ALMA [NII] 205 micron Imaging Spectroscopy of the Interacting Galaxy System BRI 1202-0725 at Redshift 4.7

Abstract

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-$\hat{I}\pm$ emitters (LAEs), all within $\sim$25 kpc of the QSO. We detect the QSO and SMG in both [NII] and continuum. At the $\sim$$1"$ (or 6.6 kpc) resolution, both QSO and SMG are resolved in [NII], with the de-convolved major axes of $\sim$9 and $\sim$14 kpc, respectively. In contrast, their continuum emissions are much more compact and unresolved even at an enhanced resolution of $\sim$$0.7"$. The ratio of the [NII] flux to the existing CO (7$-$6) flux is used to constrain the dust temperature ($T_{\mathrm{dust}}$) for a more accurate determination of the FIR luminosity $L_{\mathrm{FIR}}$. Our best estimated $T_{\mathrm{dust}}$ equals $43(\pm 2)$ K for both galaxies (assuming an emissivity index ${\hat{I}}^2=1.8$). The resulting $L_{\mathrm{CO}(7-6)}/L_{\mathrm{FIR}}$ ratios are statistically consistent with that of local luminous infrared galaxies, confirming that $L_{\mathrm{CO}(7-6)}$ 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 $(6.9)\times 10^3{M}_{\odot }$ yr${}^{-1}$ ($\pm$ 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 $(5)\times 10^{11}{M}_{\odot }$ of molecular gas in 10 $(7)\times 10^7$ years.

4 pages, 1 table, 4 figures; accepted for publication in the ApJ Letters