INTERFEROMETRIC CO OBSERVATIONS OF SUBMILLIMETER-FAINT, RADIO-SELECTED STARBURST GALAXIES AT z ∼ 2

High-redshift, dust-obscured galaxies, selected to be luminous in the radio but relatively faint at 850 mu m, appear to represent a different population from the ultraluminous submillimeter-bright population. They may be star-forming galaxies with hotter dust temperatures, or they may have lower far-infrared luminosities and larger contributions from obscured active galactic nuclei (AGNs). Here we present observations of three z similar to 2 examples of this population, which we term submillimeter-faint radio galaxies'' (SFRGs; RG J163655, RG J131236, and RG J123711) in CO(3-2) using the IRAM Plateau de Bure Interferometer to study their gas and dynamical properties. We estimate the molecular gas mass in each of the three SFRGs (8.3 x 10(9), < 5.6 x 10(9), and 15.4 x 10(9) M-circle dot, respectively) and, in the case of RG J163655, a dynamical mass by measurement of the width of the CO(3-2) line (8 x 10(10) csc(2)i M-circle dot). While these gas masses are substantial, on average they are 4 times lower than submillimeter-selected galaxies (SMGs). Radio-inferred star formation rates (SFRradio > = 970 M-circle dot yr(-1)) suggest much higher star formation efficiencies than are found for SMGs and shorter gas depletion timescales (similar to 11 Myr), much shorter than the time required to form their current stellar masses (similar to 160 Myr; similar to 10(11) M-circle dot). By contrast, star formation rates (SFRs) may be overestimated by factors of a few, bringing the efficiencies in line with those typically measured for other ultraluminous star-forming galaxies and suggesting that SFRGs are more like ultraviolet-selected (UV-selected) star-forming galaxies with enhanced radio emission. A tentative detection of RG J163655 at 350 mu m suggests hotter dust temperatures, and thus gas-to-dust mass fractions, similar to the SMGs.