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

TL;DR

This study analyzes the HI content and optical properties of nearby field galaxies from the ALFALFA survey, establishing a control sample of gas-rich, isolated galaxies to better understand their characteristics and the relationship between HI content and galaxy morphology.

Contribution

It provides a well-defined control sample of gas-rich, isolated galaxies from ALFALFA and SDSS data, and investigates their optical morphology and color-HI mass relations.

Findings

Isolated galaxies are not particularly more gas-rich than low-density environment galaxies.

Most gas-rich galaxies in the survey are late-type, with very few early-type systems.

The color distribution of HI emitters remains consistent with distance, unlike non-detections.

Abstract

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.