We exploit a large , complete optical diameter and HI flux limited sample of spiral galaxies with types later than S0a to derive a robust measurement of the HI mass function ( HIMF ) for masses log ( M _ { HI } / M _ { \odot } ) > 7.4 which takes into account the effects of local large scale structure .
The global HIMF derived for this optically–selected sample is well fit by a Schechter function with \alpha = -1.24 , log ( M _ { * } / M _ { \odot } ) = 9.99 , \phi _ { * } = 3.2 \times 10 ^ { -3 } Mpc ^ { -3 } .
These values match those derived from blind HI 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 can not be accounted for by morphological segregation , and must be a consequence of differences among galaxies of the same morphological type but 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 HI deficiency , an expectation consistent with gas removal mechanisms such as ram pressure stripping .