  We report results from a study of the HI content and stellar properties of
nearby galaxies detected by the Arecibo Legacy Fast ALFA blind 21-cm line
survey and the Sloan Digital Sky Survey in two declination strips covering a
total area of 9 hr X 16 deg. Our analysis seeks to assemble a control sample of
galaxies suitable for providing absolute measures of the HI content of gaseous
objects. From a database of ~15,000 HI detections, we have assembled three
samples of gas-rich galaxies expected to show little or no evidence of
interaction with their surroundings that could provide adequate HI standards.
The most reliable results are obtained with a sample of 5647 sources found in
low density environments, as defined by a nearest neighbor approach. The other
two samples contain several hundred relatively isolated galaxies each, as
determined from standard isolation algorithms. We find that isolated objects
are not particularly gas-rich compared to their low-density-environment
counterparts, while they suffer from selection bias and span a smaller dynamic
range. All this makes them less suitable for defining a reference for HI
content. We have explored the optical morphology of gaseous galaxies in quiet
environments finding that, within the volume surveyed, the vast majority of
them display unequivocal late-type galaxy features. In contrast, bona fide
gas-rich early-type systems account only for a negligible fraction of the 21-cm
detections. We argue that HI emission provides the most reliable way to
determine the morphological population to which a galaxy belongs. We have also
observed that the color distribution of flux-limited samples of
optically-selected field HI emitters does not vary significantly with
increasing distance, while that of non-detections becomes notably redder. This
result suggests that the colors and HI masses of gas-rich galaxies cannot be
very closely related.
