Varying [CII]/[NII] line ratios in the interacting system BR1202-0725 at
  z=4.7
  We study the properties of the interstellar medium in the interacting system
BR1202-0725 at z=4.7 via its [NII] and [CII] fine-structure line emission. This
system consists of a QSO, a sub-mm galaxy (SMG), and two Ly-alpha emitters
(LAEs). Such a diversity in galaxy properties makes BR1202-0725 a unique
laboratory of star formation and galaxy evolution at high redshift. We present
ionized nitrogen ([NII] 205 micron) observations of this system, obtained with
the IRAM Plateau de Bure Interferometer. We find no [NII] emission at the
quasar location, but tentative [NII] line detections associated with the SMG
and one of the LAEs. Together with available ionized carbon ([CII] 158 micron)
ALMA observations of this system, we find the following: The [CII]/[NII]
luminosity ratio is >5.5 for the QSO and the SMG, but it is as low as ~2 in the
LAE, suggesting that, in this source, most of the [CII] emission is associated
with the ionized medium (HII regions) rather than the neutral one (PDRs). This
study demonstrates the importance of combined studies of multiple
fine--structure lines in order to pin down the physical properties of the
interstellar medium in distant galaxies.
