arXiv subjects

Keiichi Wada

Publications and source records attributed to Keiichi Wada.

At least 19 recordsLinked to original sources

arXiv2026

Suppression of Radiative Cooling in Galaxy Cluster Cores by the Combination of AGN Heating and Sloshing

Recent XRISM observations suggest that gas mixing induced by sloshing contributes to core heating. We systematically investigate the suppression of cooling flows in galaxy cluster cool cores through three-dimensional hydrodynamic simulations that incorporate both sloshing-driven turbulence and active galactic nucleus (AGN) heating. The AGN heating is modeled as thermal energy input that mimics cosmic-ray heating. Sloshing is represented by simple waves with amplitudes \alpha = 0, 0.15, and 0.3 times the sound speed and wavelengths \lambda = 200, 1000, and 2000 kpc. We evolve each model from an isothermal initial condition to t = 8 Gyr. Without AGN heating, sloshing suppresses cooling, but it cannot stop it completely unless the core is fully disrupted. Longer wavelengths promote deeper mixing and greater suppression. Sloshing can cause cooler gas to move more quickly than hotter gas. This phenomenon has been observed in a few clusters by XRISM. When AGN heating is included, the dense central gas is heated efficiently, substantially delaying or preventing the onset of a cooling flow. However, for intermediate wave lengths, sloshing can displace the densest gas away from the AGN heating zone, reducing the feedback effect and paradoxically enhancing net cooling relative to the wave-free case. These results highlight a non-trivial coupling between sloshing and AGN feedback, with implications for interpreting XRISM velocity and temperature maps of cool-core clusters.

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arXiv2026

Vaulting the barrier: An intrinsic mechanism to fuel the gas beyond the nuclear ring into the central region of barred galaxies

Gas delivery to galactic centers powers nuclear starbursts and active galactic nuclei (AGNs), yet bar-driven inflow is generally expected to stall in a nuclear ring a few hundred parsecs across. Using three-dimensional Lagrangian hydrodynamic simulations in a fixed barred potential, we identify a bypass channel in which a fraction of the inflowing gas acquires vertical momentum, vaults across the ring, and reaches the inner few tens of parsecs. This pathway is absent in two-dimensional calculations, which instead predict long-lived stagnation at the ring. We find that the circumnuclear material within $\sim 50$ pc originates from gas initially located outside the ring ($\gtrsim 300$ pc), rather than from secondary inflow out of the ring itself. Successful delivery requires both a sufficiently large vertical excursion, $|z| \sim 100$ pc before encountering the ring, and substantial loss of azimuthal angular momentum $L_z$. The resulting inflow is organized rather than chaotic: center-reaching trajectories are confined to a limited spatial region set by the scale height of the ring gas. Most bar-driven gas still accumulates near the resonance and fuels star formation in the nuclear ring, but the vaulting stream selects a modest yet sufficient fraction that penetrates to the circumnuclear disk. These results suggest that intrinsically three-dimensional gas motions help link nuclear starbursts, AGN fueling, and the frequent misalignment of nuclear disks with respect to their host galaxies.

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arXiv2026

Multiphase Gas Structure in the Circumnuclear Region of NGC 5506 Observed with ALMA

We present a study of the multiphase gas structure and kinematics of the circumnuclear disk (CND) of NGC 5506, a nearby edge-on Seyfert galaxy, at a spatial resolution of $\sim20$ pc. Observations of [C I](1-0), CO(3-2), and HCO$^{+}$(4-3) obtained with the Atacama Large Millimeter/submillimeter Array reveal the CND dominated by rotational motion on scales of several hundred parsecs. No significant differences in geometrical thickness or velocity structure are found between [C I](1-0) and CO(3-2) across the CND, whereas HCO$^{+}$(4-3) emission is more concentrated toward the disk plane. The ratio of velocity dispersion to rotational velocity, a proxy for disk scale height-to-radius ratio, is high ($\gtrsim0.9$) in the central region ($\lesssim30$ pc) for both [C I](1-0) and CO(3-2), indicating geometrically thick structures in both tracers. Regions where the [C I](1-0)/CO(3-2) ratio exceeds the CND average are spatially correlated with the [O III]$\lambda$5007 bicone observed with the Hubble Space Telescope, suggesting that CO is preferentially dissociated by the AGN-driven biconical ionized outflow. The observed CND scale height and velocity dispersions traced by [C I](1-0) and CO(3-2) are consistent with a model in which supernova-driven turbulence provides the vertical support for the CND.

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arXiv2026

Unification Model of Active Galactic Nuclei by Photoionization Equilibrium Calculation Based on Radiative Hydrodynamic Simulations

To investigate the origin of the dependence of the covering factor on the Eddington ratio suggested by X-ray observations, we examined the angular distribution of HI and HII based on two-dimensional radiative hydrodynamic simulations. To calculate the Compton-thin covering factor $C_{22}$ and Compton-thick covering factor $C_{24}$ of HI alone, we performed one-dimensional photoionization equilibrium calculations with the XSTAR code based on radiative hydrodynamic simulations. The results obtained are as follows. (1) The Compton-thin covering factor $C_{22}$ of HI and HII is independent of the Eddington ratio and is approximately $70\%$, while $C_{22}$ of HI alone is also independent of the Eddington ratio and is approximately $30\%$. (2) The Compton-thick covering factor $C_{24}$ of HI has the same value as $C_{22}$ of HI. (3) Our $C_{24}$ is consistent with that obtained from X-ray observations. (4) Our $C_{22}$ agrees with that obtained from X-ray observations in a high Eddington ratio, while our $C_{22}$ is smaller than that from X-ray observations in a low Eddington ratio. (5) To explain the difference between $C_{22}$ obtained from theoretical calculations and that inferred from X-ray observations, a Compton-thin gas is required in regions extending at least $10~\mathrm{pc}$ beyond the current computational regions.

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arXiv2026

Broad line regions behind haze: Intrinsic shape of Br$\gamma$ line and its origin in a type-1 Seyfert galaxy

The broad-line region (BLR) of active galactic nuclei (AGN) is an essential component, yet its small size keeps its origin, structure, and kinematics uncertain. Infrared interferometry with VLTI/GRAVITY is now resolving BLR-scale emission, with data for NGC 3783 consistent with a rotating, geometrically thick configuration. However, the processes shaping the spectra remain poorly constrained, and the cloud models are tuned phenomenologically rather than derived from first-principles predictions. We address this by coupling three-dimensional radiation-hydrodynamic (RHD) simulations of gas around a supermassive black hole with radiative-transfer calculations using Cloudy, comparing the results to the SINFONI Br$\gamma$ profile of NGC 3783. We find that Br$\gamma$ arises from ionized gas in the surface of the rotating thin disk, with electron temperatures of approximately $T_e \approx 10^4$ K and number densities of $n_e \approx 10^8-10^{11}$ cm$^{-3}$. However, the intrinsic line profile produced by the RHD kinematics is narrower than observed and displays substructure. An approximate treatment of the electron scattering suggests that scattering in surrounding diffuse ionized gas significantly broadens and smooths the intrinsic Br$\gamma$ profile, making it consistent with the observed profile. This scattering medium has an electron temperature of $10^4 - 10^5$ K, and a number density of $n \lesssim 10^8$ cm$^{-3}$. Although a best-fit viewing angle of $\approx 15$ deg is suggested, the scattered line is notably less sensitive to inclination than the intrinsic line. The observed BLR profiles may be understood as the intrinsic emission viewed through an electron-scattering haze, such that some spectral detail is plausibly redistributed rather than seen directly.

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arXiv2026

The Origin of Spin-Alignment of Dark Matter Subhalos

Subhalo spin is essential for modeling galaxy formation and controlling systematic uncertainties in intrinsic alignment (IA) studies. However, the physical mechanisms governing subhalo spin acquisition within the tidal environments of host halos remain poorly understood. In this work, we investigate the alignment between subhalo and host halo spins using the high-resolution cosmological $N$-body simulation, Shin-Uchuu. We find that the spin alignment between subhalos and host halos becomes increasingly pronounced toward the central regions. Our analysis reveals that subhalos typically acquire spin in the same direction as their orbital angular momentum. Since the orbital angular momentum of most subhalos is aligned with the host halo spin, an overall alignment between subhalo and host spins emerges. When classified by orbital orientation, however, subhalo spins in the inner regions are found to be oriented perpendicularly or anti-parallel to the host spin for polar and retrograde orbits, respectively. These results provide strong evidence that subhalo spins are acquired through torques exerted by the tidal field of the host halo. Furthermore, we demonstrate that the mass ratio and the radial distance from the host center are the primary parameters governing subhalo spin alignment, while the dependence on the accretion redshift plays a less significant role compared to the radial distance and mass ratio.

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arXiv2025

Dynamical Mechanism of Episodic Gas Accretion to the Central Region of Spiral Galaxies

We performed \textit{N}-body/SPH simulations of isolated spiral galaxies with various bulge-to-disk mass ratios ($M_{\rm bulge}/M_{\rm disk}$) from 0.02 to 0.2 to investigate mass transport from galactic scales (10 kpc) down to circumnuclear disk scales ($\lesssim$ 100 pc). Our analysis revealed these main findings, (1) Gravitational torque from stellar spiral arms causes gas accretion with $\sim1$ $M_\odot$ ${\rm yr}^{-1}$ along the gas spiral arms from a few kpc to a few 100 pc scale. The density of accreting gas is a few 100 ${\rm cm^{-3}}$, comparable to the gas arms. The pressure gradient force is over an order of magnitude weaker than the stellar gravitational torque. (2) Gravitational torque from barred structure causes episodic gas clump accretion with $\sim1$ $M_\odot$ ${\rm yr}^{-1}$ on timescales of 10 Myr from kpc to a few 100 pc scale. The densities of these clumps exceed 700 ${\rm cm^{-3}}$, and this accretion occurs along elliptical orbits with a delayed phase relative to the bar potential \citep{wada1994}. (3) Episodic gas clumpy accretion is important for galactic center instability, confirmed by $M_{\rm bulge}/M_{\rm disk}$ = 0.02 but not by $M_{\rm bulge}/M_{\rm disk}$ = 0.1 and 0.2. This difference occurs because in the bulge-dominated potentials, bar instability is suppressed, and rapid gas clumps accretion does not occur. These findings suggest that gas clump accretion events driven by bars could be a source of high-density gas to the galactic center of the spiral galaxy, potentially promoting temporary activity in the galactic center.

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arXiv2025

On the origin of short-lived cocoon in 3C84: powered by tidal disruption events ?

We evaluated the jet power and the density of ambient matter in 3C 84 by using the momentum balance along the jet axis and the transonic condition for the cocoons observed at two different scales (approximately 1 and 6 parsec scales). For the inner cocoon, we precisely determined the ratio of jet power to ambient density $L_{\rm j}/n_{\rm a}$ to be $(0.3-0.7)\times 10^{43}\,{\rm erg}\,{\rm s}^{-1}\,{\rm cm}^3$. Similarly, for the outer cocoon, we found that this value is more than an order of magnitude larger at $(0.9-3.7)\times 10^{44}\,{\rm erg}\,{\rm s}^{-1}\,{\rm cm}^3$. This indicates that the outer cocoon is formed by a powerful jet that propagates through an ambient density of $20-300\,{\rm cm}^{-3}$ with a jet power of $10^{45-46.5}\,{\rm erg}\,{\rm s}^{-1}$. On the other hand, the inner cocoon is formed by a weaker jet with a power of $10^{43-44}\,{\rm erg}\,{\rm s}^{-1}$, propagating through a relatively low-density environment of $6-20\, {\rm cm}^{-3}$. These results suggest that: 1) with respect to the difference in $n_{\rm a}$, it appears to support the hypothesis that the inner cocoon, recently formed about 10 years ago, is expanding in the low-density cocoon created by the jet emitted about 25-50 years ago. 2) to achieve the short-lived and high $L_{\rm j}$ that generated the outer cocoon, a large mass accretion rate must be required over a short period to activate the jet. These may imply the extreme accretion event driven by the tidal disruption events (TDEs) of massive stars and/or the disk instability.

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arXiv2025

X-Ray Radiative Transfer Calculation Based on a Physics-based Model of the Sub-parsec-scale Gases around an Active Galactic Nucleus and Its Application to NGC 3783

Although the X-ray spectra of Seyfert 1 galaxies exhibit absorption lines of He-like iron and H-like iron at blue-shifted velocities of approximately $500 \ \mathrm{km} \ \mathrm{s}^{-1}$, the physical origin of these absorption lines remains uncertain. In this study, we performed X-ray radiative transfer based on the sub-parsec-scale thermally driven outflows. The initial step involved calculating the photoionization equilibrium using the Cloudy code, which is based on three-dimensional radiative hydrodynamic simulations. Subsequently, X-ray radiative transfer was performed using the Monte Carlo simulation for astrophysics and cosmology code. Our findings indicate that when the angle of inclination ranges from $55 \ \mathrm{degrees}$ to $65 \ \mathrm{degrees}$, the transmitted component of the X-ray spectrum displays absorption lines of He-like and H-like iron, exhibiting a blue shift of approximately $500 \ \mathrm{km} \ \mathrm{s}^{-1}$. The results suggest that the absorption lines are generated by a photoionized gas within $0.005 \ \mathrm{pc}$. Additionally, the results indicate that the scattered component of the X-ray spectrum exhibits emission lines originating from neutral iron fluorescence, He-like iron, and H-like iron. The emission lines are broadened by approximately $7000 \ \mathrm{km} \ \mathrm{s}^{-1}$ due to the Keplerian rotation. Furthermore, the model reproduced the H-like iron and H-like iron absorption lines in NGC 3783 observed by the Chandra High Energy Transmission Grating.

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arXiv2024

Multiphase Gas Nature in the Sub-parsec Region of the Active Galactic Nuclei. III. Eddington Ratio Dependence on the Structures of Dusty and Dust-free Outflows

We investigated the influence of the Eddington ratio on sub-parsec-scale outflows in active galactic nuclei (AGNs) with supermassive black holes (SMBHs) masses of $10^7$ M$_{\odot}$ using two-dimensional radiation hydrodynamics simulations. When the range of Eddington ratio, $\gamma_{\rm Edd} > 10^{-3}$, the radiation force exceeds the gas pressure, leading to stronger outflows and larger dust sublimation radius. Although the sub-parsec-scale outflows is a time-dependence phenomena, our simulations demonstrated that the radial distributions can be well explained by the steady solutions of the spherically symmetric stellar winds. The dynamic structure of sub-parsec-scale outflows is influenced by the dust sublimation radius and the critical radii determined by the dynamical equilibrium condition. Although significantly affecting the outflow velocity, the Eddington ratio exerts minimal effects on temperature and number density distribution. Furthermore, our analytical solutions highlight the importance of the dust sublimation scale as a crucial determinant of terminal velocity and column density in dusty outflows. Through comparisons of our numerical model with the obscuring fraction observed in nearby AGNs, we revealed insights into the Eddington ratio dependence and the tendency towards the large obscuring fraction of the dusty and dust-free gases. The analytical solutions are expected to facilitate an understanding of the dynamical structure and radiation structures along the line of sight and their viewing angles from observations of ionized outflows.

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arXiv2024

Intrinsic line profiles for X-ray fluorescent lines in SKIRT

We included the intrinsic line profiles of the strongest fluorescent lines in the X-ray radiative transfer code SKIRT to model the cold-gas structure and kinematics based on high-resolution line observations from XRISM/Resolve and Athena/X-IFU. The intrinsic line profiles of the Ka and Kb lines of Cr, Mn, Fe, Co, Ni, and Cu were implemented based on a multi-Lorentzian parameterisation and line energies are sampled from these Lorentzian components during the radiative transfer routine. In the optically thin regime, the SKIRT results match the intrinsic line profiles as measured in the laboratory. With a more complex 3D model that also includes kinematics, we find that the intrinsic line profiles are broadened and shifted to an extent that will be detectable with XRISM/Resolve; this model also demonstrates the importance of the intrinsic line shapes for constraining kinematics. We find that observed line profiles directly trace the cold-gas kinematics, without any additional radiative transfer effects. With the advent of the first XRISM/Resolve data, this update to the X-ray radiative transfer framework of SKIRT is timely and provides a unique tool for constraining the velocity structure of cold gas from X-ray microcalorimeter spectra.

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arXiv2024

A spectacular galactic scale magnetohydrodynamic powered wind in ESO 320-G030

How galaxies regulate nuclear growth through gas accretion by supermassive black holes (SMBHs) is one of the most fundamental questions in galaxy evolution. One potential way to regulate nuclear growth is through a galactic wind that removes gas from the nucleus. It is unclear whether galactic winds are powered by jets, mechanical winds, radiation, or via magnetohydrodynamic (MHD) processes. Compact obscured nuclei (CONs) represent a significant phase of galactic nuclear growth. These galaxies hide growing SMBHs or unusual starbursts in their very opaque, extremely compact (r $<$ 100 pc) centres. They are found in approximately 30 % of the luminous and ultra-luminous infrared galaxy (LIRG and ULIRG) population. Here, we present high-resolution ALMA observations ($\sim$30 mas, $\sim$5 pc) of ground-state and vibrationally excited HCN towards ESO 320-G030 (IRAS 11506-3851). ESO 320-G030 is an isolated luminous infrared galaxy known to host a compact obscured nucleus and a kiloparsec-scale molecular wind. Our analysis of these high-resolution observations excludes the possibility of a starburst-driven wind, a mechanically or energy driven active galactic nucleus (AGN) wind, and exposes a molecular MDH wind. These results imply that the nuclear evolution of galaxies and the growth of SMBHs are similar to the growth of hot cores or protostars where gravitational collapse of the nuclear torus drives a MHD wind. These results mean galaxies are capable, in part, of regulating the evolution of their nuclei without feedback.

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arXiv2024

Circumnuclear Multi-phase Gas in the Circinus Galaxy. VI. Detectability of Molecular Inflow and Atomic Outflow

Recent submillimeter observations have revealed signs of pc-scale molecular inflow and atomic outflow in the nearest Seyfert 2 galaxy, the Circinus galaxy. To verify the gas kinematics suggested by these observations, we performed molecular and atomic line transfer calculations based on a physics-based 3D radiation-hydrodynamic model, which has been compared with multi-wavelength observations in this paper series. The major axis position-velocity diagram (PVD) of CO(3-2) reproduces the observed faint emission at the systemic velocity, and our calculations confirm that this component originates from failed winds falling back to the disk plane. The minor-axis PVD of [CI]($^3P_1$-$^3P_0$), when created using only the gas with positive radial velocities, presents a sign of blue- and redshifted offset peaks similar to those in the observation, suggesting that the observed peaks indeed originate from the outflow, but that the model may lack outflows as strong as those in the Circinus galaxy. Similar to the observed HCN(3-2), the similar dense gas tracer HCO$^+$(3-2) can exhibit nuclear spectra with inverse P-Cygni profiles with $\sim$0.5 pc beams, but the line shape is azimuthally dependent. The corresponding continuum absorbers are inflowing clumps at 5-10 pc from the center. To detect significant absorption with a high probability, the inclination must be fairly edge-on ($\gtrsim$85$^\circ$), and the beam size must be small ($\lesssim$1 pc). These results suggest that HCN or HCO$^+$ and [CI] lines are effective for observing pc-scale inflows and outflows, respectively.

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arXiv2024

[O IV] and [Ne V]-weak AGNs Hidden by Compton-thick Material in Late Mergers

We study "buried" active galactic nuclei (AGNs) almost fully covered by circumnuclear material in ultra-/luminous infrared galaxies (U/LIRGs), which show weak ionized lines from narrow line regions. Employing an indicator of [O IV] 25.89-um or [Ne V] 14.32-um line to 12-um AGN luminosity ratio, we find 17 buried AGN candidates that are [O IV]-weak ($L_{\rm [O\,IV]}$/$L_{\rm 12,AGN} \leq -$3.0) or [Ne V]-weak ($L_{\rm [Ne\,V]}$/$L_{\rm 12,AGN} \leq -$3.4) among 30 AGNs in local U/LIRGs. For the [O IV]-weak AGNs, we estimate their covering fractions of Compton-thick (CT; $N_{\rm H} \geq 10^{24}$ cm$^{-2}$) material with an X-ray clumpy torus model to be $f^{\rm (spec)}_{\rm CT} = 0.55\pm0.19$ on average. This value is consistent with the fraction of CT AGNs ($f^{\rm (stat)}_{\rm CT} = 53\pm12$%) among the [O IV]-weak AGNs in U/LIRGs and much larger than that in Swift/BAT AGNs ($23\pm6$%). The fraction of [O IV]-weak AGNs increases from $27^{+13}_{-10}$% (early) to $66^{+10}_{-12}$% (late mergers). Similar results are obtained with the [Ne V] line. The [O IV] or [Ne V]-weak AGNs in late mergers show larger $N_{\rm H}$ and Eddington ratios ($λ_{\rm Edd}$) than those of the Swift/BAT AGNs, and the largest $N_{\rm H}$ is $\gtrsim$10$^{25}$ cm$^{-2}$ at ${\log}λ_{\rm Edd} \sim -$1, close to the effective Eddington limit for CT material. These suggest that (1) the circumnuclear material in buried AGNs is regulated by the radiation force from high-$λ_{\rm Edd}$ AGNs on the CT obscurers, and (2) their dense material with large $f^{\rm (spec)}_{\rm CT}$ ($\sim 0.5 \pm 0.1$) in U/LIRGs is a likely cause of a unique structure of buried AGNs, whose amount of material may be maintained through merger-induced supply from their host galaxies.

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arXiv2023

Apparent effect of dust extinction on the observed outflow velocity of ionized gas in galaxy mergers

In this study, we examine photoionization outflows during the late stages of galaxy mergers, with a specific focus on the relation between observed velocity of outflowing gas and the apparent effects of dust extinction. We used the N-body/smoothed particle hydrodynamics (SPH) code ASURA for galaxy merger simulations. These simulations concentrated on identical galaxy mergers featuring supermassive black holes (SMBHs) of 10$^8$ M$_\odot$ and gas fractions of 30\% and 10 \%. From the simulation data, we derived velocity and velocity dispersion diagrams for the AGN-driven ionized outflowing gas. Our findings show that high-velocity outflows with velocity dispersions of 500 km s$^{-1}$ or greater can be observed in the late stages of galactic mergers. Particularly, in buried AGNs, both the luminosity-weighted outflow velocity and velocity dispersion increase owing to the apparent effects of dust extinction. Owing to these effects, the velocity--velocity dispersion diagrams display a noticeable blue-shifted tilt in models with higher gas fractions. Crucially, this tilt is not influenced by the AGN luminosity but emerges from the observational impacts of dust extinction. Our results imply that the observed high-velocity \OIII outflow exceeding 1000 km s$^{-1}$ in buried AGNs may be linked to the dust extinction that occurs during the late stages of gas-rich galaxy mergers.

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arXiv2023

Supermassive black hole feeding and feedback observed on sub-parsec scales

Active galaxies contain a supermassive black hole at their center, which grows by accreting matter from the surrounding galaxy. The accretion process in the central ~10 parsecs has not been directly resolved in previous observations, due to the small apparent angular sizes involved. We observed the active nucleus of the Circinus Galaxy using sub-millimeter interferometry. A dense inflow of molecular gas is evident on sub-parsec scales. We calculate that less than 3% of this inflow is accreted by the black hole, with the rest being ejected by multiphase outflows, providing feedback to the host galaxy. The observations also reveal a dense gas disk surrounding the inflow; the disk is gravitationally unstable which drives the accretion into the central ~1 parsec.

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arXiv2023

Circumnuclear Multi-phase Gas in the Circinus Galaxy. V. The Origin of the X-Ray Polarization in the Circinus Galaxy

The Imaging X-ray Polarimetry Explorer (IXPE) detected X-ray polarization in the nearest Seyfert 2 galaxy, the Circinus galaxy, for the first time. To reproduce the IXPE results, we computed the degree of polarization based on two types of radiative hydrodynamic simulations: a parsec-scale three-dimensional model and a sub-parsec-scale axisymmetric model with a higher spatial resolution. In a series of papers, we confirmed that these models naturally explain the multi-wavelength observations of the Circinus galaxy from radio to X-rays. We used a Monte Carlo Simulation for Astrophysics and Cosmology code to compute the linear polarization of continuum emission. We found that the degree of polarization based on the parsec-scale radiation-driven fountain model was smaller than that observed with the IXPE. The degree of polarization based on the sub-parsec-scale model depends on the hydrogen number density of the disk ($d$), and the degree of polarization obtained from our simulation is consistent with that observed with the IXPE in the case of $\log d/\mathrm{cm}^{-3} \geq 13$. We investigate where the photons are Compton scattered and imply that the origin of the X-ray polarization in the Circinus galaxy is the outflow inside $0.01 \ \mathrm{pc}$. In this case, the degree of polarization may change over a timescale of approximately ten years.

astro-ph.HE↗

arXiv2023

Multi-phase gas nature in the sub-pc region of the active galactic nuclei II: Possible origins of the changing-state AGNs

Multi-wavelength observations of active galactic nuclei (AGNs) often reveal various time scales of variability. Among these phenomena, "changing-look AGNs" are extreme cases where broad emission lines become faint/bright or even disappear/emerge between multi-epoch observations, providing crucial information about AGN internal structures. We here focus on "changing-state" AGNs, specifically investigating the transition of optical spectra over years to tens of years. Based on the axisymmetric radiation-hydrodynamical simulations (Paper I) for the gas dynamics within the dust-sublimation radius, we investigate the spectral properties of ionized gas exposed to the radiation from an AGN with a 10^7 Msun supermassive black hole. We find significant time-dependent variations in the Balmer emission lines by utilizing post-process pseudo-three-dimensional calculations and the spectral synthesis code CLOUDY. The equivalent width of Halpha and Hbeta changes by a factor of 3, or the emission lines even disappear during 30 years for the same viewing angle. The time-dependent behaviour arises primarily from gas dynamics, particularly the formation of non-steady, radiation-driven outflows within the innermost region of the disc (r <10^-3 pc). The intricate interplay between non-spherical radiation sources at the core of AGNs and the dynamic behavior of gas within the dust sublimation radius gives rise to radiation-driven outflows. This non-steady outflow potentially contributes to the observed variability in Balmer line emissions over multi-year timescales in certain AGNs.

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