Voids in the 2dF Galaxy Redshift Survey

Abstract

We present an analysis of voids in the 2dF Galaxy Redshift Survey (2dFGRS). This analysis includes identification of void regions and measurement of void statistics. The 2dFGRS is the largest completed redshift survey to date, including a total of 245,591 galaxies covering 1500 deg$^2$ to a median depth of z~0.11. We use the voidfinder algorithm to identify a total of 289 voids in the 2dFGRS with radius larger than 10h^{-1}Mpc. These voids have an average effective radius of 14.89+- 2.65 h^{-1}Mpc in the North Galactic Pole region (NGP) and 15.61+- 2.84h^{-1}Mpc in the South Galactic Pole region (SGP). These voids are extremely underdense, with average density contrast of delta rho/rho= -0.94+-0.02. The centers of voids are even emptier, because the few galaxies within the voids typically lie close to the edges. The total volume of the universe filled by these void regions is approximately 40%. We measure the Void Probability Function (VPF) of the 2dFGRS for volume-limited samples with limiting absolute magnitudes from -16 to -21 in b_J. We measure the Underdensity Probability Function for samples with limiting absolute magnitudes from -18 to -21. We find that the SGP is more underdense than the NGP for all but the brightest sample under consideration. There is good agreement between the VPF's of the Center for Astrophysics Survey and the 2dFGRS. Comparison of VPF's measured for the 2dFGRS with the distribution of simulated dark-matter halos of similar number density indicates that voids in the matter distribution in LCDM simulations are not empty enough. However, semi-analytic models of galaxy formation that include feedback effects yield VPF's that show excellent agreement with the data.