A Luminous Quasar at Redshift 7.642

Abstract

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.

Figures (1)

• Figure 1: Upper panel: image cutouts ($20^{\prime\prime} \times 20^{\prime\prime}$, north is up and east is to the left) for J0313--1806 in PS1 $z$, PS1 $y$, DELS $z$, VISTA $J$, VISTA $K{\rm s}$, WISE W1 and WISE W2 bands. The photometry is given in Table \ref{['tbl']}. Lower panel: the final stacked spectrum of J0313--1806. In the figure, we re-binned the spectrum by two spectral pixels ($\sim 173~ {\rm km~ s^{-1}}$) for illustration purposes. The black and gray lines represent the Galactic extinction-corrected spectrum and the error vector, respectively. The blue line denotes the quasar composite spectrum constructed with SDSS $z\sim2$ quasars having similar C$\;$ blueshifts and line strengths. The purple dashed line denotes the power-law continuum. The orange points are flux densities determined from photometry in the $J$ and $K$s-bands. The inset panel shows the Mg$\;$ line fitting with the purple dot-dashed line denoting the power-law continuum, the green dashed line denoting the pseudo-continuum model (the sum of power law continuum, Fe$\;$ emission, and Balmer continuum), the orange line representing the Gaussian fitting of the Mg$\;$ line and the red line representing the total fit of pseudo-continuum and Mg$\;$ line. The thin grey lines in the insert panel represent the spectral fitting of 100 mock spectra as described in § \ref{['sec_bh']}.