HI CONTENT AND OPTICAL PROPERTIES OF FIELD GALAXIES FROM THE ALFALFA SURVEY. I. SELECTION OF A CONTROL SAMPLE

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. We consider two declination strips covering a total area of 9 hr x 16 deg in the general direction of the Virgo Cluster. The present analysis focuses on gas-rich galaxies expected to show little or no evidence of interaction with their surroundings. We seek to assemble a control sample suitable for providing absolute measures of the HI content of gaseous objects, as well as to study the relationship between HI emission and widely used optical measures of morphology. From a database which includes more than 15,000 HI detections, we have assembled three samples 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 based either on a combination of spectroscopic and photometric information or solely on photometric data. 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.