The HIPASS catalogue: ΩHI and environmental effects on the H I mass function of galaxies

We use the catalogue of 4315 extragalactic H I 21-cm emission line detections from HIPASS to calculate the most accurate measurement of the H I mass function (HIMF) of galaxies to date. The completeness of the HIPASS sample is well characterised, which enables an accurate calculation of space densities. The HIMF is fitted with a Schechter function with parameters: α = −1.37 ± 0.03 ± 0.05, log(M HI /M⊙) = 9.80 ± 0.03 ± 0.03 h −2 75 , and θ = (6.0± 0.8± 0.6)× 10−3h375 Mpc −3 (random and systematic uncertainties at 68% CL), in good agreement with calculations based on the HIPASS Bright Galaxy Catalogue, which is a complete, but smaller, sub-sample of galaxies. The cosmological mass density of H I in the local universe is found to be ΩHI = (3.5 ± 0.4 ± 0.4)× 10h −1 75 . This large homogeneous sample allows us to test whether the shape of the HIMF depends on local galaxy density. We find tentative evidence for environmental effects in the sense that the HIMF becomes steeper toward higher density regions, ranging from α ≈ −1.2 in the lowest density environments to α ≈ −1.5 in the highest density environments probed by this blind H I survey. This effect appears stronger when densities are measured on larger scales.