Australia Telescope National Facility

The HI Mass Function from HIPASS
V. Kilborn ,
R. L. Webster ,
L. Staveley-Smith ,
, PASA, 16 (1), in press.

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Contents Page: Volume 16, Number 1

Introduction

The neutral hydrogen (HI) mass function (HIMF) describes the number density of neutral hydrogen as a function of HI mass at the present epoch. Measuring the HIMF is important for understanding the evolution of galaxies and for understanding the incidence of low-redshift Ly- $\alpha$ damped and `forest' lines against background ultraviolet sources. Until recently, the data used to determine the HIMF has come from very small samples of HI-selected objects (e.g. Zwaan et al. 1997, Schneider et al. 1998), or from HI observations of optically-selected galaxies (e.g. Solanes et al. 1996). This means that HIMFs typically have poor statistics in the extreme mass bins, or are inherently biased by the optical nature of the sample selection. Optical samples tend to miss low surface-brightness systems (e.g. Chengalur et al. 1995) and tidal gas (e.g. Schneider et al. 1983). However, an extensive blind HI survey such as HIPASS gives both a large and an unbiased sample and will increase the number counts for the low and high mass ends of the HIMF. Presently, the low-mass end of the HIMF is somewhat controversial with contrary views being expressed as to the relative importance of low-mass and high-mass galaxies to the overall HI density of the Universe. There is some evidence for an upturn at the low-luminosity end of the optical luminosity function (e.g. Loveday 1997). It is important to establish whether a similar upturn exists at the low-mass end of the HIMF. Most derivations of the HIMF have found a shallow faint-end slope (Schechter parameter,

$\alpha \approx 1.2$ - e.g. Zwaan et al. 1997), implying that low-mass galaxies contribute relatively little to the total HI density. Recently Schneider et al. (1998) claim to see an upturn below masses of $\sim10^8$ M $_{\odot}$ . However, their result is from a sample with only 2 galaxies in the lowest mass bin, so it remains to be seen whether this will persist in statistically larger samples. In this paper, we derive the HIMF for a sample of 263 HI-selected galaxies. In §2, we give the particulars of the HIPASS survey and the data selection process; in §3 we derive the HIMF. In §4, we briefly discuss the results. A value of