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 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 2 examples of this population, which we term ‘‘submillimeter-faint radio galaxies’’ (SFRGs; RG J163655, RG J131236, and RG J123711) in CO(3Y2) using the IRAMPlateau de Bure Interferometer to study their gas and dynamical properties.We estimate the molecular gas mass in each of the three SFRGs (8:3 ; 10, <5:6 ; 10, and 15:4 ; 10 M , respectively) and, in the case of RG J163655, a dynamical mass by measurement of the width of the CO(3Y2) line (8 ; 10 csci M ). While these gas masses are substantial, on average they are 4 times lower than submillimeter-selected galaxies (SMGs). Radio-inferred star formation rates ( SFRradio h i 1⁄4 970M yr ) suggest much higher star formation efficiencies than are found for SMGs and shorter gas depletion timescales ( 11Myr), much shorter than the time required to form their current stellar masses ( 160 Myr; 10 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 m suggests hotter dust temperatures, and thus gas-to-dust mass fractions, similar to the SMGs. Subject headingg s: cosmology: observations — galaxies: evolution — galaxies: formation — galaxies: high-redshift — galaxies: starburst Online material: color figures