Voids in the Two‐Degree Field Galaxy Redshift Survey

We present an analysis of voids in the 2dF Galaxy Redshift Survey (2dFGRS).This analysis includes identification of void regions and measurement of voidstatistics. The 2dFGRS is the largest completed redshift survey to date,including a total of 245,591 galaxies covering 1500 deg$^2$ to a median depthof z~0.11. We use the voidfinder algorithm to identify a total of 289 voids inthe 2dFGRS with radius larger than 10h^{-1}Mpc. These voids have an averageeffective 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). Thesevoids 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 galaxieswithin the voids typically lie close to the edges. The total volume of theuniverse filled by these void regions is approximately 40%. We measure the VoidProbability Function (VPF) of the 2dFGRS for volume-limited samples withlimiting absolute magnitudes from -16 to -21 in b_J. We measure theUnderdensity Probability Function for samples with limiting absolute magnitudesfrom -18 to -21. We find that the SGP is more underdense than the NGP for allbut the brightest sample under consideration. There is good agreement betweenthe VPF's of the Center for Astrophysics Survey and the 2dFGRS. Comparison ofVPF's measured for the 2dFGRS with the distribution of simulated dark-matterhalos of similar number density indicates that voids in the matter distributionin LCDM simulations are not empty enough. However, semi-analytic models ofgalaxy formation that include feedback effects yield VPF's that show excellentagreement with the data.Comment: Submitted to Ap