Dwarf galaxy formation with H₂-regulated star formation. II. gas-rich dark galaxies at redshift 2.5

We present a cosmological hydrodynamic simulation of the formation of dwarf galaxies at redshifts z ≳ 2.5 using a physically motivated model for H₂-regulated star formation. Our simulation, performed using the Enzo code and reaching a peak resolution of 109 proper parsecs at z = 2.5, extends the results of Kuhlen et al. to significantly lower redshifts. We show that a star formation prescription regulated by the local H₂ abundance leads to the suppression of star formation in dwarf galaxy halos with M _h ≲ 10¹⁰ M_O and to a large population of gas-rich "dark galaxies" at z = 2.5 with low star formation efficiencies and gas depletion timescales >20 Gyr. The fraction of dark galaxies is 60% at M _h ≃ 10¹⁰ M_O and increases rapidly with decreasing halo mass. Dark galaxies form late and their gaseous disks never reach the surface densities, ≳ 5700 M_O pc^-2 (Z/10^-3 Z _o)^-0.88, that are required to build a substantial molecular fraction. Despite this large population of dark galaxies, we show that our H₂-regulated simulation is consistent with both the observed luminosity function of galaxies and the cosmological mass density of neutral gas at z ≳ 2.5. Moreover, our results provide a theoretical explanation for the recent detection in fluorescent Lyα emission of gaseous systems at high redshift with little or no associated star formation. We further propose that H₂-regulation may offer a fresh solution to a number of outstanding "dwarf galaxy problems" in ΛCDM. In particular, H₂-regulation leads galaxy formation to become effectively stochastic on mass scales of M _h ∼ 10 ¹⁰M_O, and thus these massive dwarfs are not "too big to fail."