The Fate of the First Galaxies. III. Properties of Primordial Dwarf Galaxies and their Impact on the Intergalactic Medium

TL;DR

This paper uses advanced simulations to explore the properties of primordial dwarf galaxies and their influence on the intergalactic medium, revealing their faintness, structure, and role in metal enrichment during early cosmic times.

Contribution

It presents the first detailed simulation-based analysis of primordial dwarf galaxies, including their formation, properties, and impact on the intergalactic medium, with feedback-regulated models and inhomogeneous metal enrichment.

Findings

Primordial dwarf galaxies are numerous but faint, forming in chain structures.

Halos smaller than a critical mass are baryon-depleted due to feedback.

Metal enrichment of the IGM is highly inhomogeneous with low volume filling factors.

Abstract

In two previous papers, we presented simulations of the first galaxies in a representative volume of the Universe. The simulations are unique because we model feedback-regulated galaxy formation, using time-dependent, spatially-inhomogeneous radiative transfer coupled to hydrodynamics. Here, we study the properties of simulated primordial dwarf galaxies with masses <2x10^8 Msolar and investigate their impact on the intergalactic medium. While many primordial galaxies are dark, about 100--500 per comoving Mpc^3 are luminous but relatively faint. They form preferentially in chain structures, and have low surface brightness stellar spheroids extending to 20% of the virial radius. Their interstellar medium has mean density n_H~10--100 cm^-3, metallicity Z~ 0.01--0.1 Zsolar and can sustain a multi-phase structure. With large scatter, the mean efficiency of star formation scales with halo mass, <f_*> \propto M_dm^2, independent of redshift. Because of feedback, halos smaller than a critical mass, M_crit(z), are devoid of most of their baryons. More interestingly, we find that dark halos have always a smaller M_crit(z) than luminous ones. Metal enrichment of the intergalactic medium is inhomogeneous, with only a 1%--10% volume filling factor of enriched gas with [Z/H]>-3.0 and 10%--50% with [Z/H]>-5.0. At z=10, the fraction of stars with metallicity Z<10^-3 Zsolar is 10^-6 of the total stellar mass. Although detections of high-redshift dwarf galaxies with the James Webb Space Telescope will be a challenge, studies of their fossil records in the local Universe are promising because of their large spatial density.