On the evolution of the molecular gas fraction of star forming galaxies

TL;DR

This study measures molecular gas in star-forming galaxies at z=0.4, revealing significant evolution in gas fraction since z~2, consistent with galaxy formation models within a Lambda-CDM universe.

Contribution

First direct CO(1-0) detections of z=0.4 star-forming galaxies, demonstrating evolution of molecular gas fraction consistent with theoretical predictions.

Findings

Molecular gas makes up about 20% of baryonic mass at z=0.4.

Gas fraction evolves as (1+z)^{2 +/- 0.5} since z~2.

Observations support semi-analytic galaxy formation models.

Abstract

We present IRAM Plateau de Bure interferometric detections of CO(1-0) emission from a 24um-selected sample of star-forming galaxies at z=0.4. The galaxies have PAH 7.7um-derived star formation rates of SFR~30-60 M_Sun/yr and stellar masses M*~10^{11} M_Sun. The CO(1-0) luminosities of the galaxies imply that the disks still contain a large reservoir of molecular gas, contributing ~20% of the baryonic mass, but have star-formation 'efficiencies' similar to local quiescent disks and gas-dominated disks at z~1.5-2. We reveal evidence that the average molecular gas fraction has undergone strong evolution since z~2, with f_gas ~ (1+z)^{2 +/- 0.5}. The evolution of f_gas encodes fundamental information about the relative depletion/replenishment of molecular fuel in galaxies, and is expected to be a strong function of halo mass. We show that the latest predictions for the evolution of the molecular gas fraction in semi-analytic models of galaxy formation within a LCDM Universe are supported by these new observations.