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

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the [C II] 157.7 mu m 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-circle dot 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) x 10(9) L-circle dot and L-[C II] = (6.5 +/- 1.0) x 10(9) L-circle dot, respectively. We estimate a luminosity ratio L-[C II]/L-FIR = (8.3 +/- 1.2) x 10(-4) for the starburst SMG to the north and L-[C II]/L-FIR = (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 Ly alpha 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.