The Molecular Gas Content of z = 3 Lyman Break Galaxies: Evidence of a Non-evolving Gas Fraction in Main-sequence Galaxies at z > 2
We present observations of the CO[J = 3 → 2] emission toward two massive and infrared luminous Lyman break galaxies (LBGs) at z = 3.21 and z = 2.92, using the IRAM Plateau de Bure Interferometer, placing first constraints on the molecular gas masses (M gas) of non-lensed LBGs. Their overall properties are consistent with those of typical (main-sequence) galaxies at their redshifts, with specific star formation rates ~1.6 and ~2.2 Gyr-1, despite their large infrared luminosities (L IR ≈ (2-3) × 1012 L ☉) derived from Herschel. With one plausible CO detection (spurious detection probability of 10-3) and one upper limit, we investigate the evolution of the molecular gas-to-stellar mass ratio (M gas/M *) with redshift. Our data suggest that the steep evolution of M gas/M * of normal galaxies up to z ~ 2 is followed by a flattening at higher redshifts, providing supporting evidence for the existence of a plateau in the evolution of the specific star formation rate at z > 2.5.Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). Based also on observations carried out by the Herschel space observatory. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.