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

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.

arXiv commentTo appear in Nature Astronomy, 9 pages

References in corpus (31)

• The CO-to-H2 Conversion Factor
• CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey
• CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey - The Hubble Space Telescope Observations, Imaging Data Products and Mosaics
• Cold streams in early massive hot haloes as the main mode of galaxy formation
• Galactic star formation and accretion histories from matching galaxies to dark matter haloes
• GOODS-Herschel: an infrared main sequence for star-forming galaxies
• The Star Formation Rate and Dense Molecular Gas in Galaxies
• High molecular gas fractions in normal massive star forming galaxies in the young Universe
• PHIBSS: molecular gas content and scaling relations in z~1-3 normal star forming galaxies
• Dusty Star-Forming Galaxies at High Redshift
• Different star formation laws for disks versus starbursts at low and high redshifts
• Determining Star Formation Rates for Infrared Galaxies
• The KMOS^3D Survey: design, first results, and the evolution of galaxy kinematics from 0.7<z<2.7
• COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies: I. Relations between H2, HI, stellar content and structural properties
• Improved Estimates of the Milky Way's Stellar Mass and Star Formation Rate from Hierarchical Bayesian Meta-Analysis
• The rapid formation a large rotating disk galaxy three billion years after the Big Bang
• A General Model for the CO-H2 Conversion Factor in Galaxies with Applications to the Star Formation Law
• Evolution of the dusty infrared luminosity function from z=0 to z=2.3 using observations from Spitzer
• The metallicity dependence of the CO {\rightarrow} H_2 conversion factor in z>1 star forming galaxies
• CO excitation of normal star forming galaxies out to z=1.5 as regulated by the properties of their interstellar medium
• The Mass-Metallicity and Luminosity-Metallicity Relation from DEEP2 at z ~ 0.8
• Dust temperature and CO-to-H2 conversion factor variations in the SFR-M* plane
• The dominant epoch of star formation in the Milky Way formed the thick disc
• Galaxy Evolution and Star Formation Efficiency at 0.2 < z < 0.6
• A fundamental metallicity relation for galaxies at z = 0.84 - 1.47 from HiZELS
• CO(1-0) line imaging of massive star-forming disc galaxies at z=1.5-2.2
• The mid-life crisis of the Milky Way and M31
• On the effect of cosmological inflow on turbulence and instability in galactic discs
• The Spatially-Resolved Star Formation History of the M31 Outer Disc
• Delayed star formation in high-redshift stream-fed galaxies
• An improved probabilistic approach for linking progenitor and descendant galaxy populations using comoving number density