An Imaging and Spectroscopic Survey of Galaxies within Prominent Nearby Voids. II. Morphologies, Star Formation, and Faint Companions

We assess the morphology-density relation, H-alpha linewidth-densityrelation, and the effects of nearby companions for 300 galaxies selected fromlow-density environments of the Center for Astrophysics Redshift Survey. Boththe morphological mix and the EW(H-alpha) distribution of galaxies at modestunderdensites are indistinguishable from our control sample at modestoverdensities (early-type and absorption-line fractions both ~35%). At thelowest densities: the absorption-line fraction drops to 15%; more galaxies haveactive star formation; and there are more irregular and peculiar morphologies. Our magnitude-limited redshift survey of 250 fainter projected companionsreveals that the void galaxies are no less likely to have a neighbor inredshift space. However, the typical velocity separation of close pairs dropssharply at densities below half the mean, from >200 km/s to ~100 km/s. TheEW(H-alpha) distribution for galaxies lacking nearby companions varies littlewith density, while galaxies with nearby companions are much more likely tohave strong current star formation (H-alpha linewidths >20 \AA) in the voids. These results and the luminosity- and color-density relations of this sample(astro-ph/9910073) suggest that the formation and evolution of field galaxiesare only sensitive to large-scale underdensity below a threshold of roughlyhalf the mean density. The differences in galaxy properties at the lowerdensities may be explained by: 1) a relative scarcity of the small-scaleprimordial density enhancements needed to form massive early-type/absorption-line galaxies; 2) more effective galaxy encounters due to the lower velocitydispersion on small scales; and 3) more gas and dust available for triggeredstar formation, presuming that luminous void galaxies formed more recently.Comment: 22 pages, 10 figures; AJ in press (Jan. 2000); minor revisions from original submissio