Voids in the 2dFGRS and ΛCDM simulations:: spatial and dynamical properties

We perform a statistical study on the distribution and dynamics of voids in the 2dF Galaxy Redshift Survey (2dFGRS). Our statistics are tested and calibrated using mock 2dFGRS catalogues. We analyse the dynamics around voids in mock and real 2dFGRS surveys. The void-galaxy cross-correlation redshift-space distortions show evidence of the continuing growth of voids, confirming another prediction of the hierarchical clustering scenario. A non-linear outflow model can be used to provide quantitative estimates of the outflow velocities around 2dFGRS voids. These results are consistent with maximum outflows of 110, 210 and 270 km s(-1) for voids of < r(void)> = 7.5,12.5 and 17.5 Mpc h(-1), assuming a galaxy bias of b = 1. As an application for future surveys, our study of the mock catalogues shows that direct measurements of the expansion of voids can be obtained using peculiar velocity data. We find that it would also be possible to detect differences in the velocity dispersion of galaxies in the directions parallel and perpendicular to the void walls.