2012ApJ...752L..30W

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the [C II] 157.7 µm fine structure line and thermal dust continuum emission from a pair of gas-rich galaxies at z = 4.7, BR1202-0725. This system consists of a luminous quasar host galaxy and a bright submillimeter galaxy (SMG), while a fainter star-forming galaxy is also spatially coincident within a 4'' (25 kpc) region. All three galaxies are detected in the submillimeter continuum, indicating FIR luminosities in excess of 10¹³ L_☉for the two most luminous objects. The SMG and the quasar host galaxy are both detected in [C II] line emission with luminosities L_[C II]_= (10.0±1.5) x10⁹ L_☉ and L_[C II]_= (6.5±1.0)x10⁹ L_☉, respectively. We estimate a luminosity ratio L_[C II]_/LFIR_= (8.3±1.2)x10^–4 for the starburst SMG to the north and L_[C II]_/LFIR_= (2.5±0.4) x 10^–4 for the quasar host galaxy, in agreement with previous high-redshift studies that suggest lower [C II]-to-FIR luminosity ratios in quasars than in starburst galaxies. The third fainter object with a flux density S_340 GHz_= 1.9±0.3 mJy is coincident with a Lyα emitter and is detected in HST ACS F775W and F814W images but has no clear counterpart in the H band. Even if this third companion does not lie at a redshift similar to BR1202-0725, the quasar and the SMG represent an overdensity of massive, infrared luminous star-forming galaxies within 1.3 Gyr of the big bang.