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

Abstract

We assess the morphology-density relation, H-alpha linewidth-density relation, and the effects of nearby companions for 300 galaxies selected from low-density environments of the Center for Astrophysics Redshift Survey. Both the morphological mix and the EW(H-alpha) distribution of galaxies at modest underdensites are indistinguishable from our control sample at modest overdensities (early-type and absorption-line fractions both ~35%). At the lowest densities: the absorption-line fraction drops to 15%; more galaxies have active star formation; and there are more irregular and peculiar morphologies. Our magnitude-limited redshift survey of 250 fainter projected companions reveals that the void galaxies are no less likely to have a neighbor in redshift space. However, the typical velocity separation of close pairs drops sharply at densities below half the mean, from >200 km/s to ~100 km/s. The EW(H-alpha) distribution for galaxies lacking nearby companions varies little with density, while galaxies with nearby companions are much more likely to have 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 galaxies are only sensitive to large-scale underdensity below a threshold of roughly half the mean density. The differences in galaxy properties at the lower densities may be explained by: 1) a relative scarcity of the small-scale primordial density enhancements needed to form massive early-type/absorption- line galaxies; 2) more effective galaxy encounters due to the lower velocity dispersion on small scales; and 3) more gas and dust available for triggered star formation, presuming that luminous void galaxies formed more recently.