Pōniuā'ena: A Luminous z = 7.5 Quasar Hosting a 1.5 Billion Solar Mass Black Hole

We report the discovery of a luminous quasar, J1007+2115 at z = 7.515 ("Pōniuā'ena"), from our wide-field reionization-era quasar survey. J1007+2115 is the second quasar now known at z > 7.5, deep into the reionization epoch. The quasar is powered by a (1.5 ± 0.2) × 10⁹ M_⊙ supermassive black hole (SMBH), based on its broad Mg II emission-line profile from Gemini and Keck near-IR spectroscopy. The SMBH in J1007+2115 is twice as massive as that in quasar J1342+0928 at z = 7.54, the current quasar redshift record holder. The existence of such a massive SMBH just 700 million years after the Big Bang significantly challenges models of the earliest SMBH growth. Model assumptions of Eddington-limited accretion and a radiative efficiency of 0.1 require a seed black hole of ≳10⁴ M_⊙ at z = 30. This requirement suggests either a massive black hole seed as a result of direct collapse or earlier periods of rapid black hole growth with hyper-Eddington accretion and/or a low radiative efficiency. We measure the damping wing signature imprinted by neutral hydrogen absorption in the intergalactic medium (IGM) on J1007+2115's Lyα line profile, and find that it is weaker than that of J1342+0928 and two other z ≳ 7 quasars. We estimate an IGM volume-averaged neutral fraction $\langle {X}_{{\rm{H}}{\rm{I}}}\rangle ={0.39}_{-0.13}^{+0.22}$ . This range of values suggests a patchy reionization history toward different IGM sightlines. We detect the 158 μm [C II] emission line in J1007+2115 with the Atacama Large Millimeter/submillimeter Array; this line centroid yields a systemic redshift of z = 7.5149 ± 0.0004 and indicates a star formation rate of ∼210 M_⊙ yr^-1 in its host galaxy.