We present the results from Atacama Large Millimeter/submillimeter Array
(ALMA) imaging in the [NII] 205 micron fine-structure line (hereafter [NII])
and the underlying continuum of BRI 1202-0725, an interacting galaxy system at
z= 4.7, consisting of an optical QSO, a sub-millimeter galaxy (SMG) and two
Lyman-α emitters (LAEs), all within ∼25 kpc of the QSO. We detect
the QSO and SMG in both [NII] and continuum. At the ∼1" (or 6.6 kpc)
resolution, both QSO and SMG are resolved in [NII], with the de-convolved major
axes of ∼9 and ∼14 kpc, respectively. In contrast, their continuum
emissions are much more compact and unresolved even at an enhanced resolution
of ∼0.7". The ratio of the [NII] flux to the existing CO (7−6) flux is
used to constrain the dust temperature (Tdust) for a more accurate
determination of the FIR luminosity LFIR. Our best estimated Tdust equals 43(±2) K for both galaxies (assuming an emissivity index
β=1.8). The resulting LCO(7−6)/LFIR ratios are
statistically consistent with that of local luminous infrared galaxies,
confirming that LCO(7−6) traces the star formation (SF) rate (SFR) in
these galaxies. We estimate that the on-going SF of the QSO (SMG) has a SFR of
5.1 (6.9)×103M⊙ yr−1 (± 30%) assuming Chabrier
initial mass function, takes place within a diameter (at half maximum) of 1.3
(1.5) kpc, and shall consume the existing 5 (5)×1011M⊙ of
molecular gas in 10 (7)×107 years.Comment: 4 pages, 1 table, 4 figures; accepted for publication in the ApJ
Letter