2012ApJ...758L...9M

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 G/yr, despite their large infrared luminosities (L_IR~ (2-3)x10¹² 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.