ALMA Observations of the Sub-kpc Structure of the Host Galaxy of a z = 6.5 Lensed Quasar: A Rotationally Supported Hyper-Starburst System at the Epoch of Reionization

Abstract

We report Atacama Large Millimeter/Submillimeter Array (ALMA) observations of the dust continuum and [C ii ] emission of the host galaxy of J0439+1634, a gravitationally lensed quasar at z = 6.5. Gravitational lensing boosts the source-plane resolution to ∼0.″15 (∼0.8 kpc). The lensing model derived from the ALMA data is consistent with the fiducial model in Fan et al. based on HST imaging. The host galaxy of J0439+1634 can be well-fitted by a Sérsic profile consistent with an exponential disk, both in the far-infrared (FIR) continuum and the [C ii ] emission. The overall magnification is 4.53 ± 0.05 for the continuum and 3.44 ± 0.05 for the [C ii ] line. The host galaxy of J0439+1634 is a compact ultraluminous infrared galaxy, with a total star formation rate (SFR) of 1.56 × 10 3 M ⊙ yr −1 after correcting for lensing and an effective radius of 0.74 kpc. The resolved regions in J0439+1634 follow the “[C ii ] deficit,” where the [C ii ]-to-FIR ratio decreases with FIR surface brightness. The reconstructed velocity field of J0439+1634 appears to be rotation-like. The maximum line-of-sight rotation velocity is 130 km s −1 at a radius of 2 kpc. However, our data cannot be fit by an axisymmetric thin rotating disk, and the inclination of the rotation axis, i , remains unconstrained. We estimate the dynamical mass of the host galaxy to be 7.9 sin − 2 ( i ) × 10 9 M ⊙ . J0439+1634 is likely to have a high gas-mass fraction and an oversized SMBH compared to local relations. The SFR of J0439+1634 reaches the maximum possible values, and the SFR surface density is close to the highest value seen in any star-forming galaxy currently known in the universe.