Vigorous star formation with low efficiency in massive disk galaxies at z=1.5

We present the first detection of molecular gas cooling CO emission lines from ordinary massive galaxies at z = 1.5. Two sources were observed with the IRAM Plateau de Bure Interferometer, selected to lie in the mass-p star formation rate correlation at their redshift, thus being representative of massive high-z galaxies. Both sources were detected with high confidence, yielding L-CO' similar to 2 x 10(10) K km s(-1) pc(2). For one of the sources we find evidence of velocity shear, implying CO sizes of similar to 10 kpc. With an infrared luminosity of L-FIR similar to 10(12) L-circle dot, these disklike galaxies are borderline ULIRGs but with star formation efficiency similar to that of local spirals, and an order of magnitude lower than that in submillimeter galaxies. This suggests a CO to total gas conversion factor similar to local spirals, gas consumption timescales approaching 1 Gyr or longer, and molecular gas masses reaching similar to 10(11) M-circle dot, comparable to or larger than the estimated stellar masses. These results support a major role of in situ gas consumption over cosmological timescales and with relatively low star formation efficiency, analogous to that of local spiral disks, for the formation of today's most massive galaxies and their central black holes. Given the high space density of similar galaxies, similar to 10(-4) Mpc(-3), this implies a widespread presence of gas-rich galaxies in the early universe, many of which might be within reach of detailed investigations of current and planned facilities.