[CII] line emission in massive star-forming galaxies at z=4.7
  We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
of the [CII] 157.7micron 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 submm 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 submm continuum,
indicating FIR luminosities in excess of 10^13 Lsun for the two most luminous
objects. The SMG and the quasar host galaxy are both detected in [CII] line
emission with luminosities, L([CII]) = (10.0 +/- 1.5)x10^9 Lsun and L([CII]) =
(6.5+/-1.0)x10^9 Lsun, respectively. We estimate a luminosity ratio,
L([CII])/L(FIR) = (8.3+/-1.2)x10^-4 for the starburst SMG to the North, and
L([CII])/L(FIR) = (2.5+/-0.4)x10^-4 for the quasar host galaxy, in agreement
with previous high-redshift studies that suggest lower [CII]-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 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 similar redshift
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.
