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 \pm 0.2 ) \times 10 ^ { 9 } M _ { \odot } supermassive black hole ( SMBH ) , based on its broad \ion Mg2 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 \gtrsim 10 ^ { 4 } M _ { \odot } 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 Ly \alpha line profile , and find that it is weaker than that of J1342+0928 and two other z \gtrsim 7 quasars .
We estimate an IGM volume-averaged neutral fraction \langle x { } _ { HI } \rangle = 0.39 ^ { +0.22 } _ { -0.13 } .
This range of values suggests a patchy reionization history toward different IGM sightlines .
We detect the 158 \mu m [ \ion C2 ] emission line in J1007+2115 with ALMA ; this line centroid yields a systemic redshift of z = 7.5149 \pm 0.0004 and indicates a star formation rate of \sim 210 M _ { \sun } yr ^ { -1 } in its host galaxy .