Low-luminosity Quasars Research Articles

Reconstruction of Cosmic Black Hole Growth and Mass Distribution from Quasar Luminosity Functions at z > 4: Implications for Faint and Low-mass Populations in JWST

The evolution of the quasar luminosity function (QLF) is fundamental to understanding the cosmic evolution of black holes (BHs) through their accretion phases. In the era of the James Webb Space Telescope (JWST), Euclid, and Nancy Grace Roman Space Telescope, their unprecedented detection sensitivity and wide survey area can unveil the low-luminosity quasar and low-mass BH population, and provide new insights into quasar host galaxies. We present a theoretical model describing BH growth from initial seeding at z ≳ 20 to ∼ 4, incorporating the duration of accretion episodes, the distribution of Eddington ratios, and the mass dependency of BH accretion rates. By constraining the model parameters with the observed QLFs at 4 ≤ z ≤ 6 across a wide UV luminosity range, we find that the high-redshift BH population grows rapidly at z ≳ 6, and decelerates the pace in subsequent epochs. Toward lower redshifts (z < 6), mass-dependent accretion inhibits the growth of high-mass BHs with M • > 108 M ⊙, leading to mass saturation at M • ≳ 1010 M ⊙. We predict the BH mass function down to M • ∼ 106 M ⊙ for both unobscured and obscured quasar populations at 4 ≤ z ≤ 11, offering a benchmark for future observational tests. Our model accounts for the presence of both bright and faint quasars at z > 4, including those discovered by JWST. Furthermore, our findings suggest two distinct pathways for the early assembly of the BH–galaxy mass correlation: the population with a BH-to-stellar-mass ratio near the local value of M •/M ⋆ ≃ 5 × 10−3 maintains proximity to the relation via moderate growth, while the population that begins to grow above the local relation becomes as overmassive as M •/M ⋆ ∼ 0.01–0.1 by z ∼ 6 via rapid mass accretion.

Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). XVII. Black Hole Mass Distribution at z ∼ 6 Estimated via Spectral Comparison with Low-z Quasars

We report the distribution of black hole (BH) masses and Eddingont ratios estimated for a sample of 131 low luminosity quasars in the early cosmic epoch (5.6 < z < 7.0). Our work is based on the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which has constructed a low luminosity quasar sample down to M 1450 ∼ − 21 mag, exploiting the survey data of Hyper Suprime-Cam installed on Subaru Telescope. The discovery spectra of these quasars are limited to the rest-frame wavelengths of ∼1200–1400 Å, which contain no emission lines that can be used as BH mass estimators. In order to overcome this problem, we made use of low-z counterpart spectra from the Sloan Digital Sky Survey, which are spectrally matched to the high-z spectra in overlapping wavelengths. We then combined the C iv emission line widths of the counterparts with the continuum luminosity from the SHELLQs data to estimate BH masses. The resulting BH mass distribution has a range of ∼107–10 M ⊙, with most of the quasars having BH masses ∼108 M ⊙ with sub-Eddington accretion. The present study provides not only a new insight into normal quasars in the reionization epoch, but also a new promising way to estimate BH masses of high-z quasars without near-infrared spectroscopy.

A JWST/NIRSpec First Census of Broad-line AGNs at z = 4–7: Detection of 10 Faint AGNs with M BH ∼ 106–108 M ⊙ and Their Host Galaxy Properties

We present the first statistical sample of faint type 1 AGNs at z > 4 identified by JWST/NIRSpec deep spectroscopy. Among the 185 galaxies at z spec = 3.8–8.9 confirmed with NIRSpec, our systematic search for broad-line emission reveals 10 type 1 AGNs at z = 4.015–6.936 whose broad component is only seen in the permitted Hα line and not in the forbidden [O iii]λ5007 line that is detected with greater significance than Hα. The broad Hα line widths of FWHM ≃ 1000–6000 km s−1 suggest that the AGNs have low-mass black holes with M BH ∼ 106–108 M ⊙, remarkably lower than those of low-luminosity quasars previously identified at z > 4 with ground-based telescopes. JWST and Hubble Space Telescope high-resolution images reveal that the majority of them show extended morphologies indicating significant contribution to the total lights from their host galaxies, except for three compact objects two of which show red spectral energy distributions, probably in a transition phase from faint AGNs to low luminosity quasars. Careful AGN-host decomposition analyses show that their host’s stellar masses are systematically lower than the local relation between the black hole mass and the stellar mass, implying a fast black hole growth consistent with predictions from theoretical simulations. A high fraction of the broad-line AGNs (∼5%), higher than z ∼ 0, indicates that the number density of such faint AGNs is higher than an extrapolation of the quasar luminosity function, implying a large population of AGNs in the early universe. Such faint AGNs contribute to cosmic reionization, while the total contribution is not large, up to ∼50% at z ∼ 6, because of their faint nature.

Revisiting X-Ray-bright Optically Normal Galaxies with the Chandra Source Catalog

X-ray bright optically normal galaxies (XBONGs) are galaxies with X-ray luminosities consistent with those of active galactic nuclei (AGNs) but no evidence of AGN optical emission lines. Crossmatching the Chandra Source Catalog version 2 with the Sloan Digital Sky Survey sample of spectroscopically classified galaxies, we have identified 817 XBONG candidates with L X > 1042 erg s−1 and X-ray to optical flux ratio F XO > 0.1. Comparisons with Wide-field Infrared Survey Explorer colors and near-IR, optical, UV, and radio luminosities show that the loci of XBONGs are in-between those of control samples of normal galaxies and quasars and are consistent with low-luminosity quasars. We find that 43% of the XBONG sample have X-ray colors suggesting N H > 1022 cm−2, double the fraction in the QSO sample, suggesting that a large fraction of XBONG are highly obscured AGNs. However, ∼50% of the XBONGs are not obscured and have X-ray colors harder than those of normal galaxies. Some of these XBONGs have spatially extended X-ray emission. These characteristics suggest that they may be unidentified galaxy groups and clusters. Using the X-ray luminosity functions of QSOs, galaxies, groups and clusters, we estimate the approximate fraction of extended XBONGs to be <20%. We also assess the approximate fraction of XBONGs whose AGN signatures are diluted by stellar light of host galaxies to be ∼30%, based on their redshift and deviation from the extrapolation of the QSO L X–L r relation.

Quasar Luminosity Function at z = 7

We present the quasar luminosity function (LF) at z = 7, measured with 35 spectroscopically confirmed quasars at 6.55 < z < 7.15. The sample of 22 quasars from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, combined with 13 brighter quasars in the literature, covers an unprecedentedly wide range of rest-frame ultraviolet magnitudes over −28 < M 1450 < −23. We found that the binned LF flattens significantly toward the faint end populated by the SHELLQs quasars. A maximum likelihood fit to a double power-law model has a break magnitude , a characteristic density Gpc−3 mag−1, and a bright-end slope , when the faint-end slope is fixed to α = −1.2 as observed at z ≤ 6. The overall LF shape remains remarkably similar from z = 4 to 7, while the amplitude decreases substantially toward higher redshifts, with a clear indication of an accelerating decline at z ≥ 6. The estimated ionizing photon density, 1048.2±0.1 s−1 Mpc−3, is less than 1% of the critical rate to keep the intergalactic medium ionized at z = 7, and thus indicates that quasars are not a major contributor to cosmic reionization.

Trinity II: The luminosity-dependent bias of the supermassive black hole mass–galaxy mass relation for bright quasars at z = 6

ABSTRACT Using recent empirical constraints on the dark matter halo–galaxy–supermassive black hole (SMBH) connection from z = 0–7, we infer how undermassive, typical, and overmassive SMBHs contribute to the quasar luminosity function (QLF) at z = 6. We find that beyond Lbol = 5 × 1046 erg s−1, the z = 6 QLF is dominated by SMBHs that are at least 0.3 dex above the z = 6 median M•–M* relation. The QLF is dominated by typical SMBHs (i.e. within ±0.3 dex around the M•–M* relation) at Lbol ≲ 1045 erg s−1. At z ∼ 6, the intrinsic M•–M* relation for all SMBHs is slightly steeper than the z = 0 scaling, with a similar normalization at $M_* \sim 10^{11} \, \mathrm{M}_\odot$. We also predict the M•–M* relation for z = 6 bright quasars selected by different bolometric luminosity thresholds, finding very good agreement with observations. For quasars with Lbol &amp;gt; 3 × 1046 (1048) erg s−1, the scaling relation is shifted upwards by ∼0.35 (1.0) dex for 1011M⊙ galaxies. To accurately measure the intrinsic M•–M* relation, it is essential to include fainter quasars with Lbol ≲ 1045 erg s−1. At high redshifts, low-luminosity quasars are thus the best targets for understanding typical formation paths for SMBHs in galaxies.

X-ray emission from a rapidly accreting narrow-line Seyfert 1 galaxy at z = 6.56

Context. The space density of X-ray-luminous, blindly selected active galactic nuclei (AGN) traces the population of rapidly accreting super-massive black holes through cosmic time. It is encoded in the X-ray luminosity function, whose bright end remains poorly constrained in the first billion years after the Big Bang as X-ray surveys have thus far lacked the required cosmological volume. With the eROSITA Final Equatorial-Depth Survey (eFEDS), the largest contiguous and homogeneous X-ray survey to date, X-ray AGN population studies can now be extended to new regions of the luminosity–redshift space (L2 − 10 keV &gt; 1045 erg s−1 and z &gt; 6). Aims. The current study aims at identifying luminous quasars at z &gt; 5.7 among X-ray-selected sources in the eFEDS field in order to place a lower limit on black hole accretion well into the epoch of re-ionisation. A secondary goal is the characterisation of the physical properties of these extreme coronal emitters at high redshifts. Methods. Cross-matching eFEDS catalogue sources to optical counterparts from the DESI Legacy Imaging Surveys, we confirm the low significance detection with eROSITA of a previously known, optically faint z = 6.56 quasar from the Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs) survey. We obtained a pointed follow-up observation of the source with the Chandra X-ray telescope in order to confirm the low-significance eROSITA detection. Using new near-infrared spectroscopy, we derived the physical properties of the super-massive black hole. Finally, we used this detection to infer a lower limit on the black hole accretion density rate at z &gt; 6. Results. The Chandra observation confirms the eFEDS source as the most distant blind X-ray detection to date. The derived X-ray luminosity is high with respect to the rest-frame optical emission of the quasar. With a narrow MgII line, low derived black hole mass, and high Eddington ratio, as well as its steep photon index, the source shows properties that are similar to local narrow-line Seyfert 1 galaxies, which are thought to be powered by young super-massive black holes. In combination with a previous high-redshift quasar detection in the field, we show that quasars with L2 − 10 keV &gt; 1045 erg s−1 dominate accretion onto super-massive black holes at z ∼ 6.

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) – XV. Constraining the cosmic reionization at 5.5 &amp;lt; z &amp;lt; 7

ABSTRACT Revealing the cosmic hydrogen reionization history is one of the main goals of the modern cosmology. z &amp;gt; 5 quasars (QSOs) have been used as back-lights to investigate the evolution of the intervening intergalactic medium (IGM) during the cosmic reionization since their first discovery. However, due to the small population of luminous QSOs (∼130 QSOs known to date), a tight constraint on the reionization history has not yet been placed. In this work, we aim to tighten the constraint using the 93 QSOs (5.5 &amp;lt; z &amp;lt; 7.1) recently discovered in the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQS) project. This is the largest QSO sample used to constrain the epoch of reionization. We measure the mean IGM Ly α transmission and the QSO near-zone size using the UV spectra of these QSOs. The mean IGM Ly α transmission rises above zero at z ≲ 6, indicating the end of the reionization. The near-zone sizes of the SHELLQs QSOs are consistent with sizes spanned by QSOs of lifetime tq ∼ 1–100 Myr in simulations. Due to the scatter created by the low signal-to-noise spectra and large Ly α redshift uncertainty, we cannot conclude whether the redshift evolution of the near-zone size is affected by the reionization effect.

What is the origin of the stacked radio emission in radio-undetected quasars?

Radio emission in the brightest radio quasars can be attributed to processes inherent to active galactic nuclei (AGN) powered by super massive black holes (SMBHs), while the physical origins of the radio fluxes in quasars without radio detections have not been established with full certainly. Deep radio surveys carried out with the Low Frequency ARray (LOFAR) are at least one order of magnitude more sensitive for objects with typical synchrotron spectra than previous wide-area high-frequency surveys ( &gt; 1.0 GHz). With the enhanced sensitivity that LOFAR offers, we investigate the radio-infrared continuum of LOFAR radio-detected quasars (RDQs) and LOFAR radio-undetected quasars (RUQs) in the 9.3 deg2NOAO Deep Wide-field survey (NDWFS) of the Boötes field; RUQs are quasars that are individually undetected at a level of ≥5σin the LOFAR observations. To probe the nature of the radio and infrared emission, where direct detection is not possible due to the flux density limits, we used a median image stacking procedure. This was done in the radio frequencies of 150 MHz, 325 MHz, 1.4 GHz and 3.0 GHz, and in nine infrared bands between 8 and 500 μm. The stacking analysis allows us to probe the radio-luminosity for quasars that are up to one order of magnitude fainter than the ones detected directly. The radio and infrared photometry allow us to derive the median spectral energy distributions of RDQs and RUQs in four contiguous redshift bins between 0 &lt; z &lt; 6.15. The infrared photometry is used to derive the infrared star-formation rate (SFR) through SED fitting, and is compared with two independent radio-based star-formation (SF) tracers using the far-infrared radio correlation (FIRC) of star-forming galaxies. We find a good agreement between our radio and infrared SFR measurements and the predictions of the FIRC. Moreover, we use the FIRC predictions to establish the level of the contribution due to SMBH accretion to the total radio-luminosity. We show that SMBH accretion can account for ∼5−41% of the total radio-luminosity in median RUQs, while for median RDQs the contribution is ∼50−84%. This implies that vigorous SF activity is coeval with SMBH growth in our median stacked quasars. We find that median RDQs have higher SFRs that agree well with those of massive star-forming main sequence galaxies, while median RUQs present lower SFRs than RDQs. Furthermore, the behavior of the radio-loudness parameter (R = log10(Lrad/LAGN)) is investigated. For quasars withR ≥ −4.5, the radio-emission is consistent with being dominated by SMBH accretion, while for low radio luminosity quasars withR &lt; −4.5 the relative contribution of SF to the radio fluxes increases as the SMBH component becomes weaker. We also find signatures of SF suppression due to negative AGN feedback in the brightest median RDQs at 150 MHz. Finally, taking advantage of our broad spectral coverage, we studied the radio spectra of median RDQs and RUQs. The spectral indices of RUQs and RDQs do not evolve significantly with redshift, but they become flatter towards lower frequencies.

Joint Survey Processing. I. Compact Oddballs in the COSMOS Field—Low-luminosity Quasars at z > 6?

The faint-end slope of the quasar luminosity function at z ∼ 6 and its implication on the role of quasars in reionizing the intergalactic medium at early times has been an outstanding problem for some time. The identification of faint high-redshift quasars with luminosities of <1044.5 erg s−1 is challenging. They are rare (few per square degree), and the separation of these unresolved quasars from late-type stars and compact star-forming galaxies is difficult from ground-based observations alone. In addition, source confusion becomes significant at >25 mag, with ∼30% of sources having their flux contaminated by foreground objects when the seeing resolution is ∼0″.7. We mitigate these issues by performing a pixel-level joint processing of ground and space-based data from Subaru/Hyper-SuprimeCam (HSC) and Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). We create a deconfused catalog over the 1.64 deg2 of the COSMOS field, after accounting for spatial varying point-spread functions and astrometric differences between the two data sets. We identify twelve low-luminosity (M UV ∼ −21 mag) z > 6 quasar candidates through (i) their red color measured between ACS/F814W and HSC/i band and (ii) their compactness in the space-based data. Nondetections of our candidates in Hubble DASH data argues against contamination from late-type stars. Our constraints on the faint end of the quasar luminosity function at z ∼ 6.4 suggest a negligibly small contribution to reionization compared to the star-forming galaxy population. The confirmation of our candidates and the evolution of number density with redshift could provide better insights into how supermassive galaxies grew in the first billion years of cosmic time.

Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). XVI. 69 New Quasars at 5.8 &lt; z &lt; 7.0

Abstract We present the spectroscopic discovery of 69 quasars at 5.8 &lt; z &lt; 7.0, drawn from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) imaging survey data. This is the 16th publication from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, and it completes identification of all but the faintest candidates (i.e., i-band dropouts with z AB &lt; 24 and y-band detections, and z-band dropouts with y AB &lt; 24) with Bayesian quasar probability P Q B &gt; 0.1 in the HSC-SSP third public data release (PDR3). The sample reported here also includes three quasars with P Q B &lt; 0.1 at z ∼ 6.6, which we selected in an effort to completely cover the reddest point sources with simple color cuts. The number of high-z quasars discovered in SHELLQs has now grown to 162, including 23 type II quasar candidates. This paper also presents identification of seven galaxies at 5.6 &lt; z &lt; 6.7, an [O iii] emitter at z = 0.954, and 31 Galactic cool stars and brown dwarfs. High-z quasars and galaxies compose 75% and 16%, respectively, of all the spectroscopic SHELLQs objects that pass our latest selection algorithm with the PDR3 photometry. That is, a total of 91% of the objects lie at z &gt; 5.6. This demonstrates that the algorithm has very high efficiency, even though we are probing an unprecedentedly low luminosity population down to M 1450 ∼ −21 mag.

First black hole mass estimation for the quadruple lensed system WGD2038-4008

Context. The quadruple lensed system WGD2038-4008 (zs = 0.777 ± 0.001) has recently been discovered with the help of new techniques and observations. Black hole masses have been estimated for lensed quasars, but they have mostly been calculated for one broad emission line of one image. However, the images could be affected by microlensing, which changes the results. Aims. We present black hole mass (MBH) estimations for images A and B of WGD2038-4008 using the three most prominent broad emission lines (Hα, Hβ, and Mg II) obtained in one single-epoch spectra. This is the first time the mass has been estimated in a lensed quasar in two images, allowing us to disentangle the effects of microlensing. The high S/N of our spectra allows us to get reliable results that can be compared with the existing data in the literature. Methods. We used the X-shooter instrument mounted on the Very Large Telescope at Paranal Observatory to observe this system, taking advantage of its wide spectral range (UVB, VIS, and NIR). The sky emission correction was performed using principal component analysis as the nodding was small compared to the image separation. We compared the lines profiles to identify the microlensing in the broad-line region and corrected each spectra by the image magification. Using the flux ratio of the continuum to the core of the emission lines, we analyzed whether microlensing was present in the continuum source. Results. We obtained MBH using the single-epoch method with the Hα and Hβ emission lines from the monochromatic luminosity and the velocity width. The luminosity at 3000 Å was obtained using the spectral energy distribution of image A, while the luminosity at 5100 Å was estimated directly from the spectra. The average MBH between the images obtained was log10(MBH/M⊙) = 8.27 ± 1.05, 8.25 ± 0.32, and 8.59 ± 0.35 for Mg II, Hβ, and Hα, respectively. We find Eddington ratios similar to those measured in the literature for unlensed low-luminosity quasars. Microlensing of −0.16 ± 0.06 mag in the continuum was found, but the induced error in the MBH is minor compared to that associated with the macromodel magnification. We also obtained the accretion disk size using the MBH for the three emission lines, obtaining an average value of log10(rs/cm)=15.3 ± 0.63, which is in agreement with theoretical estimates.

Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). XIII. Large-scale Feedback and Star Formation in a Low-luminosity Quasar at z = 7.07 on the Local Black Hole to Host Mass Relation

Abstract We present Atacama Large Millimeter/submillimeter Array [C ii] 158 μm line and underlying far-infrared (FIR) continuum emission observations (0.″70 × 0.″56 resolution) toward HSC J124353.93+010038.5 (J1243+0100) at z = 7.07, the only low-luminosity (M 1450 &gt; −25 mag) quasar currently known at z &gt; 7. The FIR continuum is bright (1.52 mJy) and resolved with a total luminosity of L FIR = 3.5 × 1012 L ⊙. The spatially extended component is responsible for ∼40% of the emission. The area-integrated [C ii] spectrum shows a broad wing (FWHM = 997 km s−1, L [C ii] = 1.2 × 109 L ⊙), as well as a bright core (FWHM = 235 km s−1, L [C ii] = 1.9 × 109 L ⊙). This wing is the first detection of a galactic-scale quasar-driven outflow (atomic outflow rate &gt;447 M ⊙ yr−1) at z &gt; 7. The estimated large mass-loading factor of the total outflow (e.g., ≳9 relative to the [C ii]-based star formation rate) suggests that this outflow will soon quench the star formation of the host. The core gas dynamics are governed by rotation, with a rotation curve suggestive of a compact bulge (∼3.3 × 1010 M ⊙), although it is not yet spatially resolved. Finally, we found that J1243+0100 has a black hole mass–to–dynamical mass (and –to–bulge mass) ratio of ∼0.4% (∼1%), consistent with the local value within the uncertainties. Our results therefore suggest that the black hole–host coevolution relation is already in place at z ∼ 7 for this object.

Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). XII. Extended [C ii] Structure (Merger or Outflow) in a z = 6.72 Red Quasar

Abstract We present Atacama Large Millimeter/submillimeter Array [C ii] 158 μm line and far-infrared (FIR) continuum emission observations toward HSC J120505.09−000027.9 (J1205−0000) at z = 6.72 with a beam size of ∼0.″8 × 0.″5 (or 4.1 kpc × 2.6 kpc), the most distant red quasar known to date. Red quasars are modestly reddened by dust and are thought to be in rapid transition from an obscured starburst to an unobscured normal quasar, driven by powerful active galactic nucleus (AGN) feedback that blows out a cocoon of interstellar medium. The FIR continuum of J1205−0000 is bright, with an estimated luminosity of L FIR ∼ 3 × 1012 L ⊙. The [C ii] line emission is extended on scales of r ∼ 5 kpc, greater than that of the FIR continuum. The line profiles at the extended regions are complex and broad (FWHM ∼ 630–780 km s−1). Although it is not practical to identify the nature of this extended structure, possible explanations include (i) companion/merging galaxies and (ii) massive AGN-driven outflows. For the case of (i), the companions are modestly star-forming (∼10 M ⊙ yr−1) but are not detected by our Subaru optical observations (y AB,5σ = 24.4 mag). For the case of (ii), our lower limit to the cold neutral outflow rate is ∼100 M ⊙ yr−1. The outflow kinetic energy and momentum are both much lower than predicted in energy-conserving wind models, suggesting that the AGN feedback in this quasar is not capable of completely suppressing its star formation.

The Faint End of the Quasar Luminosity Function at z ∼ 5 from the Subaru Hyper Suprime-Cam Survey

Abstract We present the quasar luminosity function at z ∼ 5 derived from the optical wide-field survey data obtained as a part of the Subaru strategic program (SSP) with the Hyper Suprime-Cam (HSC). From a ∼81.8 deg2 area in the Wide layer of the HSC-SSP survey, we selected 224 candidates of low-luminosity quasars at z ∼ 5 by adopting the Lyman-break method down to i = 24.1 mag. Based on our candidates and spectroscopically confirmed quasars from the Sloan Digital Sky Survey (SDSS), we derived the quasar luminosity function at z ∼ 5, covering a wide luminosity range of −28.76 &lt; M 1450 &lt; −22.32 mag. We found that the quasar luminosity function is fitted by a double power-law model with a break magnitude of mag. The inferred number density of low-luminosity quasars is lower, and the derived faint-end slope, , is flatter than those of previous studies at z ∼ 5. A compilation of the quasar luminosity function at 4 ≤ z ≤ 6 from the HSC-SSP suggests that there is little redshift evolution in the break magnitude and in the faint-end slope within this redshift range, although previous studies suggest that the faint-end slope becomes steeper at higher redshifts. The number density of low-luminosity quasars decreases more rapidly from z ∼ 5 to z ∼ 6 than from z ∼ 4 to z ∼ 5.

Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). XI. Proximity Zone Analysis for Faint Quasar Spectra at z ∼ 6

Abstract We present measurements of the size of the quasar proximity zone (R p ) for 11 low-luminosity ( ) quasars at z ∼ 6, discovered by the Subaru High-z Exploration of Low-Luminosity Quasars project. Our faint quasar sample expands the R p measurement down to mag, where more common quasar populations dominate at the epoch. We restrict the sample to quasars whose systemic redshifts have been precisely measured by [C ii] 158 μm or Mg ii λ2798 emission lines. We also update the R p measurements for 26 luminous quasars presented in Eilers et al. by using the latest systemic redshift results. The luminosity dependence on R p is found to be consistent with the theoretical prediction assuming a highly ionized intergalactic medium. We find a shallow redshift evolution of the luminosity-corrected R p , ( ) over . This trend is steeper than that of Eilers et al., but significantly shallower than those of the earlier studies. Our results suggest that R p,corr is insensitive to the neutral fraction of the universe at z ∼ 6. Four quasars show exceptionally small ( proper Mpc), which could be the result of their young age (&lt;104 yr) in the reionization epoch, though statistics on this are still scarce.

The frequency of extreme X-ray variability for radio-quiet quasars

ABSTRACT We analyse 1598 serendipitous Chandra X-ray observations of 462 radio-quiet quasars to constrain the frequency of extreme amplitude X-ray variability that is intrinsic to the quasar corona and innermost accretion flow. The quasars in this investigation are all spectroscopically confirmed, optically bright (mi ≤ 20.2), and contain no identifiable broad absorption lines in their optical/ultraviolet spectra. This sample includes quasars spanning z ≈ 0.1–4 and probes X-ray variability on time-scales of up to ≈12 rest-frame years. Variability amplitudes are computed between every epoch of observation for each quasar and are analysed as a function of time-scale and luminosity. The tail-heavy distributions of variability amplitudes at all time-scales indicate that extreme X-ray variations are driven by an additional physical mechanism and not just typical random fluctuations of the coronal emission. Similarly, extreme X-ray variations of low-luminosity quasars seem to be driven by an additional physical mechanism, whereas high-luminosity quasars seem more consistent with random fluctuations. The amplitude at which an X-ray variability event can be considered extreme is quantified for different time-scales and luminosities. Extreme X-ray variations occur more frequently at long time-scales (Δt ≳ 300 d) than at shorter time-scales and in low-luminosity quasars compared to high-luminosity quasars over a similar time-scale. A binomial analysis indicates that extreme intrinsic X-ray variations are rare, with a maximum occurrence rate of $\lt 2.4{{\ \rm per\ cent}}$ of observations. Finally, we present X-ray variability and basic optical emission-line properties of three archival quasars that have been newly discovered to exhibit extreme X-ray variability.

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). IX. Identification of two red quasars at z &amp;gt; 5.6

Abstract We present the first discovery of dust-reddened quasars (red quasars) in the high-z universe (z &amp;gt; 5.6). This is a result from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which is based on the sensitive multi-band optical imaging data produced by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. We identified four red quasar candidates from the 93 spectroscopically confirmed high-z quasars in the SHELLQs sample, based on detections in the Wide-field Infrared Survey Explorer (WISE) data at 3.4 and 4.6 μm (rest-frame ∼5000–6500 Å). The amount of dust reddening was estimated with spectral energy distribution (SED) fits over optical and mid-infrared wavelengths. Two of the four candidates were found to be red quasars with dust reddening of E(B − V) &amp;gt; 0.1. The remaining SHELLQs quasars without individual WISE detections are significantly fainter in the WISE bands and bluer than the red quasars, although we did detect them in the W1 band in a stacked image. We also conducted the same SED fits for high-z optically-luminous quasars, but no red quasar was found. This demonstrates the power of Subaru HSC to discover high-z red quasars, which are fainter than the limiting magnitudes of past surveys in the rest-frame ultraviolet, due to dust extinction.

Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). X. Discovery of 35 Quasars and Luminous Galaxies at 5.7 ≤ z ≤ 7.0

Abstract We report the discovery of 28 quasars and 7 luminous galaxies at 5.7 ≤ z ≤ 7.0. This is the tenth in a series of papers from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the deep multiband imaging data produced by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The total number of spectroscopically identified objects in SHELLQs has now grown to 93 high-z quasars, 31 high-z luminous galaxies, 16 [O iii] emitters at z ∼ 0.8, and 65 Galactic cool dwarfs (low-mass stars and brown dwarfs). These objects were found over 900 deg2, surveyed by HSC between 2014 March and 2018 January. The full quasar sample includes 18 objects with very strong and narrow Lyα emission, whose stacked spectrum is clearly different from that of other quasars or galaxies. While the stacked spectrum shows N v λ1240 emission and resembles that of lower-z narrow-line quasars, the small Lyα width may suggest a significant contribution from the host galaxies. Thus, these objects may be composites of quasars and star-forming galaxies.

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). VIII. A less biased view of the early co-evolution of black holes and host galaxies

Abstract We present ALMA [C ii] line and far-infrared (FIR) continuum observations of three $z \gt 6$ low-luminosity quasars ($M_{\rm 1450} \gt -25$ mag) discovered by our Subaru Hyper Suprime-Cam (HSC) survey. The [C ii] line was detected in all three targets with luminosities of $(2.4\mbox{--}9.5) \times 10^8\, L_{\odot }$, about one order of magnitude smaller than optically luminous ($M_{\rm 1450} \lesssim -25$ mag) quasars. The FIR continuum luminosities range from $\lt 9 \times 10^{10}\, L_{\odot }$ (3 $\sigma$ limit) to ${\sim } 2 \times 10^{12}\, L_{\odot }$, indicating a wide range in star formation rates in these galaxies. Most of the HSC quasars studied thus far show [C ii]/ FIR luminosity ratios similar to local star-forming galaxies. Using the [C ii]-based dynamical mass ($M_{\rm dyn}$) as a surrogate for bulge stellar mass ($M_{\rm\, bulge}$), we find that a significant fraction of low-luminosity quasars are located on or even below the local $M_{\rm\, BH}$–$M_{\rm\, bulge}$ relation, particularly at the massive end of the galaxy mass distribution. In contrast, previous studies of optically luminous quasars have found that black holes are overmassive relative to the local relation. Given the low luminosities of our targets, we are exploring the nature of the early co-evolution of supermassive black holes and their hosts in a less biased way. Almost all of the quasars presented in this work are growing their black hole mass at a much higher pace at $z \sim 6$ than the parallel growth model, in which supermassive black holes and their hosts grow simultaneously to match the local $M_{\rm\, BH}$–$M_{\rm\, bulge}$ relation at all redshifts. As the low-luminosity quasars appear to realize the local co-evolutionary relation even at $z \sim 6$, they should have experienced vigorous starbursts prior to the currently observed quasar phase to catch up with the relation.