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N.A. Grogin (STScI), M.J. Geller (Harvard-Smithsonian CfA)
We present the first results from an optical imaging and spectroscopic survey of ~300 galaxies in and around three prominent voids within the Center for Astrophysics Redshift Survey (CfA2). We investigate the H-alpha linewidths, the luminosity, color, and morphological distributions, and the nearby fainter companions of galaxies in low-density regions as determined by a 5/h Mpc-smoothed density estimator applied to the CfA2 survey volume.
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.
Likewise, the luminosity and B-R color distributions vary little with density in regions above half the mean, but shift significantly at the lowest densities: the luminosity function steepens to \alpha ~-1.4 ; and the mean B-R color is bluer by 0.3 mag.
Our redshift survey of 250 projected companions with mR \leq 16.1 reveals that the void galaxies are no less likely to have a neighbor in redshift space (within 1000 km/s and 110 kpc/h projected). 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 in the voids.
The differences in galaxy properties which emerge at the lowest 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.