We present observations of the CO [ J=3 \rightarrow 2 ] emission towards two massive and infrared luminous Lyman Break Galaxies 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 \sim 1.6 and \sim 2.2 Gyr ^ { -1 } , despite their large infrared luminosities ( L _ { IR } \approx 2–3 \times 10 ^ { 12 } L _ { \odot } ) 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 _ { \ast } ) with redshift .
Our data suggest that the steep evolution of M _ { gas } / M _ { \ast } of normal galaxies up to z \sim 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 .