2020ApJ...891...64F

Gravitational lensing sometimes dominates the observed properties of apparently very bright objects. We present morphological properties in the high-resolution (FWHM ∼ 0.''15) Atacama Large Millimeter/submillimeter Array (ALMA) 1 mm map for an ultraluminous quasar at z = 6.30, SDSS J010013.02+280225.8 (hereafter J0100+2802), whose black hole (BH) mass M_BH is the most massive (∼1.2 x 10¹⁰M_☉) at z > 6 ever known. We find that the continuum emission of J0100+2802 is resolved into a quadruple system within a radius of 0.''2, which can be interpreted as either multiple dusty star-forming regions in the host galaxy or multiple images due to strong gravitational lensing. The Mg II absorption and the potential Lyα line features have been identified at z = 2.33 in the near-infrared spectroscopy toward J0100+2802, and a simple mass model fitting well reproduces the positions and flux densities of the quadruple system, both of which are consistent with the latter interpretation. Although a high-resolution map taken in the Advanced Camera for Survey on board Hubble Space Telescope (HST) shows a morphology with an apparently single component, in our fiducial lens mass model it can simply be explained by a ∼50 pc scale offset between the ALMA and HST emission regions. In this case, the magnification factor for the observed HST emission is obtained to ∼450, reducing the intrinsic M_BH estimate to below 10⁹ M_☉. The confirmation or the rejection of the gravitational lensing scenario is important for our understanding of the supermassive BHs in the early universe.