arXiv subjects

Kohei Inayoshi

Publications and source records attributed to Kohei Inayoshi.

At least 91 records · Page 5Linked to original sources

arXiv2025

Gravitational Wave Forecasts Constrained by JWST AGN Observations for Early Massive Black Hole Mergers

Massive black holes (BHs) grow by gas accretion and mergers, observable through electromagnetic (EM) and gravitational wave (GW) emission. The James Webb Space Telescope (JWST) has detected faint active galactic nuclei (AGNs), revealing an abundant population of accreting BHs with masses of $M_\bullet\sim 10^{6-8}~M_\odot$. This mass range overlaps with the detection scopes of space-based GW interferometers and approaches the upper bounds of the predicted mass of seed BHs. We model BH mass assembly in light of the new JWST findings to investigate their formation channels and predict merger events. Two types of seed BHs are considered: heavy seeds ($M_\bullet\sim 10^{2-5}~M_\odot$) formed in rare and overdense cosmic regions, and light seeds ($M_\bullet\sim 10^{1-3}~M_\odot$) formed as stellar remnants in less massive dark-matter halos. The BHs grow through episodic accretion and merger events, which we model by fitting the AGN luminosity function to observational data including JWST-identified AGNs at $z\sim 5$. We find that heavy seeds alone struggle to explain quasars and faint JWST-selected AGNs simultaneously, requiring the more abundant light seeds. The observed merger rate of BHs from heavy seeds alone is limited to $\lesssim 10^{-1}~{\rm yr}^{-1}$ for major mergers at $z\geq5$. However, the presence of light seeds increases the major merger rate by several orders of magnitude, which peaks at a total BH mass of $M_\bullet\simeq 2\times 10^3~M_\odot$ over $5<z<10$ at a rate of $\sim 30~{\rm yr}^{-1}$. These events are detectable by future GW observatories such as the Laser Interferometer Space Antenna (LISA). Precise sky localization and distance measurement of those GW events, with solid angle and luminosity distance uncertainties $ΔΩΔ\log D_L\lesssim 10^{-4}~\rm deg^2$, will enable EM identification of mergers at $z\geq5$ and multi-messenger follow-up observations.

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arXiv2025

Little Red Dots: Rapidly Growing Black Holes Reddened by Extended Dusty Flows

The James Webb Space Telescope (JWST) observations have revolutionized extragalactic research, particularly with the discovery of little red dots (LRD), which we propose are dust-reddened broad-line active galactic nuclei (AGNs). Their unique v-shape spectral feature observed through JWST/NIRCam challenges us to discern the relative contributions of the galaxy and AGN. We study a spectral energy distribution (SED) model for LRDs from rest-frame UV to infrared bands. We hypothesize that the incident radiation from an AGN, characterized by a typical SED, is embedded in an extended dusty medium with an extinction law similar to those seen in dense regions such as Orion Nebula or certain AGN environments. The UV-optical spectrum is described by dust-attenuated AGN emission, featuring a red optical continuum at $λ>4000$ A and a flat UV spectral shape established through a gray extinction curve at $λ<3000$ A, due to the absence of small-size grains. There is no need for additional stellar emission or AGN scattered light. In the infrared, the SED is shaped by an extended dust and gas distribution ($γ<1$; $ρ\propto r^{-γ}$) with a characteristic gas density of $\simeq 10-10^3~{\rm cm}^{-3}$, which allows relatively cool dust temperatures to dominate the radiation, thereby shifting the energy peak from near- to mid-infrared bands. This model, unlike the typical AGN hot torus models, can produce an infrared SED flattening that is consistent with LRD observations through JWST MIRI. Such a density structure can arise from the coexistence of inflows and outflows during the early assembly of galactic nuclei. This might be the reason why LRDs emerge preferentially in the high-redshift universe younger than one billion years.

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arXiv2025

The Rise of Faint, Red AGN at $z>4$: A Sample of Little Red Dots in the JWST Extragalactic Legacy Fields

We present a sample of 341 "little red dots" (LRDs) spanning the redshift range $z\sim2-11$ using data from the CEERS, PRIMER, JADES, UNCOVER and NGDEEP surveys. Unlike past use of color indices to identify LRDs, we employ continuum slope fitting using shifting bandpasses to sample the same rest-frame emission blueward and redward of the Balmer break. This enables the detection of LRDs over a wider redshift range and with less contamination from galaxies with strong breaks that otherwise lack a rising red continuum. The redshift distribution of our sample increases at $z<8$ and then undergoes a rapid decline at $z\sim4.5$, which may tie the emergence of these sources to the inside-out growth that galaxies experience during this epoch. We find that LRDs are $\sim1$ dex more numerous than X-ray and UV selected AGN at z~5-7. Within our sample, we have identified the first two X-ray detected LRDs. An X-ray spectral analysis confirms that these AGN are moderately obscured with $\log\,(N_{\rm H}/{\rm cm}^{2}$) of $23.3^{+0.4}_{-1.3}$ and $22.72^{+0.13}_{-0.16}$. Our analysis reveals that reddened AGN emission dominates their rest-optical light, while the rest-UV originates from their host galaxies. We also present NIRSpec observations from the RUBIES survey of 17 LRDs that show broad emission lines consistent with AGN activity. The confirmed AGN fraction of our sample is 71\% for sources with F444W<26.5. In addition, we find three LRDs with blue-shifted Balmer absorption features in their spectra, suggesting an outflow of high-density, low-ionization gas from near the central engine of these faint, red AGN.

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arXiv2024

A Search for $z=5$ H$α$ and H$β+$[O III] Dual-Line Emitting Galaxies in the JWST CEERS Field: Implications for the AGN Abundance

The James Webb Space Telescope (JWST) has enabled us to uncover faint galaxies and active galactic nuclei (AGNs) in the early universe. Taking advantage of the unique filter combination used in the Cosmic Evolution Early Release Science Survey (CEERS) program, we perform an extensive photometric search of galaxies emitting strong H$β+$[O III] and H$α$ lines. The redshift range of the galaxies is limited to $5.03\leq z\leq 5.26$ by requiring photometric excesses in NIRCam's F277W and F410M images. A total of 261 H$β+$[O III] and H$α$ dual-line emitters are found over the absolute UV magnitude $-22\lesssim M_{\mathrm{UV}}\lesssim -17$, with a mean rest-frame equivalent width of 1010 A for H$β+$[O III] and 1040 A for H$α$. This population accounts for $\sim 40\%$ of the Lyman break galaxies at this redshift range. Intriguingly, there are 58 objects (22% of the whole sample) that exhibit compact morphology at the rest-UV or optical wavelength. With an assumption that these compact dual-line emitters are dominated by AGN, their AGN bolometric luminosities are in the range of $2\times 10^{43} \lesssim L_{\rm bol}/({\rm erg~s}^{-1})\lesssim 3\times 10^{44}$. Their number density is two orders of magnitude higher than the extrapolation from the UV-selected luminous quasars, which is in good agreement with previous JWST studies of broad-line AGNs, requiring a $\sim 10\%$ of the AGN duty cycle. Moreover, our dual-line emitter sample reaches the faint end of the H$α$ and [O III] luminosity functions down to $\lesssim 10^{42}~{\rm erg~s}^{-1}$. Spectroscopic follow-up observations are planned in an approved JWST Cycle 3 program, in which we aim to confirm their nature, characterize their black hole activity, and construct their mass distribution at $10^6\lesssim M_{\rm BH}/M_\odot \lesssim 10^8$.

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arXiv2024

Exploring Low-Mass Black Holes through Tidal Disruption Events in the Early Universe: Perspectives in the Era of JWST, RST, and LSST Surveys

The James Webb Space Telescope (JWST) has recently uncovered the presence of low-luminosity active galactic nuclei (AGNs) at $z=4-11$. Spectroscopic observations have provided estimates of the nuclear black hole (BH) masses for these sources, extending the low-mass boundary down to $M_{\rm BH} \sim 10^{6-7}~M_\odot$. Despite this breakthrough, the observed lowest mass of BHs is still $\gtrsim 1-2$ orders of magnitude heavier than the predicted mass range of their seed population, thereby leaving the initial mass distribution of massive BHs poorly constrained. In this paper, we focus on UV-to-optical (in rest frame) flares of stellar tidal disruption events (TDEs) embedded in low-luminosity AGNs as a tool to explore low-mass BH populations with $\lesssim 10^{4-6}~M_\odot$. We provide an estimate of the TDE rate over $z=4-11$ associated wth the properties of JWST-detected AGN host galaxies, and find that deep and wide survey programs with JWST and Roman Space Telescope (RST) can detect and identify TDEs up to $z\simeq 4-7$. The predicted detection numbers of TDEs at $z>4$ in one year are $N_{\rm TDE} \sim 2-10~(0.2-2)$ for the JADES-Medium (and COSMOS-Web) survey with JWST, and $N_{\rm TDE} \sim 2-10~(8-50)$ for the Deep (and Wide) tier of the High-latitude Time Domain Survey with RST. We further discuss the survey strategies to hunt for the transient high-redshift TDEs in wide-field surveys with RST, as well as a joint observation campaign with the Vera C. Rubin Observatory for enhancing the detection number. The high-redshift TDE search will give us a unique opportunity to probe the mass distribution of early BH populations.

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arXiv2024

Birth of Rapidly Spinning, Overmassive Black Holes in the Early Universe

The James Webb Space Telescope (JWST) has unveiled numerous massive black holes (BHs) in faint, broad-line active galactic nuclei (AGNs). The discovery highlights the presence of dust-reddened AGN populations, referred to as "little red dots (LRDs)", more abundant than X-ray selected AGNs, which are less influenced by obscuration. This finding indicates that the cosmic growth rate of BHs within this population does not decrease but rather increases at higher redshifts beyond $z\sim 6$. The BH accretion rate density deduced from their luminosity function is remarkably higher than that from other AGN surveys in X-ray and infrared bands. To align the cumulative mass density accreted to BHs with the observed BH mass density at $z\simeq 4-5$, as derived from the integration of the BH mass function, the radiative efficiency must be doubled from the canonical 10% value, achieving significance beyond the $>3σ$ confidence level. This suggests the presence of rapid spins with 96% of the maximum limit among these BHs, maintained by prolonged mass accretion instead of chaotic accretion with randomly oriented inflows. Moreover, we derive an upper bound for the stellar mass of galaxies hosting these LRDs, ensuring consistency with galaxy formation in the standard cosmological model, where the host stellar mass is limited by the available baryonic reservoir. Our analysis gives a lower bound for the BH-to-galaxy mass ratio that exceeds the typical value known in the nearby universe and aligns with that for JWST-detected unobscured AGNs. Accordingly, we propose a hypothesis that the dense, dust-rich environments within LRDs facilitate the emergence of rapidly spinning and overmassive BH populations during the epoch of reionization. This scenario predicts a potential association between relativistic jets and other high-energy phenomena with overmassive BHs in the early universe.

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arXiv2024

Towards a universal analytical model for Population III star formation: interplay between feedback and fragmentation

JWST has brought us new insights into Cosmic Dawn with tentative detection of the unique signatures of metal-free Population III (Pop III) stars, such as strong HeII emission, extremely blue UV spectrum, and enhanced nitrogen abundance. Self-consistent theoretical predictions of the formation rates, sites, and masses of Pop III stars are crucial for interpreting the observations, but are challenging due to complex physical processes operating over the large range of length scales involved. One solution is to combine analytical models for the small-scale star formation process with cosmological simulations that capture the large-scale physics such as structure formation, radiation backgrounds, and baryon-dark matter streaming motion that regulate the conditions of Pop III star formation. We build an analytical model to predict the final masses of Pop III stars/clusters from the properties of star-forming clouds, based on the key results of small-scale star formation simulations and stellar evolution models. Our model for the first time considers the interplay between feedback and fragmentation and covers different modes of Pop III star formation ranging from ordinary small ($\sim 10-2000\ \rm M_\odot$) clusters in molecular-cooling clouds to massive ($\gtrsim 10^{4}\ \rm M_\odot$) clusters containing supermassive ($\sim 10^{4}-3\times 10^{5}\ \rm M_\odot$) stars under violent collapse of atomic-cooling clouds. As an example, the model is applied to the Pop III star-forming clouds in the progenitors of typical haloes hosting high-$z$ luminous quasars, which shows that formation of Pop III massive clusters is common ($\sim 20-70\%$) in such biased ($\sim4σ$) regions, and the resulting heavy black hole seeds from supermassive stars can account for a significant fraction of observed luminous ($\gtrsim 10^{46}\ \rm erg\ s^{-1}$) quasars at $z\sim 6$.

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arXiv2024

Radiation hydrodynamical simulations of super-Eddington mass transfer and black hole growth in close binaries

Radiation-driven outflows play a crucial role in extracting mass and angular momentum from binary systems undergoing rapid mass transfer at super-Eddington rates. To study the mass transfer process from a massive donor star to a stellar-mass black hole (BH), we perform multi-dimensional radiation-hydrodynamical simulations that follow accretion flows from the first Lagrange point down to about a hundred times the Schwarzschild radius of the accreting BH. Our simulations reveal that rapid mass transfer occurring at over a thousand times the Eddington rate leads to significant mass loss from the accretion disk via radiation-driven outflows. Consequently, the inflow rates at the innermost radius are regulated by two orders of magnitude smaller than the transfer rates. We find that convective motions within the accretion disk drive outward energy and momentum transport, enhancing the radiation pressure in the outskirts of the disk and ultimately generating large-scale outflows with sufficient energy to leave the binary. Furthermore, we observe strong anisotropy in the outflows, which occur preferentially toward both the closest and furthest points from the donor star. However, when averaged over all directions, the specific angular momentum of the outflows is nearly comparable to the value predicted in the isotropic emission case. Based on our simulation results, we propose a formula that quantifies the mass growth rates on BHs and the mass loss rates from binaries due to radiation-driven outflows. This formula provides important implications for the binary evolution and the formation of merging binary BHs.

astro-ph.HE↗

arXiv2024

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 $z\sim 4$,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 $4\leq z\leq6$ across a wide UV luminosity range ($-29 10^8~M_\odot$, leading to mass saturation at $M_\bullet\gtrsim 10^{10}~M_\odot$. We predict the BH mass function down to $M_{\bullet}\sim 10^6~M_\odot$ for both unobscured and obscured quasar populations at $4\leq z \leq 11$, 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 $M_\bullet/M_{\star}\simeq5\times10^{-3}$ maintains a proximity to the relation through its evolution via moderate growth, while the population that begins to grow above the local relation accretes mass rapidly and becomes as overmassive as $M_\bullet/M_\star \sim 0.01-0.1$ by $z\sim 6$.

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arXiv2024

A SPectroscopic survey of biased halos In the Reionization Era (ASPIRE): Impact of Galaxies on the CGM Metal Enrichment at z > 6 Using the JWST and VLT

We characterize the multiphase circumgalactic medium and galaxy properties at z = 6.0-6.5 in four quasar fields from the James Webb Space Telescope A SPectroscopic survey of biased halos In the Reionization Era (ASPIRE) program. We use the Very Large Telescope/X-shooter spectra of quasar J0305-3150 to identify one new metal absorber at z = 6.2713 with multiple transitions (OI, MgI, FeII and CII). They are combined with the published absorbing systems in Davies et al. (2023a) at the same redshift range to form of a sample of nine metal absorbers at z = 6.03 to 6.49. We identify eight galaxies within 1000 km s$^{-1}$ and 350 kpc around the absorbing gas from the ASPIRE spectroscopic data, with their redshifts secured by [OIII]($λλ$4959, 5007) doublets and H$β$ emission lines. Our spectral energy distribution fitting indicates that the absorbing galaxies have stellar mass ranging from 10$^{7.2}$ to 10$^{8.8}M_{\odot}$ and metallicity between 0.02 and 0.4 solar. Notably, the z = 6.2713 system in the J0305-3150 field resides in a galaxy overdensity region, which contains two (tentatively) merging galaxies within 350 kpc and seven galaxies within 1 Mpc. We measure the relative abundances of $α$ elements to iron ([$α$/Fe]) and find that the CGM gas in the most overdense region exhibits a lower [$α$/Fe] ratio. Our modeling of the galaxy's chemical abundance favors a top-heavy stellar initial mass function, and hints that we may be witnessing the contribution of the first generation Population III stars to the CGM at the end of reionization epoch.

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arXiv2023

Black hole mass and Eddington ratio distributions of less-luminous quasars at $z\sim4$ in the Subaru Hyper Suprime-Cam Wide field

We investigate the black hole mass function (BHMF) and Eddington ratio distribution function (ERDF) of broad-line AGNs at z=4, based on a sample of 52 quasars with i<23.2 at 3.50 < z < 4.25 from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) S16A-Wide2 dataset, and 1,462 quasars with i<20.2 in the same redshift range from the Sloan Digital Sky Survey (SDSS) DR7 quasar catalog. Virial BH masses of quasars are estimated using the width of the CIV 1549Å line and the continuum luminosity at 1350Å. To obtain the intrinsic broad-line AGN BHMF and ERDF, we correct for the incompleteness in the low-mass and/or low-Eddington-ratio ranges caused by the flux-limited selection. The resulting BHMF is constrained down to $\log M_{\rm BH}/M_{\odot}\sim7.5$. In comparison with broad-line AGN BHMFs at z=2 in the literature, we find that the number density of massive SMBHs peaks at higher redshifts, consistent with the "down-sizing" evolutionary scenario. Additionally, the resulting ERDF shows a negative dependence on BH mass, suggesting more massive SMBHs tend to accrete at lower Eddington ratios at z=4. With the derived intrinsic broad-line AGN BHMF, we also evaluate the active fraction of broad-line AGNs among the entire SMBH population at z=4. The resulting active fraction may suggest a positive dependence on BH mass. Finally, we examine the time evolution of broad-line AGN BHMF between z=4 and 6 through solving the continuity equation. The results suggest that the broad-line AGN BHMFs at z=4-6 only show evolution in their normalization, but with no significant changes in their shape.

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arXiv2023

Detection of stellar light from quasar host galaxies at redshifts above 6

The detection of starlight from the host galaxies of quasars during the reionization epoch ($z>6$) has been elusive, even with deep HST observations. The current highest redshift quasar host detected, at $z=4.5$, required the magnifying effect of a foreground lensing galaxy. Low-luminosity quasars from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) mitigate the challenge of detecting their underlying, previously-undetected host galaxies. Here we report rest-frame optical images and spectroscopy of two HSC-SSP quasars at $z>6$ with JWST. Using NIRCam imaging at 3.6$μ$m and 1.5$μ$m and subtracting the light from the unresolved quasars, we find that the host galaxies are massive (stellar masses of $13\times$ and $3.4\times$ $10^{10}$ M$_{\odot}$, respectively), compact, and disk-like. NIRSpec medium-resolution spectroscopy shows stellar absorption lines in the more massive quasar, confirming the detection of the host. Velocity-broadened gas in the vicinity of these quasars enables measurements of their black hole masses ($1.4\times 10^9$ and $2.0\times$ $10^{8}$ M$_{\odot}$, respectively). Their location in the black hole mass - stellar mass plane is consistent with the distribution at low redshift, suggesting that the relation between black holes and their host galaxies was already in place less than a billion years after the Big Bang.

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arXiv2023

The Assembly of Black Hole Mass and Luminosity Functions of High-redshift Quasars via Multiple Accretion Episodes

The early evolution of the quasar luminosity function (QLF) and black hole mass function (BHMF) encodes key information on the physics determining the radiative and accretion processes of supermassive black holes (BHs) in high-$z$ quasars. Although the QLF shape has been constrained by recent observations, it remains challenging to develop a theoretical model that explains its redshift evolution associated with BH growth self-consistently. In this study, based on a semi-analytical model for the BH formation and growth, we construct the QLF and BHMF of the early BH population that experiences multiple accretion bursts, in each of which a constant Eddington ratio is assigned following a Schechter distribution function. Our best-fit model to reproduce the observed QLF and BHMF at $z\simeq 6$ suggests that several episodes of moderate super-Eddington accretion occur and each of them lasts for $τ\simeq 20-30$ Myr. The average duty cycle in super-Eddington phases is $\simeq 15\%$ for massive BHs that reach $\gtrsim 10^8~M_\odot$ by $z\simeq 6$, which is nearly twice that of the entire population. We also find that the observed Eddington-ratio distribution function is skewed to a log-normal shape owing to detection limits of quasar surveys. The predicted redshift evolution of the QLF and BHMF suggests a rapid decay of their number and mass density in a cosmic volume toward $z\gtrsim 6$. These results will be unveiled by future deep and wide surveys with the James Webb Space Telescope, Roman Space Telescope, and Euclid.

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arXiv2023

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 $M^*_{1450} = -25.60^{+0.40}_{-0.30}$, a characteristic density $Φ^* = 1.35^{+0.47}_{-0.30}$ Gpc$^{-3}$ mag$^{-1}$, and a bright-end slope $β= -3.34^{+0.49}_{-0.57}$, when the faint-end slope is fixed to $α= -1.2$ as observed at $z \le 6$. 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 $z \ge 6$. The estimated ionizing photon density, $10^{48.2 \pm 0.1}$ 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.

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arXiv2023

eROSITA Final Equatorial-Depth Survey (eFEDS): eFEDS X-ray view of WERGS Radio Galaxies selected by the Subaru/HSC and VLA/FIRST survey

We construct the eROSITA X-ray catalog of radio galaxies discovered by the WERGS survey that is made by the cross-matching of the wide-area Subaru/HSC optical survey and VLA/FIRST 1.4 GHz radio survey. We find 393 eROSITA detected radio galaxies in the 0.5--2 keV band in the eFEDS field covering 140~deg$^2$. Thanks to the wide and medium depth eFEDS X-ray survey, the sample contains the rare and most X-ray luminous radio galaxies above the knee of the X-ray luminosity function, spanning 44 22$. Those obscured and radio AGN reside in $0.4<z<3.2$, indicating that they are obscured counterparts of the radio-loud quasar, which are missed in the previous optical quasar surveys. By combining radio and X-ray luminosities, the jet production efficiency $η_\mathrm{jet}$ is investigated, and we find 14 sources with extremely high jet production efficiency at $η_\mathrm{jet}\approx1$. This high $η_\mathrm{jet}$ value might be a result of 1) the decreased radiation efficiency of $η_\mathrm{rad}<0.1$ due to the low accretion rate for those sources and/or 2) the boosting due to the decline of $L_\mathrm{bol}$ by a factor of 10--100 by keeping $P_\mathrm{jet}$ constant in the previous Myr, indicating the experience of the AGN feedback. Finally, inferring the BH masses from the stellar-mass, we find that X-ray luminous sources show the excess of the radio emission with respect to the value estimated from the fundamental plane. Such radio emission excess cannot be explained by the Doppler booming alone, and therefore disk-jet connection of X-ray luminous eFEDS-WERGS is fundamentally different from the conventional fundamental plane which mainly covers low accretion regime.

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arXiv2023

Hidden Little Monsters: Spectroscopic Identification of Low-Mass, Broad-Line AGN at $z>5$ with CEERS

We report on the discovery of two low-luminosity, broad-line AGN at $z>5$ identified using JWST NIRSpec spectroscopy from the CEERS Survey. We detect broad H$α$ emission from both sources, with FWHM of $2038\pm286$ and $1807\pm207$ km s$^{-1}$, resulting in black hole (BH) masses that are 1-2 dex below that of existing samples of luminous quasars at $z>5$. The first source, CEERS 1670 at $z=5.242$, is 2-3 dex fainter than known quasars at similar redshifts and was previously identified as a candidate low-luminosity AGN based on its rest-frame optical SED. We measure a BH mass of $M_{\rm BH}=1.3\pm0.4\times 10^{7}~M_{\odot}$, confirming that this AGN is powered by the least-massive BH known in the universe at the end of cosmic reionization. The second source, CEERS 3210 at $z=5.624$, is inferred to be a heavily obscured, broad-line AGN caught in a transition phase between a dust-obscured starburst and an unobscured quasar. We estimate its BH mass to be $M_{\rm BH}\simeq 0.9-4.7 \times 10^{7}~M_{\odot}$, depending on the level of dust obscuration assumed. We derive host stellar masses, $M_\star$, allowing us to place constraints on the BH-galaxy mass relationship in the lowest mass range yet probed in the early universe. The $M_{\rm BH}/M_\star$ ratio for CEERS 1670, in particular, is consistent with or higher than the empirical relationship seen in massive galaxies at $z=0$. We examine the emission-line ratios of both sources and find that their location on the BPT and OHNO diagrams is consistent with model predictions for low-metallicity AGN with $Z/Z_\odot \simeq 0.2-0.4$. The spectroscopic identification of low-luminosity, broad-line AGN at $z>5$ with $M_{\rm BH}\simeq 10^{7}~M_{\odot}$ demonstrates the capability of JWST to push BH masses closer to the range predicted for the BH seed population and provides a unique opportunity to study the early stages of BH-galaxy assembly.

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arXiv2022

Systematic Broad-band X-ray Study of super-Eddington Accretion onto Supermassive Black Holes. I. X-ray Continuum

We present the first systematic broad-band X-ray study of super-Eddington accretion onto SMBHs with simultaneous {\it NuSTAR} and {\it XMM-Newton} or {\it Swift}/XRT observations of a sample of 8 super-Eddington accreting AGN with Eddington ratio $1<λ_{\rm Edd}<426$. We find that the SEAMBHs show a steep primary continuum slope as expected for sources accreting in the super Eddington regime, mostly dominated by relativistic reflection. The Iron K$α$ emission lines of the sources of our sample show relativistic broadening. In addition the equivalent widths of the narrow components of the Iron K$α$ lines follow the 'X-ray Baldwin' effect, also known as the 'Iwasawa-Taniguchi' effect. We found a statistically significant correlation between the photon-index of the primary power-law and the Eddington ratio, consistent with past studies. Moreover, as expected for super-Eddington sources, the median value of the reflection fraction of the sources we analysed is a factor $\sim 2$ higher than the median reflection fraction value of the type\,1 AGN from the BASS sample. We are able to estimate the coronal temperature for three sources of our sample: Mrk\,382 ($kT_e=7.8$\,keV), PG\,0026+129 ($kT_e=39$\,keV) and IRAS\,04416+1215 ($kT_e=3$\,keV). Looking at the position of the SEAMBHs sources of our sample in the compactness-temperature diagram it appears that in super-Eddington AGN, as for lower Eddington ratio AGN, the X-ray corona is controlled by pair production and annihilation.

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arXiv2022

A Candidate of a Least-Massive Black Hole at the First 1.1 Billion Years of the Universe

We report a candidate of a low-luminosity active galactic nucleus (AGN) at z=5 that was selected from the first near-infrared images of the JWST CEERS project. This source, named CEERS-AGN-z5-1 at absolute 1450 A magnitude M1450=-19.5 +/- 0.3, was found via a visual selection of compact sources from a catalog of Lyman break galaxies at z>4, taking advantage of the superb spatial resolution of the JWST/NIRCam images. The 20 photometric data available from CFHT, HST, Spitzer, and JWST suggest that the continuum shape of this source is reminiscent of that for an unobscured AGN, and there is a clear color excess in the filters where the redshifted Hbeta+[OIII] and Halpha are covered. The estimated line luminosity is L_Hbeta+[OIII] =10^43.0 erg s-1 and L_Halpha =10^42.9 erg s-1 with the corresponding rest-frame equivalent width EW_{Hbeta+[OIII]} =1100 A and EW_Halpha =1600 A, respectively. Our SED fitting analysis favors the scenario that this object is either a strong broad-line emitter or even a super-Eddington accreting black hole (BH), although a possibility of an extremely young galaxy with moderate dust attenuation is not completely ruled out. The bolometric luminosity, L_bol=2.5 +/- 0.3 \times 10^44 erg s-1, is consistent with those of z<0.35 broad-line AGNs with M_BH = 10^6 M_sun accreting at the Eddington limit. This new AGN population at the first 1.1 billion years of the universe may close the gap between the observed BH mass range at high redshift and that of BH seeds. Spectroscopic confirmation is awaited to secure the redshift and its AGN nature.

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