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

Abstract: We report the discovery of a luminous quasar, J1007+2115 at $z=7.515$ ("P={o}niu={a}'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 $(1.5\pm0.2)\times10^9$ $M_{\odot}$ supermassive black hole (SMBH), based on its broad MgII 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 $z\gtrsim7$ quasars. We estimate an IGM volume-averaged neutral fraction $\langle x\rm_{HI}\rangle=0.39^{+0.22}_{-0.13}$. This range of values suggests a patchy reionization history toward different IGM sightlines. We detect the 158 $\mu$m [C II] emission line in J1007+2115 with ALMA; this line centroid yields a systemic redshift of $z=7.5149\pm0.0004$ and indicates a star formation rate of $\sim210$ $M_{\odot}$ yr$^{-1}$ in its host galaxy.

P={o}niu={a}`ena: A Luminous Quasar at High Redshift

The study by Yang et al. presents the discovery of a remarkably luminous quasar, J1007+2115, designated in the Hawaiian language as "P={o}niu={a}`ena," which is observed at a redshift of $z=7.5149$. This quasar is characterized by a prominent feature—a supermassive black hole (SMBH) with a mass of approximately $(1.5 \pm 0.2) \times 10^9 M_{\odot}$, making it the second quasar known at $z > 7.5$ and significantly deep into the epoch of reionization.

Observational Findings and Methodology

The quasar was discovered as part of a wide-field reionization-era quasar survey leveraging data from a combination of optical and infrared photometric surveys including DECaLS, PS1, UHS, and WISE. Spectroscopic confirmation and subsequent detailed study were conducted using instruments such as Gemini/GNIRS, Magellan/FIRE, Keck/NIRES, and ALMA.

The intrinsic properties of the quasar's spectrum were derived using principal component analysis (PCA), which facilitated the modeling of the quasar's Ly$\alpha$ region and quantification of IGM absorption features. Detection of the 158 $\mu$m C \textsc{ii} emission line with ALMA established a systemic redshift of $z=7.5149 \pm 0.0004$, and from this line, the authors deduced a star formation rate of $\sim210$ $M_{\odot}$\,yr${}^{-1}$ in the quasar's host galaxy.

Implications for Early SMBH Growth

The sheer mass of the black hole, exceeding contemporaneous records such as that of the quasar J1342+0928, imposes stringent constraints on theoretical models of early black hole growth. Accounting for such a massive black hole at an epoch only 700 million years post-Big Bang challenges the assumptions of seed mass and accretion efficiency. Under models assuming Eddington-limited accretion and radiative efficiency of 0.1, the existence of J1007+2115 would imply black hole seeds with masses $\gtrsim 10^{4} M_{\odot}$ at $z=30$, suggesting scenarios of direct collapse as viable initial conditions or necessitating alternate paths such as early periods of hyper-Eddington accretion or reduced radiative efficiencies.

Insights into Cosmic Reionization

In the context of cosmic evolution, the quasar notably exhibits a weaker damping wing signature as measured by neutral hydrogen absorption in its Ly$\alpha$ line profile compared to similar high-redshift quasars. This observation enabled Yang et al. to infer an intergalactic medium (IGM) volume-averaged neutral fraction $\langle x\rm_{HI}\rangle=0.39^{+0.22}_{-0.13}$, supporting the notion of a heterogeneous or patchy reionization driven by the interactions of early massive objects like J1007+2115.

Conclusions and Future Directions

The detection of J1007+2115 contributes essential data to comprehend the evolution of SMBHs and the reionization epoch. The findings underscore the complexities involved with black hole growth models at high redshifts, exhibiting stark discrepancies between observational data and theoretical predictions. Future studies could benefit from expanding the sample size of $z > 7$ quasars, providing comprehensive coverage of different sightlines to further constrain models of early universe conditions. Enhanced observations with upcoming facilities, such as the James Webb Space Telescope, dark matter simulations, and advanced modeling techniques could potentially elucidate the mechanisms governing rapid black hole growth and IGM reionization, enriching our understanding of the early universe.