We report the identification of radio (1.4 and 3 GHz) and mid-infrared, far-infrared, and sub-mm (24-850$mu$m) emission at the position of one of 41 UV-bright ($mathrm{M_{UV}^{}}lesssim-21.25$) $zsimeq6.6-6.9$ Lyman-break galaxy candidates in the 1.5
deg$^2$ COSMOS field. This source, COS-87259, exhibits a sharp flux discontinuity (factor $>$3) between two narrow/intermediate bands at 9450 $mathring{A}$ and 9700 $mathring{A}$ and is undetected in all nine bands blueward of 9600 $mathring{A}$, as expected from a Lyman-alpha break at $zsimeq6.8$. The full multi-wavelength (X-ray through radio) data of COS-87529 can be self-consistently explained by a very massive (M$_{ast}=10^{10.8}$ M$_{odot}$) and extremely red (rest-UV slope $beta=-0.59$) $zsimeq6.8$ galaxy with hyperluminous infrared emission (L$_{mathrm{IR}}=10^{13.6}$ L$_{odot}$) powered by both an intense burst of highly-obscured star formation (SFR$approx$1800 M$_{odot}$ yr$^{-1}$) and an obscured ($tau_{mathrm{9.7mu m}}=7.7pm2.5$) radio-loud (L$_{mathrm{1.4 GHz}}sim10^{25.5}$ W Hz$^{-1}$) AGN. The radio emission is compact (1.04$pm$0.12 arcsec) and exhibits an ultra-steep spectrum between 1.4-3 GHz ($alpha=-2.06^{+0.27}_{-0.25}$) with evidence of spectral flattening at lower frequencies, consistent with known $z>4$ radio galaxies. We also demonstrate that COS-87259 may reside in a significant (11$times$) galaxy overdensity at $zsimeq6.6-6.9$, as common for systems hosting radio-loud AGN. Nonetheless, a spectroscopic redshift will ultimately be required to establish the true nature of COS-87259 as we cannot yet completely rule out low-redshift solutions. If confirmed to lie at $zsimeq6.8$, the properties of COS-87259 would be consistent with a picture wherein AGN and highly-obscured star formation activity are fairly common among very massive (M$_{ast}>10^{10}$ M$_{odot}$) reionization-era galaxies.
We report ALMA observations of the dust continuum and {cii} emission of the host galaxy of J0439+1634, a gravitationally lensed quasar at $z=6.5$. Gravitational lensing boosts the source-plane resolution to $sim0farcs15$ $(sim0.8text{ kpc})$. The len
sing model derived from the ALMA data is consistent with the fiducial model in citet{fan19} based on {it HST} imaging. The host galaxy of J0439+1634 can be well-fitted by a Sersic profile consistent with an exponential disk, both in the far-infrared (FIR) continuum and the {cii} emission. The overall magnification is $4.53pm0.05$ for the continuum and $3.44pm0.05$ for the {cii} line. The host galaxy of J0439+1634 is a compact ultra-luminous infrared galaxy, with a total star formation rate (SFR) of $1.56times10^{3}M_odot/text{year}$ after correcting for lensing and an effective radius of $0.74$ kpc. The resolved regions in J0439+1634 follow the ``{cii} deficit, where the {cii}-to-FIR ratio decreases with FIR surface brightness. The reconstructed velocity field of J0439+1634 appears to be rotation-like. The maximum line-of-sight rotation velocity of 130 km/s at a radius of 2 kpc. However, our data cannot be fit by an axisymmetric thin rotating disk, and the inclination of the rotation axis, $i$, remains unconstrained. We estimate the dynamical mass of the host galaxy to be $7.9sin^{-2}(i)times10^{9}M_odot$. J0439+1634 is likely to have a high gas-mass fraction and an oversized SMBH compared to local relations. The SFR of J0439+1634 reaches the maximum possible values, and the SFR surface density is close to the highest value seen in any star-forming galaxy currently known in the universe.
We present a method of selecting quasars up to redshift $approx$ 6 with random forests, a supervised machine learning method, applied to Pan-STARRS1 and WISE data. We find that, thanks to the increasing set of known quasars we can assemble a training
set that enables supervised machine learning algorithms to become a competitive alternative to other methods up to this redshift. We present a candidate set for the redshift range 4.8 to 6.3 which includes the region around z = 5.5 where quasars are difficult to select due to photometric similarity to red and brown dwarfs. We demonstrate that under our survey restrictions we can reach a high completeness ($66 pm 7 %$ below redshift 5.6 / $83^{+6}_{-9}%$ above redshift 5.6) while maintaining a high selection efficiency ($78^{+10}_{-8}%$ / $94^{+5}_{-8}%$). Our selection efficiency is estimated via a novel method based on the different distributions of quasars and contaminants on the sky. The final catalog of 515 candidates includes 225 known quasars. We predict the candidate catalog to contain an additional $148^{+41}_{-33}$ new quasars below redshift 5.6 and $45^{+5}_{-8}$ above and make the catalog publicly available. Spectroscopic follow-up observations of 37 candidates lead us to discover 20 new high redshift quasars (18 at $4.6le zle5.5$, 2 $zsim5.7$). These observations are consistent with our predictions on efficiency. We argue that random forests can lead to higher completeness because our candidate set contains a number of objects that would be rejected by common color cuts, including one of the newly discovered redshift 5.7 quasars.
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 $zge7.5$, 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 $3.6times10^{13} L_odot$. Deep spectroscopic observations reveal a SMBH with a mass of $(1.6pm0.4) times10^9M_odot$ 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 $rmsim 200 ~M_odot ~yr^{-1}$ and a dust mass of $sim7times10^7~M_odot$. 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.
X-ray emission from quasars has been detected up to redshift $z=7.5$, although only limited to a few objects at $z>6.5$. In this work, we present new Chandra observations of five $z>6.5$ quasars. By combining with archival Chandra observations of six
additional $z>6.5$ quasars, we perform a systematic analysis on the X-ray properties of these earliest accreting supermassive black holes (SMBHs). We measure the black hole masses, bolometric luminosities ($L_{rm bol}$), Eddington ratios ($lambda_{rm Edd}$), emission line properties, and infrared luminosities ($L_{rm IR}$) of these quasars using infrared and sub-millimeter observations. Correlation analysis indicates that the X-ray bolometric correction (the factor that converts from X-ray luminosity to bolometric luminosity) decreases with increasing $L_{rm bol}$, and that the UV/optical-to-X-ray ratio, $alpha_{rm ox}$, strongly correlates with $L_{rm 2500}$, and moderately correlates with $lambda_{rm Edd}$ and blueshift of CIV emission lines. These correlations are consistent with those found in lower-$z$ quasars, indicating quasar accretion physics does not evolve with redshift. We also find that $L_{rm IR}$ does not correlate with $L_{rm 2-10 keV}$ in these luminous distant quasars, suggesting that the ratio of the SMBH growth rate and their host galaxy growth rate in these early luminous quasars are different from those of local galaxies. A joint spectral analysis of the X-ray detected $z>6.5$ quasars yields an average X-ray photon index of $ Gamma=2.32^{+0.31}_{-0.30}$, steeper than that of low-$z$ quasars. By comparing it with the $Gamma-lambda_{rm Edd}$ relation, we conclude that the steepening of $Gamma$ for quasars at $z>6.5$ is mainly driven by their higher Eddington ratios.
We report new measurements of the intergalactic medium (IGM) Ly$alpha$ and Ly$beta$ effective optical depth at $5.3<z<6.5$, using a new sample of quasar sightlines including 32 quasars at $6.308le zle7.00$. These quasars provide a large statistical s
ample to measure the IGM evolution during the transition phase of the reionization epoch. We construct a data set of deep optical spectra of these quasars using VLT, Keck, Gemini, LBT, and MMT. We measure the Ly$alpha$ effective optical depth at $5.36<z<6.57$ using the Ly$alpha$ forests of both individual spectra and the stacked spectrum. The large scatter of individual measurements is consistent with previous work, suggesting an inhomogeneous reionization process. Combining our new measurements and previous results, we obtain a best-fit for the Ly$alpha$ effective optical depth evolution at $z>5.3$, $taupropto(1+z)^{8.6pm1.0}$. We then estimate the observed Ly$beta$ effective optical depth using Ly$beta$ forests and convert them to Ly$alpha$ optical depth for comparison, which provides additional constraints on the evolution of the IGM optical depth. The Ly$beta$-based measurements are generally in agreement with the best-fit evolution obtained from Ly$alpha$ forests. Using this new sample, we identify 389 Ly$alpha$ and 50 Ly$beta$ transmission spikes at $5.5<z<6.3$. The upper limits of Ly$alpha$ optical depth estimated using transmission spikes are well consistent with our best-fit evolution. The evolution in number density of these high-redshift transmission spikes suggests a rapid transition phase at the end of the reionization. Comparison of our optical depth measurements with hydrodynamical simulations indicates a IGM neutral hydrogen fraction $langle f_{rm HI}ranglegtrsim10^{-4}$ at $z=6$.
We present a submillimetre continuum survey (SCUBA2 High rEdshift bRight quasaR surveY, hereafter SHERRY) of 54 high redshift quasars at $5.6<z<6.9$ with quasar bolometric luminosities in a range of (0.2$-$$ 5)times10^{14},L_{odot}$, using the Submil
limetre Common-User Bolometer Array-2 (SCUBA2) on the James Clerk Maxwell Telescope. About 30% (16/54) of the sources are detected with a typical 850$mu$m rms sensitivity of 1.2 $rm mJy,beam^{-1}$ ($Srm _{ u,850,mu m} = 4$-5 mJy, at $>3.5sigma$). The new SHERRY detections indicate far-infrared (FIR) luminosities of $rm 3.5times10^{12}$ to $rm 1.4times10^{13}$ $L_{odot}$, implying extreme star formation rates of 90 to 1060 $M_{odot}$ yr$^{-1}$ in the quasar host galaxies. Compared with $z =$ 2$-$5 samples, the FIR luminous quasars ($L_{rm FIR} > 10^{13},L_{odot}$) are more rare at $z sim 6$. The optical/near-infrared (NIR) spectra of these objects show 11% (6/54) of the sources have weak Ly$alpha$, emission line features, which may relate to different sub-phases of the central active galactic nuclei (AGNs). Our SCUBA2 survey confirms the trend reported in the literature that quasars with submillimeter detections tend to have weaker ultraviolet (UV) emission lines compared to quasars with nondetections. The connection between weak UV quasar line emission and bright dust continuum emission powered by massive star formation may suggest an early phase of AGN-galaxy evolution, in which the broad line region is starting to develop slowly or is shielded from the central ionization source, and has unusual properties such as weak line features or bright FIR emission.
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 $(1.5pm0.2)times10^9$ $M_{odot}$ 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+2115s Ly$alpha$ line profile, and find that it is weaker than that of J1342+0928 and two other $zgtrsim7$ quasars. We estimate an IGM volume-averaged neutral fraction $langle xrm_{HI}rangle=0.39^{+0.22}_{-0.13}$. 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 $z=7.5149pm0.0004$ and indicates a star formation rate of $sim210$ $M_{odot}$ yr$^{-1}$ in its host galaxy.
From near-infrared spectroscopic measurements of the MgII emission line doublet, we estimate the black hole (BH) mass of the quasar, SMSS J215728.21-360215.1, as being (3.4 +/- 0.6) x 10^10 M_sun and refine the redshift of the quasar to be z=4.692. S
MSS J2157 is the most luminous known quasar, with a 3000A luminosity of (4.7 +/- 0.5) x 10^47 erg/s and an estimated bolometric luminosity of 1.6 x 10^48 erg/s, yet its Eddington ratio is only ~0.4. Thus, the high luminosity of this quasar is a consequence of its extremely large BH -- one of the most massive BHs at z > 4.
We investigate the distribution of companion galaxies around quasars using {em Hubble Space Telescope} ({em HST}) Advanced Camera for Surveys Wide Field Camera (ACS/WFC) archival images. Our master sample contains 532 quasars which have been observed
by {em HST} ACS/WFC, spanning a wide range of luminosity $(-31<M_i(z=2)<-23)$ and redshift ($0.3<z<3$). We search for companions around the quasars with projected distance of $10text{ kpc}<d<100text{ kpc}$. PSF subtraction is performed to enhance the completeness for close companions. The completeness is estimated to be high $(>90%)$ even for the faintest companions of interest. The number of physical companions is estimated by subtracting a background density from the number density of projected companions. We divide all the companions into three groups (faint, intermediate and bright) according to their fluxes. A control sample of galaxies is constructed to have similar redshift distribution and stellar mass range as the quasar sample using the data from {em HST} deep fields. We find that quasars and control sample galaxies have similar numbers of faint and bright companions, while quasars show a $3.7sigma$ deficit of intermediate companions compared to galaxies. The numbers of companions in all three groups do not show strong evolution with redshift, and the number of intermediate companions around quasars decreases with quasar luminosity. Assuming that merger-triggered quasars have entered the final coalescence stage during which individual companions are no longer detectable at large separations, our result is consistent with a picture in which a significant fraction of quasars is triggered by mergers.
