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

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 / h 70 −2 M ⊙ ) = 9.86 ± 0.01, along with a normalisation of ϕ s = (6.58 ± 0.23)×10 −3 h 70 3 Mpc −3 dex −1 . This gives us the cosmic H I abundance: Ω H I = (4.54 ± 0.20) × 10 −4 h 70 −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 s 1 / h 70 −2 M ⊙ ) = 9.96 ± 0.03 and log( M s 2 / h 70 −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.