The Faint End of the HI Mass Function

We study the faint end of the HI mass function (HIMF) in order to test the predictions of the CDM theory on the number density of objects with small (dark) masses. The neutral hydrogen is much better tracer of the underlying mass distribution compared to the luminous matter and can be used to test the existence of a population of small galaxies in which the star formation has been partially or completely suppressed during cosmic evolution. Due to technical limitations, the existing HI surveys are not very sensitive to HI masses below 10 8 M⊙. We designed a blind HI survey to be sensitive to objects with small HI masses. The surveyed area is in the Canis Venatici groups of galaxies and covers in total ≈ 86 deg 2 of sky, with observed velocities in the range −350 < cz < 1400 km s −1. We detected 69 objects, 22 of them for the first time in HI. All new HI detections fall in the lower part of the mass-histogram, confirming our ability to detect galaxies with small HI masses. The calculated HIMF is flat in the faint end regime (α ∼-1), different from the steep rise predicted by CDM models. Possible effects of the environment on the estimated HIMF parameters are discussed. 1. Motivation Some works that address the discrepancy between the number of the observed dwarf galaxies in the optical and the number of those galaxies predicted by CDM models, suggest that small galaxies contain relatively less luminous matter than larger galaxies (e.g. Verde, Oh & Jimenez 2002). If that is the case, a fraction of galaxies with dark matter haloes of mass below 10 9-10 10 M ⊙ may never form stars and instead they will remain dark. Neutral hydrogen, if still within these systems, would be a much better tracer of dark galaxies. If one assumes that HI makes up a few percent of the total mass of a galaxy, dark galaxies would contain HI in the range 10 7-10 8 M ⊙ or less. The surveys carried out in HI have poor statistics for objects with masses below 10 8 M ⊙. Moreover, all HI objects detected on extragalactic scales have an optical counterpart and have masses above 10 7 M ⊙ (Briggs 2004). On the other hand, there is a population of recently discovered compact HI clouds (CHVCs) around the Milky Way and M31 …