Linked evolution of gas and star formation in galaxies over cosmic history

TL;DR

This paper investigates how gas inflow and internal galactic processes sustain star formation over cosmic time, highlighting a new mechanism involving supershells that replenishes gas and supports ongoing star formation.

Contribution

It introduces a novel mechanism where expanding supershells within galaxies facilitate gas cooling, molecular formation, and star formation, explaining sustained star formation rates over cosmic history.

Findings

Supershells can efficiently cool and recombine ionized gas.

Shells can trigger molecular cloud formation and collapse.

Replenishment rates from shells match observed gas densities over 0<z<5.

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. Pre-existing 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 pre-existing molecular clouds to trigger collapse and star formation. We show that this mechanism provides replenishment rates sufficient to maintain both the observed HI mass density and the inferred molecular gas mass density over the redshift range 0<z<5.