The 1000 Brightest Hipass Galaxies: H I Mass Function and Ω Hi

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 (Koribalski et al. 2003). This sample spans nearly four orders of magnitude in H I mass (from log(MHI⁄ M⊙) + 2logh75 = 6.8 to 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 later 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 quantify these biases. The large sample of galaxies enables an accurate measurement of the cosmological mass density of neutral gas: ΩHI = (3.8± 0.6)× 10−4h−1 75 . Low surface brightness galaxies contribute only ∼ 15% to this value, consistent with previous findings. Subject headings: Galaxies: ISM, Galaxies: Luminosity Function, Mass Function, Radio Lines: Galaxies, Surveys, ISM: General