Morphology, Environment, and the H I Mass Function

We exploit a large, complete optical diameter- and H I flux-limited sample of spiral galaxies with types later than S0a to derive a robust measurement of the H I mass function (HIMF) for masses log(M/M☉) > 7.4 that takes into account the effects of local large-scale structure. The global HIMF derived for this optically selected sample is well fitted by a Schechter function with α = -1.24, log(M*/M☉) = 9.99, and * = 3.2 × 10-3 Mpc-3. These values match those derived from blind H I surveys to within the estimated uncertainties, yet our estimated HIMF is clearly lower than most other estimates at the lowest masses. We also investigate the variation in the derived HIMF among spiral subclasses, finding a clear distinction between the Schechter parameters found for types Sa-Sc and those Scd and later in the sense that the HIMF of the latest types is rising at the low-mass end, whereas that of the main spiral classes is flat or even declining. We also explore the possible environmental dependence of the HIMF by computing it separately in regimes of differing cosmic density. The HIMFs of higher density regions are found to have flatter low-mass ends and lower values of M* than those of lower density regions, although the statistical significance of the difference is low. Because the subsamples found in different density regimes exhibit virtually the same morphological fractions, the environmental dependence cannot be accounted for by morphological segregation but must be a consequence of differences among galaxies of the same morphological type that are found in different environments. If this dependence is caused by the well-known deficiency of galaxies in clusters, then it would suggest that galaxies of small linear optical diameter are characterized by higher H I deficiency, an expectation consistent with gas removal mechanisms such as ram pressure stripping.