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 greater than or similar to 2.5 using a physically motivated model for H-2-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 H-2 abundance leads to the suppression of star formation in dwarf galaxy halos with M-h less than or similar to 10(10) M-circle dot 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 similar or equal to 10(10) M-circle dot and increases rapidly with decreasing halo mass. Dark galaxies form late and their gaseous disks never reach the surface densities, greater than or similar to 5700 M-circle dot pc(-2) (Z/10(-3) Z(circle dot))(-0.88), that are required to build a substantial molecular fraction. Despite this large population of dark galaxies, we show that our H-2-regulated simulation is consistent with both the observed luminosity function of galaxies and the cosmological mass density of neutral gas at z greater than or similar to 2.5. Moreover, our results provide a theoretical explanation for the recent detection in fluorescent Ly alpha emission of gaseous systems at high redshift with little or no associated star formation. We further propose that H-2-regulation may offer a fresh solution to a number of outstanding dwarf galaxy problems in Lambda CDM. In particular, H-2-regulation leads galaxy formation to become effectively stochastic on mass scales of M-h similar to 10(10) M-circle dot, and thus these massive dwarfs are not too big to fail.