mass slope (Δα = 0.14 ± 0.03) between the two regions, and argue that this is likely caused by the presence of a deep void in one field and the Virgo cluster in the other. Furthermore, we find that the value of the 'knee' mass within the Local Volume appears to be suppressed by 0.18 ± 0.04 dex compared to the global ALFALFA value, which explains the lower value measured by the shallower H I Parkes All Sky Survey (HIPASS). We discuss possible explanations and interpretations of these results and how they can be expanded on with future surveys.