High-Resolution ALMA Imaging for a Gravitationally-lensed Quasar at $z=6.5$: Constraining the AGN Contribution to Galactic-Scale Dust Heating

Abstract

We present high-resolution (beam size $0\farcs076\times0\farcs040$) Atacama Large Millimeter/submillimeter Array (ALMA) observations of the far-infrared $(λ_\text{rest}=162.7μ\rm{m})$ dust continuum of J0439+1634, a gravitationally lensed quasar at $z=6.52$. We perform pixelated lens modeling for the visibility data, finding that J0439+1634 is well-described by a singular isothermal ellipsoid plus an external shear lensing model. The best-fit lensing potential exhibits a naked-cusp configuration, confirming the finding in Fan et al. (2019). The reconstructed source plane continuum emission shows a compact bright core, with size $\lesssim200$ pc and peak brightness $\sim0.6 \text{ Jy arcsec}^{-2}$. The total continuum flux at 245 GHz is $3.36\pm0.02$ mJy. The flux magnification is {$4.63\pm0.03$}, indicating an average source-plane resolution of $0\farcs019$ (equivalent to 104 pc). The spatial resolution around the supermassive black hole reaches $\sim36$ pc. %Using the new lensing model, we re-fit the Hubble Space Telescope image for J0439+1634, and find that the position of the optical quasar is consistent with the brightest pixel in the dust continuum map. Leveraging the exceptional source-plane resolution, we build a radiative transfer model to describe the observed dust emission profile. The best-fit model indicates that heated dust from the active galactic nucleus (AGN) dominates the sub-millimeter emission at $r\lesssim100$ pc and that star-heated dust dominates the outer region of the host galaxy. AGN heating contributes {$\sim13\%$} to the observed sub-mm flux. Therefore, previous far-infrared-based star formation rate measurements for most high-redshift quasars are likely mildly overestimated.