Large Molecular Gas Reservoirs in Ancestors of Milky Way-Mass Galaxies 9 Billion Years Ago

TL;DR

This study detects large molecular gas reservoirs in galaxies 9 billion years ago, revealing that early Milky Way progenitors had abundant cold gas fueling star formation, with efficiencies similar to present-day galaxies.

Contribution

It provides direct CO observations of Milky Way progenitors at z=1.2-1.3, showing large gas masses and similar star-formation efficiencies to local galaxies, advancing understanding of galaxy evolution.

Findings

Galaxies at z=1.2-1.3 have molecular gas masses comparable to stellar masses.

Star-formation efficiency has remained nearly constant since z=1.2.

Gas depletion timescales are long, indicating sustained star formation.

Abstract

The gas accretion and star-formation histories of galaxies like the Milky Way remain an outstanding problem in astrophysics. Observations show that 8 billion years ago, the progenitors to Milky Way-mass galaxies were forming stars 30 times faster than today and predicted to be rich in molecular gas, in contrast with low present-day gas fractions ($<$10%). Here we show detections of molecular gas from the CO(J=3-2) emission (rest-frame 345.8 GHz) in galaxies at redshifts z=1.2-1.3, selected to have the stellar mass and star-formation rate of the progenitors of today's Milky Way-mass galaxies. The CO emission reveals large molecular gas masses, comparable to or exceeding the galaxy stellar masses, and implying most of the baryons are in cold gas, not stars. The galaxies' total luminosities from star formation and CO luminosities yield long gas-consumption timescales. Compared to local spiral galaxies, the star-formation efficiency, estimated from the ratio of total IR luminosity to CO emission,} has remained nearly constant since redshift z=1.2, despite the order of magnitude decrease in gas fraction, consistent with results for other galaxies at this epoch. Therefore the physical processes that determine the rate at which gas cools to form stars in distant galaxies appear to be similar to that in local galaxies.