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 similar to 1.6 and similar to 2.2 Gyr(-1), despite their large infrared luminosities (L-IR approximate to (2-3) x 10(12) L-circle dot) 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 similar to 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.