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

We analyze the optical properties of ∼300 galaxies within and around three prominent voids of the Center for Astrophysics Redshift Survey. We determine CCD morphologies and Hα equivalent widths from our imaging and spectroscopic survey. We also describe a redshift survey of 250 neighboring galaxies in the imaging survey fields. We assess the morphology-density relation, EW(Hα)-density relation, and the effects of nearby companions for galaxies in low-density environments selected with a smoothed large-scale (5 h-1 Mpc) galaxy number density n. Both the morphological mix and the Hα line width distribution of galaxies at modest underdensities, 0.5 < n/n̄ ≤ 1, are indistinguishable from our control sample at modest overdensities, 1 < n/n̄ ≤ 2. Both density regions contain a similar fraction of galaxies with early-type (E and S0) morphologies and with absorption-line spectra (≈35%). At the lowest densities, n/n̄ ≤ 0.5, there is a 3 σ shift in the distribution of EW(Hα) away from absorption-line systems (only ≈15%) and toward emission-line systems with active star formation—EW(Hα) ∼ 40–100 Å. There is a 2 σ shift in the morphological distribution away from early types and toward irregular and peculiar morphologies. The redshift survey of projected companions, 80% complete to mR = 16.13, demonstrates that the incidence of a close companion in redshift space is insensitive to global density over the range we investigate (0.16 < n/n̄ ≤ 2). However, the typical velocity separation of close pairs drops significantly (>3 σ) from Δcz ≳ 200 km s-1 at 0.5 < n/n̄ ≤ 2 down to Δcz = 103 ± 20 km s-1 at n ≤ 0.5n̄. In the lowest density environments, galaxies with companions clearly (∼4 σ) have stronger star formation than comparable galaxies at larger global density (0.5 < n/n̄ ≤ 2). On the other hand, the distribution of EW(Hα) for galaxies without nearby companions (closer than ≈150 h-1 kpc and 1000 km s-1) varies little over the entire density range. These results, combined with the luminosity- and color-density relations of this sample (Paper I), suggest that the formation and evolution of field galaxies are insensitive to large-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 ≤ 0.5n̄) may be explained by (1) a relative scarcity of the small-scale primordial density enhancements needed 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 (≲200 h-1 kpc) we observe in these regions. In the voids, where the luminous galaxies presumably formed more recently, there should be more gas and dust present for active star formation triggered by nearby companions.