arXiv subjects

Toshihiro Kawaguchi

Publications and source records attributed to Toshihiro Kawaguchi.

At least 55 records · Page 3Linked to original sources

arXiv2021

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). XVI. 69 New Quasars at 5.8 < z < 7.0

We present the spectroscopic discovery of 69 quasars at 5.8 < z < 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 completes identification of all but the faintest candidates (i.e., i-band dropouts with zAB < 24 and y-band detections, and z-band dropouts with yAB < 24) with Bayesian quasar probability Pq > 0.1 in the HSC-SSP third public data release (PDR3). The sample reported here also includes three quasars with Pq < 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 < z < 6.7, an [O III] emitter at z = 0.954, and 31 Galactic cool stars and brown dwarfs. High-z quasars and galaxies comprise 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 > 5.6. This demonstrates that the algorithm has very high efficiency, even though we are probing an unprecedentedly low-luminosity population down to M1450 ~ -21 mag.

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arXiv2021

Ionizing radiation from AGNs at z>3.3 with the Subaru Hyper Suprime-Cam Survey and the CFHT Large Area U-band Deep Survey (CLAUDS)

We use deep and wide imaging data from the CFHT Large Area U-band Deep Survey (CLAUDS) and the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) to constrain the ionizing radiation (Lyman Continuum; LyC) escape fraction from AGNs at $z \sim 3 - 4$. For 94 AGNs with spectroscopic redshifts at $3.3 < z < 4.0$, we use their U-band / i-band flux ratios to estimate LyC transmission of individual AGNs. The distribution of their LyC transmission shows values lower than the range of LyC transmission values for IGM of the same redshift range, which suggests that LyC escape fraction of AGNs at $z>3.3$ is considerably lower than unity in most cases. We do not find any trend in LyC transmission values depending on their UV luminosities. Based on the photometry of stacked images we find the average flux ratio of LyC and non-ionizing UV photons escaping from the objects $(f_{LyC}/f_{UV})^{out} = 0.182 \pm 0.043$ for AGNs at $3.3<z<3.6$, which corresponds to LyC escape fraction $f_{esc} = 0.303 \pm 0.072$ if we assume a fiducial intrinsic SED of AGN. Based on the estimated LyC escape fraction and the UV luminosity function of AGNs, we argue that UV-selected AGNs' contribution to the LyC emissivity at the epoch is minor, although the size of their contribution largely depends on the shape of the UV luminosity function.

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arXiv2021

The eROSITA Final Equatorial-Depth Survey (eFEDS): A multiwavelength view of WISE mid-infrared galaxies/active galactic nuclei

We investigate the physical properties--such as the stellar mass, SFR, IR luminosity, X-ray luminosity, and hydrogen column density--of MIR galaxies and AGN at $z < 4$ in the 140 deg$^2$ field observed by SRG/eROSITA through the eFEDS survey. By cross-matching the WISE 22 $μ$m (W4)-detected sample and the eFEDS X-ray point-source catalog, we find that 692 extragalactic objects are detected by eROSITA. We have compiled a multiwavelength dataset. We have also performed (i) an X-ray spectral analysis, (ii) SED fitting using X-CIGALE, (iii) 2D image-decomposition analysis using Subaru HSC images, and (iv) optical spectral fitting with QSFit to investigate the AGN and host-galaxy properties. For 7,088 WISE W4 objects that are undetected by eROSITA, we have performed an X-ray stacking analysis to examine the typical physical properties of these X-ray faint and/or probably obscured objects. We find that (i) 82% of the eFEDS-W4 sources are classified as X-ray AGN with $\log\,L_{\rm X} >$ 42 erg s$^{-1}$; (ii) 67% and 24% of the objects have $\log\,(L_{\rm IR}/L_{\odot}) > 12$ and 13, respectively; (iii) the relationship between $L_{\rm X}$ and the 6 $μ$m luminosity is consistent with that reported in previous works; and (iv) the relationship between the Eddington ratio and $N_{\rm H}$ for the eFEDS-W4 sample and a comparison with a model prediction from a galaxy-merger simulation indicates that approximately 5% of the eFEDS-W4 sources in our sample are likely to be in an AGN-feedback phase, in which strong radiation pressure from the AGN blows out the surrounding material from the nuclear region. Thanks to the wide area coverage of eFEDS, we have been able to constrain the ranges of the physical properties of the WISE W4 sample of AGNs at $z < 4$, providing a benchmark for forthcoming studies on a complete census of MIR galaxies selected from the full-depth eROSITA all-sky survey.

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arXiv2021

Destruction of the central black hole gas reservoir through head-on galaxy collisions

A massive black hole exists in almost every galaxy. They occasionally radiate a vast amount of light by releasing gravitational energy of accreting gas, with a cumulative active period of only a few $10^8$ years, so-called the duty cycle of the Active Galactic Nuclei. Namely, many galaxies today host a starving massive black hole. Although galaxy collisions have been thought to enhance nucleus activity, the origin of the duty cycle, especially the shutdown process, is a still critical issue. Here we show that galaxy collisions are also capable of suppressing black hole fueling by using an analytic model and three-dimensional hydrodynamic simulations, applying the well-determined parameter sets for the galactic collision in the Andromeda galaxy. Our models demonstrate that a central collision of galaxies can strip the torus-shaped gas surrounding the massive black hole, the putative fueling source. The derived condition for switching-off the black hole fueling indicates that a significant fraction of currently bright nuclei can become inactive, reminiscent of fading/dying active nucleus phenomena associated with galaxy merging events. Galaxy collisions may therefore be responsible both for switching-off and turning-on the nucleus activity, depending on the collision orbit (head-on or far-off-centre).

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arXiv2021

Optical Spectroscopy of Dual Quasar Candidates from the Subaru HSC-SSP program

We report on a spectroscopic program to search for dual quasars using Subaru Hyper Suprime-Cam (HSC) images of SDSS quasars which represent an important stage during galaxy mergers. Using Subaru/FOCAS and Gemini-N/GMOS, we identify three new physically associated quasar pairs having projected separations less than 20 kpc, out of 26 observed candidates. These include the discovery of the highest redshift ($z=3.1$) quasar pair with a separation $<$ 10 kpc. Based on the sample acquired to date, the success rate of identifying physically associated dual quasars is $19\%$ when excluding stars based on their HSC colors. Using the full sample of six spectroscopically confirmed dual quasars, we find that the black holes in these systems have black hole masses ($M_{BH} \sim 10^{8-9}M_{\odot}$) similar to single SDSS quasars as well as their bolometric luminosities and Eddington ratios. We measure the stellar mass of their host galaxies based on 2D image decomposition of the five-band ($grizy$) optical emission and assess the mass relation between supermassive black holes (SMBHs) and their hosts. Dual SMBHs appear to have elevated masses relative to their host galaxies. Thus mergers may not necessarily align such systems onto the local mass relation, as suggested by the Horizon-AGN simulation. This study suggests that dual luminous quasars are triggered prior to the final coalescence of the two SMBHs, resulting in early mass growth of the black holes relative to their host galaxies.

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arXiv2021

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

We present ALMA [CII] 158 $μ$m line and underlying far-infrared (FIR) continuum emission observations ($0''.70 \times 0''.56$ resolution) toward HSC J124353.93$+$010038.5 (J1243$+$0100) at $z = 7.07$, the only low-luminosity ($M_{\rm 1450} > -25$ mag) quasar currently known at $z > 7$. The FIR continuum is bright (1.52 mJy) and resolved with a total luminosity of $L_{\rm FIR} = 3.5 \times 10^{12}~L_\odot$. The spatially extended component is responsible for $\sim 40\%$ of the emission. The area-integrated [CII] spectrum shows a broad wing (${\rm FWHM} = 997$ km s$^{-1}$, $L_{\rm [CII]} = 1.2 \times 10^9~L_\odot$) as well as a bright core (${\rm FWHM} = 235$ km s$^{-1}$, $L_{\rm [CII]} = 1.9 \times 10^9~L_\odot$). This wing is the first detection of a galactic-scale quasar-driven outflow (atomic outflow rate $> 447~M_\odot$ yr$^{-1}$) at $z > 7$. The estimated large mass loading factor of the total outflow (e.g., $\gtrsim 9$ relative to the [CII]-based SFR) 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 ($\sim 3.3 \times 10^{10}~M_\odot$), although it is not yet spatially resolved. Finally, we found that J1243$+$0100 has a black hole mass-to-dynamical mass ratio (and -to-bulge mass ratio) of $\sim 0.4\%$ ($\sim 1\%$), consistent with the local value within uncertainties. Our results therefore suggest that the black hole-host co-evolution relation is already in place at $z \sim 7$ for this object.

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arXiv2021

The eROSITA Final Equatorial-Depth Survey (eFEDS): An X-ray bright, extremely luminous infrared galaxy at z = 1.87

In this study, we investigate the X-ray properties of WISE J090924.01+000211.1 (WISEJ0909+0002), an extremely luminous infrared (IR) galaxy (ELIRG) at $z_{\rm spec}$= 1.871 in the eROSITA final equatorial depth survey (eFEDS). WISEJ0909+0002 is a WISE 22 $μ$m source, located in the GAMA-09 field, detected by eROSITA during the performance and verification phase. The corresponding optical spectrum indicates that this object is a type-1 active galactic nucleus (AGN). Observations from eROSITA combined with Chandra and XMM-Newton archival data indicate a very luminous ($L$ (2--10 keV) = ($2.1 \pm 0.2) \times 10^{45}$ erg s$^{-1}$) unobscured AGN with a power-law photon index of $Γ$ = 1.73$_{-0.15}^{+0.16}$, and an absorption hydrogen column density of $\log\,(N_{\rm H}/{\rm cm}^{-2}) < 21.0$. The IR luminosity was estimated to be $L_{\rm IR}$ = (1.79 $\pm$ 0.09) $\times 10^{14}\, L_{\odot}$ from spectral energy distribution modeling based on 22 photometric data (X-ray to far-IR) with X-CIGALE, which confirmed that WISEJ0909+0002 is an ELIRG. A remarkably high $L_{\rm IR}$ despite very low $N_{\rm H}$ would indicate that we are witnessing a short-lived phase in which hydrogen gas along the line of sight is blown outwards, whereas warm and hot dust heated by AGNs still exist. As a consequence of eROSITA all-sky survey, $6.8_{-5.6}^{+16}\times 10^2$ such X-ray bright ELIRGs are expected to be discovered in the entire extragalactic sky ($|b| > 10^\circ$). This can potentially be the key population to constrain the bright-end of IR luminosity functions.

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arXiv2021

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

We present ALMA [C II] 158 $μ$m line and far-infrared (FIR) continuum emission observations toward HSC J120505.09$-$000027.9 (J1205$-$0000) at $z = 6.72$ with the beam size of $\sim 0''.8 \times 0''.5$ (or 4.1 kpc $\times$ 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 which blows out a cocoon of interstellar medium (ISM). The FIR continuum of J1205$-$0000 is bright, with an estimated luminosity of $L_{\rm FIR} \sim 3 \times 10^{12}~L_\odot$. The [C II] line emission is extended on scales of $r \sim 5$ kpc, greater than the FIR continuum. The line profiles at the extended regions are complex and broad (FWHM $\sim 630-780$ 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 ($\sim 10~M_\odot$ yr$^{-1}$), but are not detected by our Subaru optical observations ($y_{\rm AB,5σ} = 24.4$ mag). For the case of (ii), our lower-limit to the cold neutral outflow rate is $\sim 100~M_\odot$ yr$^{-1}$. The outflow kinetic energy and momentum are both much smaller than what predicted in energy-conserving wind models, suggesting that the AGN feedback in this quasar is not capable of completely suppressing its star formation.

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arXiv2020

NuSTAR Non-detection of a Faint Active Galactic Nucleus in an Ultraluminous IR Galaxy with Kpc-scale Fast Wind

Large-scale outflows are generally considered as a possible evidence that active galactic nuclei (AGNs) can severely affect their host galaxies. Recently an ultraluminous IR galaxy (ULIRG) at $z=0.49$, AKARI J0916248+073034, was found to have a galaxy-scale [OIII] $λ$5007 outflow with one of the highest energy-ejection rates at $z<1.6$. However, the central AGN activity estimated from its torus mid-IR (MIR) radiation is weak relative to the luminous [OIII] emission. In this work we report the first NuSTAR hard X-ray follow-up of this ULIRG to constrain its current AGN luminosity. The intrinsic 2-10 keV luminosity shows a 90% upper-limit of $3.0\times10^{43}$ erg s$^{-1}$ assuming Compton-thick obscuration ($N_{\rm H}=1.5\times10^{24}$ cm$^{-2}$), which is only 3.6% of the luminosity expected from the extinction corrected [OIII] luminosity. With the NuSTAR observation, we succeed to identify that this ULIRG has a most extreme case of X-ray deficit among local ULIRGs. A possible scenario to explain the drastic declining in both of the corona (X-ray) and torus (MIR) is that the primary radiation from the AGN accretion disk is currently in a fading status, as a consequence of a powerful nuclear wind suggested by its powerful ionized outflow in the galaxy scale.

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arXiv2020

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

We present the quasar luminosity function at $z \sim 5$ derived from the optical wide-field survey data obtained as a part of the Subaru strategic program (SSP) with Hyper Suprime-Cam (HSC). From $\sim$81.8 deg$^2$ area in the Wide layer of the HSC-SSP survey, we selected 224 candidates of low-luminosity quasars at $z \sim 5$ 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 $z \sim 5$ covering a wide luminosity range of $-28.76 < M_{\rm 1450} < -22.32$ mag. We found that the quasar luminosity function is fitted by a double power-law model with a break magnitude of $M^{*}_{1450} = -25.05^{+0.10}_{-0.24}$ mag. The inferred number density of low-luminosity quasars is lower, and the derived faint-end slope, $-1.22^{+0.03}_{-0.10}$, is flatter than those of previous studies at $z \sim 5$. A compilation of the quasar luminosity function at $4 \leq z \leq 6$ 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 $z \sim 5$ to $z \sim 6$ than from $z \sim 4$ to $z \sim 5$.

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arXiv2020

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

We present measurements of the size of the quasar proximity zone ($R_p$) for eleven low-luminosity ($-26.16\leq M_{1450}\leq-22.83$) quasars at $z\sim6$, discovered by the Subaru High-$z$ Exploration of Low-Luminosity Quasars project (SHELLQs). Our faint quasar sample expands the $R_p$ measurement down to $M_{1450}=-22.83$ mag, where more common quasar populations dominate at the epoch. We restrict the sample to quasars whose systemic redshifts have been precisely measured by [CII] 158 $μ$m or MgII $λ$2798 emission lines. We also update the $R_p$ measurements for 26 luminous quasars presented in Eilers et al. (2017)(arXiv:1703.02539) by using the latest systemic redshift results. The luminosity dependence on $R_p$ is found to be consistent with the theoretical prediction assuming highly ionized intergalactic medium. We find a shallow redshift evolution of the luminosity-corrected $R_p$, $R_{p,{\rm corr}}^{-25}$ ($R_{p, \rm corr}^{-25}\propto(1+z)^{-3.79\pm1.72}$) over $5.8\lesssim z \lesssim6.6$. This trend is steeper than that of Eilers et al. (2017) but significantly shallower than those of the earlier studies. Our results suggest that $R_{p,\rm corr}$ is insensitive to the neutral fraction of the universe at $z\sim6$. Four quasars show exceptionally small $R_{p,\rm corr}^{-25}$ ($ \lesssim0.90$ proper Mpc), which could be the result of their young age ($<10^4$ yr) in the reionization epoch, though statistics is still small.

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arXiv2020

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). IX. Identification of Two Red Quasars at z > 5.6

We present the first discovery of dust-reddened quasars (red quasars) in the high-z universe (z >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 spectroscopically confirmed 93 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 um (rest-frame ~5000-6500 A). 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) > 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.

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arXiv2020

Subaru Hyper Suprime-Cam View of Quasar Host Galaxies at z < 1

Active galactic nuclei (AGNs) are key for understanding the coevolution of galaxies and supermassive black holes (SMBHs). AGN activity is thought to affect the properties of their host galaxies, via a process called "AGN feedback", which drives the co-evolution. From a parent sample of 1151 z < 1 type-1 quasars from the Sloan Digital Sky Survey quasar catalog, we detected host galaxies of 862 of them in the high-quality grizy images of the Subaru Hyper Suprime-Cam (HSC) survey. The unprecedented combination of the survey area and depth allows us to perform a statistical analysis of the quasar host galaxies, with small sample variance. We fit the radial image profile of each quasar as a linear combination of the point spread function and the Sersic function, decomposing the images into the quasar nucleus and the host galaxy components. We found that the host galaxies are massive, with stellar mass Mstar > 10^(10) Msun, and are mainly located on the green valley. This trend is consistent with a scenario in which star formation of the host galaxies is suppressed by AGN feedback, that is, AGN activity may be responsible for the transition of these galaxies from the blue cloud to the red sequence. We also investigated the SMBH mass to stellar mass relation of the z < 1 quasars, and found a consistent slope with the local relation, while the SMBHs may be slightly undermassive. However, the above results are subject to our sample selection, which biases against host galaxies with low masses and/or large quasar-to-host flux ratios.

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arXiv2020

Tracing the co-evolution path of super massive black holes and spheroids with AKARI-selected ultra-luminous IR galaxies at intermediate redshifts

We present the stellar population and ionized-gas outflow properties of ultra-luminous IR galaxies (ULIRGs) at $z=$ 0.1-1.0, which are selected from AKARI FIR all-sky survey. We construct a catalog of 1077 ULIRGs to examine feedback effect after major mergers. 202 out of the 1077 ULIRGs are spectroscopically identified by SDSS and Subaru/FOCAS observations. Thanks to deeper depth and higher resolution of AKARI compared to the previous IRAS survey, and reliable identification from WISE MIR pointing, the sample is unique in identifying optically-faint (i$\sim$20) IR-bright galaxies, which could be missed in previous surveys. A self-consistent spectrum-SED decomposition method, which constrains stellar population properties in SED modeling based on spectral fitting results, has been employed for 149 ULIRGs whose optical continua are dominated by host galaxies. They are massive galaxies ($M_{\rm star}\sim10^{11}$-$10^{12}$ M$_{\odot}$), associated with intense star formation activities (SFR $\sim$ 200-2000 M$_{\odot}$ yr$^{-1}$). The sample covers a range of AGN bolometric luminosity of $10^{10}$-$10^{13}$ L$_{\odot}$, and the outflow velocity measured from [OIII] 5007A line shows a correlation with AGN luminosity. Eight galaxies show extremely fast outflows with velocity up to 1500-2000 km s$^{-1}$. However, the co-existence of vigorous starbursts and strong outflows suggests the star formation has not been quenched during the ULIRG phase. By deriving stellar mass and mass fraction of young stellar population, we find no significant discrepancies between stellar properties of ULIRGs with weak and powerful AGNs. The results are not consistent with the merger-induced evolutionary scenario, which predicts that SF-dominated ULIRGs show smaller stellar mass and younger stellar populations compared to AGN-dominated ULIRGs.

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arXiv2020

Dual supermassive black holes at close separation revealed by the Hyper Suprime-Cam Subaru Strategic Program

The unique combination of superb spatial resolution, wide-area coverage, and deep depth of the optical imaging from the Hyper Suprime-Cam (HSC) Subaru Strategic Program is utilized to search for dual quasar candidates. Using an automated image analysis routine on 34,476 known SDSS quasars, we identify those with two (or more) distinct optical point sources in HSC images covering 796 deg^2. We find 421 candidates out to a redshift of 4.5 of which one hundred or so are more likely after filtering out contaminating stars. Angular separations of 0.6 - 4.0" correspond to projected separations of 3 - 30 kpc, a range relatively unexplored for population studies of luminous dual quasars. Using Keck-I/LRIS and Gemini-N/NIFS, we spectroscopically confirm three dual quasar systems at z < 1, two of which are previously unknown out of eight observed, based on the presence of characteristic broad emission lines in each component, while highlighting that the continuum of one object in one of the pairs is reddened. In all cases, the [OIII]5007 emission lines have mild velocity offsets, thus the joint [OIII] line profile is not double-peaked. We find a dual quasar fraction of 0.26+/-0.18% and no evidence for evolution. A comparison with the Horizon-AGN simulation seems to support the case of no evolution in the dual quasar fraction when broadly matching the quasar selection. These results may indicate a scenario in which the frequency of the simultaneous triggering of luminous quasars is not as sensitive as expected to the cosmic evolution of the merger rate or gas content of galaxies.

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arXiv2020

Semi-analytic modelling of AGNs: auto-correlation function and halo occupation

The spatial clustering of active galactic nuclei (AGNs) is considered to be one of the important diagnostics for the understanding of the underlying processes behind their activities complementary to measurements of the luminosity function (LF). We analyse the AGN clustering from a recent semi-analytic model performed on a large cosmological $N$-body simulation covering a cubic gigaparsec comoving volume. We have introduced a new time-scale of gas accretion on to the supermassive black holes to account for the loss of the angular momentum on small scales, which is required to match the faint end of the observed X-ray LF. The large simulation box allows us accurate determination of the auto-correlation function of the AGNs. The model prediction indicates that this time-scale plays a significant role in allowing massive haloes to host relatively faint population of AGNs, leading to a higher bias factor for those AGNs. The model predictions are in agreement with observations of X-ray selected AGNs in the luminosity range $10^{41.5}~\mathrm{erg} \ \mathrm{s}^{-1} \leq L_{2-10\mathrm{keV}} \leq 10^{44.5}~\mathrm{erg} \ \mathrm{s}^{-1}$, with the typical host halo mass of $10^{12.5-13.5} h^{-1}\,{\rm M}_{\odot}$ at $z \lesssim 1$. This result shows that the observational clustering measurements impose an independent constraint on the accretion time-scale complementary to the LF measurements. Moreover, we find that not only the effective halo mass corresponding to the overall bias factor, but the extended shape of the predicted AGN correlation function shows remarkable agreement with those from observations. Further observational efforts towards the low luminosity end at $z \sim 1$ would give us stronger constraints on the triggering mechanisms of AGN activities through their clustering.

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arXiv2020

Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). III. Discovery of a z = 4.72 Radio Galaxy with Lyman Break Technique

We report a discovery of $z = 4.72$ radio galaxy, HSC J083913.17+011308.1, by using the Lyman break technique with the Hyper Suprime-Cam Subaru Strategic Survey (HSC-SSP) catalog for VLA FIRST radio sources. The number of known high-$z$ radio galaxies (HzRGs) at $z > 3$ is quite small to constrain the evolution of HzRGs so far. The deep and wide-area optical survey by HSC-SSP enables us to apply the Lyman break technique to a large search for HzRGs. For an HzRG candidate among pre-selected $r$-band dropouts with a radio detection, a follow-up optical spectroscopy with GMOS/Gemini has been performed. The obtained spectrum presents a clear Ly$α$ emission line redshifted to $z=4.72$. The SED fitting analysis with the rest-frame UV and optical photometries suggests the massive nature of this HzRG with $\log{M_*/M_{\odot}} = 11.4$. The small equivalent width of Ly$α$ and the moderately red UV colors indicate its dusty host galaxy, implying a chemically evolved and dusty system. The radio spectral index does not meet a criterion for an ultra-steep spectrum: $α^{325}_{1400}$ of $-1.1$ and $α^{150}_{1400}$ of $-0.9$, demonstrating that the HSC-SSP survey compensates for a sub-population of HzRGs which are missed in surveys focusing on an ultra-steep spectral index.

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arXiv2020

Revisiting Soltan's argument based on a semi-analytical model for galaxy and black hole evolution

We show the significance of the super-Eddington accretion for the cosmic growth of supermassive black holes (SMBHs) with a semi-analytical model for galaxy and black hole evolution. The model explains various observed properties of galaxies and active galactic nuclei at a wide redshift range. By tracing the growth history of individual SMBHs, we find that the fraction of the SMBH mass acquired during the super-Eddington accretion phases to the total SMBH mass becomes larger for less massive black holes and at higher redshift. Even at z = 0, SMBHs with > 1e+9 Msun have acquired more than 50% of their mass by super-Eddington accretions, which is apparently inconsistent with classical Soltan's argument. However, the mass-weighted radiation efficiency of SMBHs with > 1e+8 Msun obtained with our model, is about 0.08 at z = 0, which is consistent with Soltan's argument within the observational uncertainties. We, therefore, conclude that Soltan's argument cannot reject the possibility that SMBHs are grown mainly by super-Eddington accretions.

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