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

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 ≲2 × 10⁸ M_☉ and investigate their impact on the intergalactic medium. While many primordial galaxies are dark, about 100-500 per comoving Mpc³ 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 Z_☉, and can sustain a multiphase structure. With large scatter, the mean efficiency of star formation scales with halo mass, ⟨ f_*⟩ ∝ M²_DM, 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 Z_☉ is 10⁻⁶ of the total stellar mass. However, this study focuses on the effects of radiative feedback: mechanical feedback from SN explosions is only included in two of the seven simulations we have analyzed. 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.