An Imaging and Spectroscopic Survey of Galaxies within Prominent Nearby Voids. I. The Sample and Luminosity Distribution

We study the optical properties of a large sample of galaxies in low-density regions of the nearby universe. We make a 5h −1 Mpc-smoothed map of the galaxy density throughout the Center for Astrophysics Redshift Survey (CfA2, Geller & Huchra 1989) to identify galaxies within three prominent nearby “voids” with diameter ∼> 30h −1 Mpc. We augment the CfA2 void galaxy sample with fainter galaxies found in the same regions from the deeper Century (Geller et al. 1997) and 15R (Geller et al. 2000) Redshift Surveys. We obtain B and R CCD images and high signal-to-noise longslit spectra for the resulting sample of 149 void galaxies, as well as for an additional 131 galaxies on the periphery of these voids. Here we describe the photometry for the sample, including B isophotal magnitudes and B−R colors. For the 149 galaxies which lie in regions below the mean survey density, the luminosity functions in B and R are well-fit by Schechter functions with respective parameters (αB = −0.5 ± 0.3, B ∗ = −18.9 ± 0.2) and (αR = −0.9 ± 0.3, R ∗ = −20.4 ± 0.3). The B luminosity function (LF) is consistent with typical survey LFs (e.g. the Southern Sky Redshift Survey; Da Costa et al. 1998), and the R LF is consistent with the Century Survey. The B and R LFs of 131 galaxies in the “void periphery”, regions between the mean density and twice the mean, have similar Schechter parameters. The CfA2 LF is inconsistent with both samples at the 3.5σ level. When we narrow our analysis to the 46 galaxies in regions below half the mean density, the LF is significantly steeper: α ∼ −1.4 ±0.5. The typical survey LFs are inconsistent with this subsample at the ∼ 2σ level. The B−R color distribution of galaxies in the lowest-density regions is also shifted significantly (∼3σ) blue-ward of the higher density samples. The most luminous red galaxies (R ∼ < −21) are absent from the lowest density regions at the 2.5σ level. Subject headings: large-scale structure of universe — galaxies: luminosity function, mass function — galaxies: photometry – 2 – 1.