The Dawn of Galaxies

Abstract

The development of primordial inhomogeneities into the non-linear regime and the formation of the first astrophysical objects within dark matter halos mark the transition from a simple, neutral, cooling universe -described by just a few parameters -to a messy ionized one -the realm of radiative, hydrodynamic, and star formation processes. The recent measurement by the WMAP satellite of a large optical depth to electron scattering implies that this transition must have begun very early, and that the universe was reionized at redshift z ion = 17 ± 5. It is an early generation of extremely metal-poor massive stars and/or 'seed' accreting black holes in subgalactic halos that may have generated the ultraviolet radiation and mechanical energy that reheated and reionized most of the hydrogen in the cosmos. The detailed thermal, ionization, and chemical enrichment history of the universe during the crucial formative stages around z = 10 − 20 depends on the power-spectrum of density fluctuations on small scales, the stellar initial mass function and star formation efficiency, a complex network of poorly understood 'feedback' mechanisms, and remains one of the crucial missing links in galaxy formation and evolution studies. *

Number and entropy of halo black holes

“…Such Pop-III stars are not well understood but we expect they can be very massive, 10 5 M ⊙ , to live for a short time, less that a million years, then explode leaving black holes which have a total mass that is a significant fraction of the original star's mass. Nevertheless, it is very unlikely [11] that a sufficent number of Pop-III stars can form to make all dark matter. Thus, the IMBHs may have formed in the early universe as primordial black holes.…”

“…Finally, if we truncate to n ≤ 5, since Pop-III stars or IMBHs with higher masses seem unlikely [11], then the typical number of IMBHs per halo is ∼ 10 10 , giving about one million googols for the entropy of the universe. The majority of entropy may be concentrated in a tiny fraction of the total number of black holes as can be seen by studying examples, e.g., f 2 = f 5 = 0.5.…”