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We report results from a study of the HI content and stellar properties ofnearby galaxies detected by the Arecibo Legacy Fast ALFA blind 21-cm linesurvey and the Sloan Digital Sky Survey in two declination strips covering atotal area of 9 hr X 16 deg. Our analysis seeks to assemble a control sample ofgalaxies suitable for providing absolute measures of the HI content of gaseousobjects. From a database of ~15,000 HI detections, we have assembled threesamples of gas-rich galaxies expected to show little or no evidence ofinteraction with their surroundings that could provide adequate HI standards.The most reliable results are obtained with a sample of 5647 sources found inlow density environments, as defined by a nearest neighbor approach. The othertwo samples contain several hundred relatively isolated galaxies each, asdetermined from standard isolation algorithms. We find that isolated objectsare not particularly gas-rich compared to their low-density-environmentcounterparts, while they suffer from selection bias and span a smaller dynamicrange. All this makes them less suitable for defining a reference for HIcontent. We have explored the optical morphology of gaseous galaxies in quietenvironments finding that, within the volume surveyed, the vast majority ofthem display unequivocal late-type galaxy features. In contrast, bona fidegas-rich early-type systems account only for a negligible fraction of the 21-cmdetections. We argue that HI emission provides the most reliable way todetermine the morphological population to which a galaxy belongs. We have alsoobserved that the color distribution of flux-limited samples ofoptically-selected field HI emitters does not vary significantly withincreasing distance, while that of non-detections becomes notably redder. Thisresult suggests that the colors and HI masses of gas-rich galaxies cannot bevery closely related.

HI Content and Optical Properties of Field Galaxies from the ALFALFA Survey. I.Selection of a Control Sample