[C II] LINE EMISSION IN MASSIVE STAR-FORMING GALAXIES AT <i>z</i> = 4.7

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 10^(13) 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_([CII]) = (10.0 ± 1.5) × 10^9 L_☉ and L_([CII]) = (6.5 ± 1.0) × 10^9 L_☉, respectively. We estimate a luminosity ratio L_([CII])/L_(FIR) = (8.3 ± 1.2) × 10^(–4) for the starburst SMG to the north and L [C II]/L FIR = (2.5 ± 0.4) × 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_(340GHz) = 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.