The 1000 Brightest HIPASS Galaxies: The H I Mass Function and ΩH I

We present a new, accurate measurement of the H I mass function of galaxies from the HIPASS Bright Galaxy Catalog, a sample of 1000 galaxies with the highest H I peak flux densities in the southern (δ < 0°) hemisphere. This sample spans nearly 4 orders of magnitude in H I mass [log (MH I/M⊙) + 2 log h75 = 6.8–10.6] and is the largest sample of H I–selected galaxies to date. We develop a bivariate maximum likelihood technique to measure the space density of galaxies and show that this is a robust method, insensitive to the effects of large-scale structure. The resulting H I mass function can be fitted satisfactorily with a Schechter function with faint-end slope α = -1.30. This slope is found to be dependent on morphological type, with late-type galaxies giving steeper slopes. We extensively test various effects that potentially bias the determination of the H I mass function, including peculiar motions of galaxies, large-scale structure, selection bias, and inclination effects, and we quantify these biases. The large sample of galaxies enables an accurate measurement of the cosmological mass density of neutral gas: ΩH I = (3.8 ± 0.6) × 10-4 h. Low surface brightness galaxies contribute only ∼15% to this value, consistent with previous findings.