The Molecular Gas Content of Z = 3 Lyman Break Galaxies: Evidence of a Non-Evolving Gas Fraction in Main-Sequence Galaxies At Z > 2

Abstract : 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}$$\approx$2-3 × 10$^{12}$ 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.