Distant quasars are unique tracers to study the formation of the earliest
supermassive black holes (SMBHs) and the history of cosmic reionization.
Despite extensive efforts, only two quasars have been found at $z\ge7.5$, due
to a combination of their low spatial density and the high contamination rate
in quasar selection. We report the discovery of a luminous quasar at $z=7.642$,
J0313$-$1806, the most distant quasar yet known. This quasar has a bolometric
luminosity of $3.6\times10^{13} L_\odot$. Deep spectroscopic observations
reveal a SMBH with a mass of $(1.6\pm0.4) \times10^9M_\odot$ in this quasar.
The existence of such a massive SMBH just $\sim$670 million years after the Big
Bang challenges significantly theoretical models of SMBH growth. In addition,
the quasar spectrum exhibits strong broad absorption line (BAL) features in CIV
and SiIV, with a maximum velocity close to 20% of the speed of light. The
relativistic BAL features, combined with a strongly blueshifted CIV emission
line, indicate that there is a strong active galactic nucleus (AGN) driven
outflow in this system. ALMA observations detect the dust continuum and [CII]
emission from the quasar host galaxy, yielding an accurate redshift of $7.6423
\pm 0.0013$ and suggesting that the quasar is hosted by an intensely
star-forming galaxy, with a star formation rate of $\rm\sim 200 ~M_\odot
~yr^{-1}$ and a dust mass of $\sim7\times10^7~M_\odot$. Followup observations
of this reionization-era BAL quasar will provide a powerful probe of the
effects of AGN feedback on the growth of the earliest massive galaxies.

本研究报道了一个 $z=7.642$ 的亮度较高、质量较大的类星体，表明它被一个有强烈活动星系核驱动的气体外流所包围。研究还发现了该类星体的宿主星系本身正在积极形成恒星，并提供了第一个试图探究宇宙再电离时期黑洞生长影响的强大工具。

红移 7.642 的明亮类星体

A Luminous Quasar at Redshift 7.642

We present new measurements of the quasar luminosity function (LF) at $z \sim
6$, over an unprecedentedly wide range of the rest-frame ultraviolet luminosity
$M_{1450}$ from $-30$ to $-22$ mag. This is the fifth in a series of
publications from the Subaru High-$z$ Exploration of Low-Luminosity Quasars
(SHELLQs) project, which exploits the deep multi-band imaging data produced by
the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The LF was
calculated with a complete sample of 110 quasars at $5.7 \le z \le 6.5$, which
includes 48 SHELLQs quasars discovered over 650 deg$^2$, and 63 brighter
quasars discovered by the Sloan Digital Sky Survey and the Canada-France-Hawaii
Quasar Survey (including one overlapping object). This is the largest sample of
$z \sim 6$ quasars with a well-defined selection function constructed to date,
and has allowed us to detect significant flattening of the LF at its faint end.
A double power-law function fit to the sample yields a faint-end slope $\alpha
= -1.23^{+0.44}_{-0.34}$, a bright-end slope $\beta = -2.73^{+0.23}_{-0.31}$, a
break magnitude $M_{1450}^* = -24.90^{+0.75}_{-0.90}$, and a characteristic
space density $\Phi^* = 10.9^{+10.0}_{-6.8}$ Gpc$^{-3}$ mag$^{-1}$. Integrating
this best-fit model over the range $-18 < M_{1450} < -30$ mag, quasars emit
ionizing photons at the rate of $\dot{n}_{\rm ion} = 10^{48.8 \pm 0.1}$
s$^{-1}$ Mpc$^{-3}$ at $z = 6.0$. This is less than 10 % of the critical rate
necessary to keep the intergalactic medium ionized, which indicates that
quasars are not a major contributor to cosmic reionization.

本文基于 Hyper Suprime-Cam 超广角相机的深度多波段图像数据，利用 110 个射线星系和双指数幂律函数，重新测量了射线星系的浓度和射电百分之十的贡献率，揭示了射线星系在宇宙再电离中的主导地位。

Subaru High-z 低亮度类星体勘测（SHELLQs）。5. 类星体亮度函数和对宇宙再电离的贡献在 z = 6

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). V. Quasar  Luminosity Function and Contribution to Cosmic Reionization at z = 6

The reionization of cosmic hydrogen marks a critical juncture in the history
of structure formation in the universe. Here we present a new formulation of
the standard reionization equation for the evolution of the volume-averaged HII
fraction that is more consistent with the accepted conceptual model of
inhomogeneous intergalactic absorption. The revised equation retains the basic
terminology and simplicity of the classic calculation but explicitly accounts
for the presence of the optically thick "Lyman-limit systems" that are known to
determine the mean free path of ionizing radiation after overlap. Integration
of this equation provides a better characterization of the timing of
reionization by smoothly linking the pre-overlap with the post-overlap phases
of such process. We confirm the validity of the quasi-instantaneous
approximation as predictor of reionization completion/maintenance, and discuss
new insights on the sources of cosmic reionization using the improved
formalism. A constant emission rate into the intergalactic medium (IGM) of 3
Lyman continuum (LyC) photons per atom per Gyr leads to a reionization history
that is consistent with a number of observational constraints on the ionization
state of the z=5-9 universe and with the reduced Thomson scattering optical
depth recently reported by the Planck Collaboration. While star-forming
galaxies can dominate the reionization process if the luminosity-weighted
fraction of LyC photons that escape into the IGM, f_esc, exceeds 15% (for a
faint magnitude cut-off of the galaxy UV luminosity function of M_lim=-13 and a
LyC photon yield per unit 1500 AA luminosity of xi_ion=10^{25.3} Hz/erg, simple
models where the product of the two unknowns f_esc xi_ion is not evolving with
redshift fail to reproduce the changing neutrality of the IGM observed at these
epochs.

本文提出了一种新的宇宙氢再电离方程，以更好地描述宇宙海洋吸收的分布，该方程综合了氦的吸收以及光子在光度不透明吸收介质之间的漫游，解释了宇宙再电离的特定时间，讨论了宇宙再电离的来源并确认了几个天文量的研究