Interferometric CO Observations of Submillimeter-faint, Radio-selected Starburst Galaxies at z ~ 2

International audienceHigh-redshift, dust-obscured galaxies, selected to be luminous in the radio but relatively faint at 850 μm, 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 ~ 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 × 109, 9, and 15.4 × 109 M⊙, respectively) and, in the case of RG J163655, a dynamical mass by measurement of the width of the CO(3-2) line (8 × 1010csc2i M⊙). While these gas masses are substantial, on average they are 4 times lower than submillimeter-selected galaxies (SMGs). Radio-inferred star formation rates (radio > = 970 M⊙ yr-1) suggest much higher star formation efficiencies than are found for SMGs and shorter gas depletion timescales (~11 Myr), much shorter than the time required to form their current stellar masses (~160 Myr; ~1011 M⊙). 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 μm suggests hotter dust temperatures, and thus gas-to-dust mass fractions, similar to the SMGs