Discovery of the First Low-Luminosity Quasar at z > 7

TL;DR

This paper reports the discovery of the first low-luminosity quasar at z > 7, revealing a less luminous and moderately accreting supermassive black hole, providing insights into early universe quasar populations.

Contribution

It presents the first low-luminosity, sub-Eddington accreting quasar at z > 7, expanding understanding of early black hole growth and quasar diversity.

Findings

First low-luminosity z > 7 quasar discovered

Black hole mass is moderate, with sub-Eddington accretion

Luminosity comparable to low-redshift quasars

Abstract

We report the discovery of a quasar at z = 7.07, which was selected from the deep multi-band imaging data collected by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. This quasar, HSC J124353.93+010038.5, has an order of magnitude lower luminosity than do the other known quasars at z > 7. The rest-frame ultraviolet absolute magnitude is M1450 = -24.13 +/- 0.08 mag and the bolometric luminosity is Lbol = (1.4 +/- 0.1) x 10^{46} erg/s. Its spectrum in the optical to near-infrared shows strong emission lines, and shows evidence for a fast gas outflow, as the C IV line is blueshifted and there is indication of broad absorption lines. The Mg II-based black hole mass is Mbh = (3.3 +/- 2.0) x 10^8 Msun, thus indicating a moderate mass accretion rate with an Eddington ratio 0.34 +/- 0.20. It is the first z > 7 quasar with sub-Eddington accretion, besides being the third most distant quasar, known to date. The luminosity and black hole mass are comparable to, or even lower than, those measured for the majority of low-z quasars discovered by the Sloan Digital Sky Survey, and thus this quasar likely represents a z > 7 counterpart to quasars commonly observed in the low-z universe.

Tables

Table 1. Table 1: Coordinates 3 3 3 Coordinates are at J 𝐽 J 2000.0. The astrometric accuracy of the HSC-SSP data is estimated to be 0″.1 (Aihara et al., 2018b ) . and Brightness

R.A.	12h43m53s\@alignment@align.93
Decl.	+01∘00′38″\@alignment@align.5
$g_{\mathrm{AB}}$ (mag)	26\@alignment@align.7 (2$\sigma$)
$r_{\mathrm{AB}}$ (mag)	26\@alignment@align.5 (2$\sigma$)
$i_{\mathrm{AB}}$ (mag)	26\@alignment@align.7 (2$\sigma$)
$z_{\mathrm{AB}}$ (mag)	25\@alignment@align.8 (2$\sigma$)
$y_{\mathrm{AB}}$ (mag)	23\@alignment@align.57 $\pm$ 0
$m_{1450}$ (mag)	22\@alignment@align.82 $\pm$ 0
$M_{1450}$ (mag)	$-24.13\pm 0.08$\@alignment@align
$L_{\mathrm{bol}}$ (erg s-1)	(1\@alignment@align.4 $\pm$ 0

Table 2. Table 2: Emission line measurements

	Ly $\alpha$ + N V $\lambda$1240	C IV $\lambda$1549	C III] $\lambda$1909	Mg II $\lambda$2800
Redshift	$\cdots$	$\cdots$	$\cdots$	7.07 $\pm$ 0.01
Velocity offset (km s-1)	$\cdots$	$-$2400 $\pm$ 500	$-$800 $\pm$ 400	$\cdots$
Flux (erg s-1 cm-2)	(9.6 $\pm$ 0.4) $\times$ 10-17	(2.1 $\pm$ 0.4) $\times$ 10-16	(1.6 $\pm$ 0.5) $\times$ 10-16	(6.2 $\pm$ 1.9) $\times$ 10-17
REW (Å)	16 $\pm$ 1	48 $\pm$ 10	51 $\pm$ 15	35 $\pm$ 11
FWHM (km s-1)	$\cdots$	5500 $\pm$ 1300	4600 $\pm$ 1500	3100 $\pm$ 900
$M_{\mathrm{BH}}$ ($M_{\odot }$)	$\cdots$	$\cdots$	$\cdots$	(3.3 $\pm$ 2.0) $\times$ 108
${\lambda }_{\mathrm{Edd}}$	$\cdots$	$\cdots$	$\cdots$	0.34 $\pm$ 0.20

Full text

dark ages, reionization, first stars — galaxies: active — galaxies: high-redshift — intergalactic medium — quasars: general — quasars: supermassive black holes

††journal: ApJ

1 Introduction

Quasars residing in the first billion years of the Universe ($z>5.7$) have been used as various types of probes into early cosmic history. The progress of cosmic reionization can be estimated from H I absorption imprinted on the rest-frame ultraviolet spectrum of a high-$z$ quasar; this absorption is very sensitive to the neutral fraction of the foreground intergalactic medium (IGM; Gunn & Peterson, 1965; Fan et al., 2006a). The luminosity and mass functions of quasars reflect the seeding and growth mechanisms of supermassive black holes (SMBHs), which can be studied through comparison with theoretical models (e.g., Volonteri, 2012; Ferrara et al., 2014; Madau et al., 2014). Measurements of quasar host galaxies and surrounding environments tell us about the earliest mass assembly, possibly happening in the highest density peaks of the underlying dark matter distribution (e.g., Goto et al., 2009; Decarli et al., 2017; Izumi et al., 2018).

Quasars at the highest redshifts are of particular interest, as they have spent only a short time since their formation. The current frontier for high-$z$ quasar searches is $z>7$, where only a few quasars have been found to date. Since radiation from $z>7$ quasars is almost completely absorbed by the IGM at observed wavelengths $\lambda<$ 9700 Å, and such objects are very rare and faint, one needs wide-field deep imaging in near-infrared (IR) bands or in the $y$-band with red-sensitive CCDs to discover those quasars. The first $z>7$ quasar was discovered by Mortlock et al. (2011) at $z=7.09$, from the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS; Lawrence et al., 2007) data. The second one was discovered by Bañados et al. (2018) at $z=7.54$, by combining data from the Wide-field Infrared Survey Explorer (Wright et al., 2010), UKIDSS, and the Dark Energy Camera Legacy Survey111 http://legacysurvey.org/decamls. In addition, two quasars, both at $z=7.02$, were recently discovered (Wang et al., 2018; Yang et al., 2018) by combining datasets from several wide-field surveys, including the Dark Energy Survey (DES Collaboration, 2005), the Dark Energy Spectroscopic Instrument legacy imaging surveys (Dey et al., 2018), and the Panoramic Survey Telescope & Rapid Response System 1 (Pan-STARRS1; Chambers et al., 2016).

However, the above $z>7$ quasars are all very luminous (if they are not strongly lensed; e.g., Fan et al., 2018; Pacucci & Loeb, 2018), due to the detection limits of the imaging survey observations. These quasars harbor SMBHs with masses of roughly a billion solar masses, shining at close to the Eddington luminosity (however the black hole mass of one of the quasars at $z=7.02$ has not been measured; Yang et al., 2018). They likely represent the most extreme monsters, which are very rare at all redshifts, especially at $z>7$. To understand a wider picture of the formation and early evolution of SMBHs, it is crucial to find $z>7$ quasars of more typical luminosity, which would be direct counterparts to low-$z$ ordinary quasars.

This letter presents the discovery of a quasar at $z=7.07$, HSC $J124353.93+010038.5$ (hereafter “$J1243+0100$”), which has an order of magnitude lower luminosity than do the other known $z>7$ quasars. It harbors a SMBH with a mass of $M_{\rm BH}=(3.3\pm 2.0)\times 10^{8}M_{\odot}$ and shining at an Eddington ratio $\lambda_{\rm Edd}=0.34\pm 0.20$. We describe the target selection and spectroscopic observations in §2. The spectral properties of the quasar are measured and discussed in §3. A summary appears in §4. We adopt the cosmological parameters $H_{0}$ = 70 km s*-1* Mpc*-1*, $\Omega_{\rm M}$ = 0.3, and $\Omega_{\rm\Lambda}$ = 0.7. All magnitudes refer to point spread function (PSF) magnitudes in the AB system (Oke & Gunn, 1983), and are corrected for Galactic extinction (Schlegel et al., 1998).

2 Observations

$J1243+0100$ was selected from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) survey (Aihara et al., 2018a) data, as a part of the “Subaru High-$z$ Exploration of Low-Luminosity Quasars (SHELLQs)” project (Matsuoka et al., 2016, 2018a, 2018b, 2018c). The coordinates and brightness are summarized in Table 3. A three-color composite image around the quasar is presented in Figure 1. This source has a full-width-at-half-maximum (FWHM) size of 0″.7 on the $y$-band image, which is consistent with the PSF size estimated at the corresponding image position. We used the methods detailed in Matsuoka et al. (2018b) to select this source as a high-$z$ quasar candidate. The probability that this source was a quasar, not a Galactic brown dwarf, was $P_{\rm Q}=0.4$, based on our Bayesian probabilistic algorithm (Matsuoka et al., 2016) and the HSC $i$, $z$, and $y$-band photometry. It is among $\sim$30 $z$-band dropout objects in our quasar candidate list; we have so far conducted follow-up observations of roughly half of these candidates, and partly reported the results in the SHELLQs papers mentioned above. The highest-$z$ quasar we found and published previously is at $z\sim 6.9$ (Matsuoka et al., 2018a).