VOIDS IN THE LARGE-SCALE STRUCTURE

Voids are the most prominent feature of the large-scale structure of t he universe. Still, their incorporation into quantitative analysis of it has been relatively recent, owing essentially to the lack of an obj ective tool to identify the voids and to quantify them. To overcome th is, we present here the VOID FINDER algorithm, a novel tool for object ively quantifying voids in the galaxy distribution. The algorithm firs t classifies galaxies as either wall galaxies or field galaxies. Then, it identifies voids in the wall-galaxy distribution. Voids are define d as continuous volumes that do not contain any wall galaxies. The voi ds must be thicker than an adjustable limit, which is refined in succe ssive iterations. Iii this way, we identify the same regions that woul d be recognized as voids by the eye. Small breaches in the walls are i gnored, avoiding artificial connections between neighboring voids. We test the algorithm using Voronoi tesselations. By appropriate scaling of the parameters with the selection function, we apply it to two reds hift surveys, the dense SSRS2 and the full-sky IRAS 1.2 Jy. Both surve ys show similar properties: similar to 50% of the volume is filled by voids. The voids have a scale of at least 40 h(-1) Mpc and an average -0.9 underdensity. Faint galaxies do not fill the voids, but they do p opulate them more than bright ones, These results suggest that both op tically and IRAS-selected galaxies delineate the same large-scale stru cture. Comparison with the recovered mass distribution further suggest s that the observed Voids in the galaxy distribution correspond well t o underdense regions in the mass distribution. This confirms the gravi tational origin of the voids.