First [NII]122$\mu$m line detection in a QSO-SMG pair BRI 1202-0725 at
  $z=4.69$
  We report the first detection obtained with ALMA of the [N II] 122$\mu$m line
emission from a galaxy group BRI 1202-0725 at $z=4.69$ consisting of a QSO and
a submilimeter-bright galaxy (SMG). Combining with a detection of [N II]
205$\mu$m line in both galaxies, we constrain the electron densities of the
ionized gas based on the line ratio of [NII]122/205. The derived electron
densities are $26^{+12}_{-11}$ and $134^{+50}_{-39}$ cm$^{-3}$ for the SMG and
the QSO, respectively. The electron density of the SMG is similar to that of
the Galactic Plane and to the average of the local spirals. Higher electron
densities by up to a factor of three could, however, be possible for systematic
uncertainties of the line flux estimates. The electron density of the QSO is
comparable to high-$z$ star-forming galaxies at $z=1.5-2.3$, obtained using
rest-frame optical lines and with the lower limits suggested from stacking
analysis on lensed starbursts at $z=1-3.6$ using the same tracer of [NII]. Our
results suggest a large scatter of electron densities in global scale at fixed
star formation rates for extreme starbursts. The success of the [N II]
122$\mu$m and 205$\mu$m detections at $z=4.69$ demonstrates the power of future
systematic surveys of extreme starbursts at $z>4$ for probing the ISM
conditions and the effects on surrounding environments.
