FORMATION OF VOIDS FROM NEGATIVE DENSITY PERTURBATIONS

The formation of voids from negative density perturbations, present in the recombination era, is of appreciable interest, and a number of investigations on the subject have recently been made. These investigations, in general, include only the effect of gravity and neglect nongravitational physical processes (NPP) such as photon drag and photon cooling. In the present investigation we study the effect of NPP in the formation of voids, following the analysis published by de Araujo and Opher in 1988 made for positive density perturbations. Previous investigations made similarity analysis or postulated the magnitude of the initial density perturbation, δi. We obtain the magnitude of δi using the same mass dependence as previously used for positive perturbations (e.g., de Araujo and Opher). In general we find that the inclusion of NPP act as a drag, impeding the formation of underdense regions and increasing their present density. We study a spectrum of negative perturbations of the form δi = -(M/M0)-α(1 + zrec)-1, where M0 is the mass scale, and zrec is the redshift at recombination. We find, for example, for α = 1/2 (corresponding to a white noise spectrum) and M0 ≈ 1015 M⊙ (corresponding to a galactic cluster) that the inclusion of the NPP increases the final density of the underdense regions ∼35%-75% for M ∼ 109-1012 M⊙. Our calculations produce underdense regions over several megaparsecs. It is of interest to obtain (for a given range of redshifts), for small neighbor distances (<1°), the distribution of dwarf galaxies of mass ∼ 109 M⊙ (∼5 optical mag greater than average galaxies). Our results predict the existence of regions ≲1/10 the average density for regions of mass ∼109-1010 M⊙, and the distribution of dwarf galaxies of mass M ∼ 109 M⊙ are sensitive to these underdense regions.