The H I mass function of the Local Universe: Combining measurements from HIPASS, ALFALFA, and FASHI

¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China
² University of Chinese Academy of Sciences, Beijing 100049, China
³ Steward Observatory, University of Arizona, 933 N Cherry Ave., Tucson, AZ 85721, USA
⁴ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
⁵ Guizhou Radio Astronomical Observatory, Guizhou University, Guiyang 550000, China
⁶ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China

^⋆ Corresponding authors; guohong@shao.ac.cn; cpzhang@nao.cas.cn; mz@nao.cas.cn

Received: 13 November 2024
Accepted: 17 February 2025

Abstract

We present the first H I mass function (HIMF) measurement for the recent FAST All Sky H I (FASHI) survey and the most complete measurements of the HIMF in the Local Universe thus far. We obtained these results by combining the H I catalogues from H I Parkes All Sky Survey (HIPASS), Arecibo Legacy Fast ALFA (ALFALFA), and FASHI surveys at a redshift of 0 < z < 0.05, covering 76% of the entire sky. We adopted the same methods to estimate the distances, calculate the sample completeness, and determine the HIMF for all three surveys. The best-fit Schechter function for the total HIMF shows a low-mass slope parameter of α = −1.30 ± 0.01 and a ‘knee’ mass of log(M_s/h₇₀⁻² M_⊙) = 9.86 ± 0.01, along with a normalisation of ϕ_s = (6.58 ± 0.23)×10⁻³ h₇₀³ Mpc⁻³ dex⁻¹. This gives us the cosmic H I abundance: Ω_H I = (4.54 ± 0.20) × 10⁻⁴ h₇₀⁻¹. We find that a double Schechter function with the same slope α better describes our HIMF, where the two different ‘knee’ masses are log(M_s1/h₇₀⁻² M_⊙) = 9.96 ± 0.03 and log(M_s2/h₇₀⁻² M_⊙) = 9.65 ± 0.07. We verify that the measured HIMF is marginally affected by the choice of distance estimates. The effect of cosmic variance is significantly suppressed by combining the three surveys and this provides a unique opportunity to obtain an unbiased estimate of the HIMF in the Local Universe.