Abstract
We compare the cosmic evolution of star formation rates in galaxies with that of their neutral hydrogen densities. We highlight the need for neutral hydrogen to be continually replenished from a reservoir of ionized gas to maintain the observed star formation rates in galaxies. Hydrodynamic simulations indicate that the replenishment may occur naturally through gas infall, although measured rates of gas infall in nearby galaxies are insufficient to match consumption. We identify an alternative mechanism for this replenishment, associated with expanding supershells within galaxies. Preexisting ionized gas can cool and recombine efficiently in the walls of supershells, molecular gas can form in situ in shell walls, and shells can compress preexisting molecular clouds to trigger collapse and star formation. We show that this mechanism provides replenishment rates sufficient to maintain both the observed H I mass density and the inferred molecular gas mass density over the redshift range -->0 ≤ z 5.
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