An imaging and spectroscopic survey of galaxies within prominent nearby voids. II. Morphologies, star formation, and faint companions

We analyze the optical properties of similar to 300 galaxies within and aro und three prominent voids of the Center for Astrophysics Redshift Survey. W e determine CCD morphologies and H alpha equivalent widths from our imaging and spectroscopic survey. We also describe a redshift survey of 250 neighb oring galaxies in the imaging survey fields. We assess the morphology-densi ty relation, EW(H alpha)-density relation, and the effects of nearby compan ions for galaxies in low-density environments selected with a smoothed larg e-scale (5 h(-1) Mpc) galaxy number density n. Both the morphological mix a nd the H alpha line width distribution of galaxies at modest underdensities , 0.5 < n/(n) over bar less than or equal to 1, are indistinguishable from our control sample at modest overdensities, 1 < n/(n) over bar less than or equal to 2. Both density regions contain a similar fraction of galaxies wi th early-type (E and SO) morphologies and with absorption-line spectra (app roximate to 35%). At the lowest densities, n/(n) over bar less than or equa l to 0.5, there is a 3 sigma shift in the distribution of EW(H alpha) away from absorption-line systems (only approximate to 15%) and toward emission- line systems with active star formation-EW(H alpha) similar to 40-100 Angst rom. There is a 2 sigma shift in the morphological distribution away from e arly types and toward irregular and peculiar morphologies. The redshift survey of projected companions, 80% complete to m(R) = 16.13, demonstrates that the incidence of a close companion in redshift space is i nsensitive to global density over the range we investigate (0.16 < n/(n) ov er bar less than or equal to 2). However, the typical velocity separation o f close pairs drops significantly (>3 sigma) from Delta cz greater than or similar to 200 km s(-1) at 0.5 < n/(n) over bar less than or equal to 2 dow n to Delta cz = 103 +/- 20 km s(-1) at n less than or equal to 0.5 (n) over bar. Tn the lowest density environments, galaxies with companions clearly (similar to 4 sigma ) have stronger star formation than comparable galaxies at larger global density (0.5 < n/(n) over bar less than or equal to 2). O n the other hand, the distribution of EW(H alpha) for galaxies without near by companions (closer than approximate to 150 h(-1) kpc and 1000 km s(-1)) varies little over the entire density range. These results, combined with t he luminosity- and color-density relations of this sample (Paper I), sugges t that the formation and evolution of field galaxies are insensitive to lar ge-scale underdensity down to a threshold of roughly half the mean density. The differences in galaxy properties at the lowest global densities we can explore (n less than or equal to 0.5 (n) over bar) may be explained by (1) a relative scarcity of the small-scale primordial density enhancements nee ded to form massive early-type/absorption-line galaxies and (2) present-day galaxy encounters that are relatively more effective because of the lower velocity dispersion on small scales (less than or similar to 200 h(-1) kpc) we observe in these regions. In the voids, where the luminous galaxies pre sumably formed more recently, there should be more gas and dust present for active star formation triggered by nearby companions.