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In two previous papers, we presented simulations of the first galaxies in arepresentative volume of the Universe. The simulations are unique because wemodel feedback-regulated galaxy formation, using time-dependent,spatially-inhomogeneous radiative transfer coupled to hydrodynamics. Here, westudy the properties of simulated primordial dwarf galaxies with masses <2x10^8Msolar and investigate their impact on the intergalactic medium. While manyprimordial galaxies are dark, about 100--500 per comoving Mpc^3 are luminousbut relatively faint. They form preferentially in chain structures, and havelow 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_*> \proptoM_dm^2, independent of redshift. Because of feedback, halos smaller than acritical mass, M_crit(z), are devoid of most of their baryons. Moreinterestingly, we find that dark halos have always a smaller M_crit(z) thanluminous 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 and10%--50% with [Z/H]>-5.0. At z=10, the fraction of stars with metallicityZ<10^-3 Zsolar is 10^-6 of the total stellar mass. Although detections ofhigh-redshift dwarf galaxies with the James Webb Space Telescope will be achallenge, studies of their fossil records in the local Universe are promisingbecause of their large spatial density.

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