The Fate of the First Galaxies. II. Effects of Radiative Feedback

Abstract

We use three-dimensional cosmological simulations with radiative transfer to study the formation and evolution of the first galaxies in a ÃCDM cosmology. The simulations include continuum radiative transfer using the optically thin variable Eddington tensor (OTVET) approximation and line radiative transfer in the H 2 Lyman-Werner bands of the UV background radiation. Chemical and thermal processes are treated in detail, particularly the ones relevant for H 2 formation and destruction. We find that the first luminous objects ('' small-halo objects '') are characterized by bursting star formation (SF) that is self-regulated by a feedback process acting on cosmological instead of galactic scales. The global SF history is regulated by the mean number of ionizing photons that escape from each source, UV h f esc i. It is almost independent of the assumed SF efficiency parameter, * , and the intensity of the dissociating background. The main feedback process that regulates the SF is the reformation of H 2 in front of H ii regions and inside relic H ii regions. The H ii regions remain confined inside filaments, maximizing the production of H 2 in overdense regions through cyclic destruction/reformation of H 2 . If UV h f esc i > 10 À7 = Ã , the SF is self-regulated, photoevaporation of smallhalo objects dominates the metal pollution of the low-density intergalactic medium, and the mass of produced metals depends only on h f esc i. If UV h f esc id10 À7 = Ã , positive feedback dominates, and small-halo objects constitute the bulk of the mass in stars and metals until at least redshift z $ 10. Small-halo objects cannot reionize the universe because the feedback mechanism confines the H ii regions inside the large-scale structure filaments. In contrast to massive objects ('' large halos ''), which can reionize voids, small-halo objects partially ionize only the dense filaments while leaving the voids mostly neutral.

Simulations of Early Structure Formation: Primordial Gas Clouds

“…We continued the simulations until about 50-100 million years after the first bound object formed. Radiation from the first object(s) should, in principle, be included because it affects the chemical and thermodynamic evolution of the surrounding IGM in a large fraction of our simulated regions (Ricotti, Gnedin, & Shull 2001). Nevertheless, we do not take such radiative effects into account in the first series of our simulations, in order to isolate other dynamical effects on primordial gas cloud formation.…”

“…On the other hand, a gas with extremely large velocity gradients and disordered motion remains nearly optically thin even for column densities N H 2 $ 10 20 10 21 cm À2 (Glover & Brand 2001). Since the full treatment of three-dimensional radiative transfer for 76 LW lines, even when only the lowest energy level transitions are included, is practically intractable (see, however, Ricotti et al 2001 for one-dimensional calculations for a stationary gas), we adopt the following approximate method to estimate the maximum effect of self-shielding. We consider only shielding by gas in virialized regions and do not consider absorption by the IGM in underdense regions, assuming that the amount of residual intergalactic H 2 is negligible.…”

“…Ricotti et al (2002b) presented extensive tests using a series of simulations and examined the effect of various parameters on the global SFR. They showed that an important parameter that influences the SFR at z > 20 is, indeed, the size of the simulation volume.…”

“…Ciardi et al (2000) used outputs from N-body simulations to locate starforming systems in a cosmological volume. More recently, Ricotti, Gnedin, & Shull (2002a, 2002b performed cosmological simulations including radiative transfer to compute the star formation history at high redshift.…”

Oxygen Pumping: Probing Intergalactic Metals at the Epoch of Reionization

“…However, the recombination time for oxygen (as for hydrogen) is shorter than the Hubble time at , indicating that oxygen z տ 6 can be neutral even in regions where H has been ionized but where short-lived ionizing sources have turned off, allowing the region to recombine (Oh 2002). Numerical simulations that do not resolve the first generation of "minihalos" find a small volume filling factor of such "fossil" H ii regions (Iliev et al 2007;McQuinn et al 2006;Ricotti et al 2002). However, Oh & Haiman (2003) argued, in a semianalytical model, that minihalos are more susceptible to negative feedback and that they can produce fossil H ii regions that occupy 150% of the volume of the IGM prior to reionization.…”