Active Galactic Nuclei Environment Research Articles

The Fe Kα emission line in water megamaser galaxies

Water maser emissions at around 22 GHz from extragalactic sources are significant in astrophysics, offering a unique means to measure cosmological distances and thus constrain the Hubble parameter with high precision. They also provide the most precise method for measuring black hole masses beyond the Milky Way. The challenge of their limited detection has spurred researchers to seek clues for better characterisation. Despite these efforts, the physical environments in which these masers form still need to be clarified. Consequently, statistical studies have been extensively employed to enhance our understanding of these environments. A sample of 32 Active Galactic Nuclei (AGN)-powered water megamaser galaxies and their Fe Kα emission line at 6.4 keV data was compiled and statistically analysed to investigate any correlation between the two parameters for the first time. A Spearman correlation analysis shows a statistically significant inverse correlation between the maser luminosity (L22 GHz) and the Fe Kα luminosity (LFe Kα). Specifically, Spearman’s coefficient ρ was −0.48, with a p-value = 5 × 10−3. This anti-correlation suggests that in luminous X-ray galaxies, AGN feedback may heat the circumnuclear clouds, leading to more substantial maser emissions, as it favours warm gas while weakening Fe Kα, which is associated with cold, dense gas environments. Additionally, the obscuration and orientation of the AGN’s circumnuclear material play a significant role, with denser, edge-on tori potentially favouring maser production while diluting Fe Kα emission. The lack of significant correlations between L22 GHz and other AGN indicators, such as absorption-corrected L2−10 keV and L12 μ, combined with the previously found anti-correlation between LFe Kα and these indicators suggest that the observed L22 GHz-LFe Kα anti-correlation is driven by localised effects within the masing region rather than broader AGN properties. These findings highlight the need for further research with larger samples and more detailed modelling to disentangle the various factors influencing maser activity in AGN environments and validate the anticorrelation’s significance, as sources of uncertainty might still have affected the results.

Separating the Spectral Counterparts in NGC 1275/Perseus Cluster in X-Rays

We present a model-independent method for separating the spectral counterparts of the active galactic nucleus (AGN) NGC 1275 from the surrounding emission of the Perseus cluster, as observed by Suzaku/XIS cameras. The Perseus cluster emission extends to higher energies than typically observed in AGN environments, reaching up to 9–10 keV. This necessitates precise separation of AGN and cluster spectra. To circumvent the degeneracy arising from numerous spectral fitting parameters, including elemental abundances, thermal and Compton emissions from the nucleus, and spectral parameters of the jet synchrotron self-Compton/inverse Compton emissions, we avoid traditional spectral fitting methods. Instead, we leverage spatial resolution and employ a double background subtraction approach. We apply this procedure to the complete set of Suzaku/XIS observational data for NGC 1275, resulting in cleaned spectra and a light curve of the AGN emission in this system. To demonstrate the applicability of our method, we also utilize the available XMM-Newton/EPIC data.

The outflowing ionised gas of I Zw 1 observed by HST COS

Aims. We present an analysis of the Hubble Space Telescope Cosmic Origins Spectrograph spectrum of I Zw 1 aiming to probe the absorbing medium associated with the active galactic nucleus (AGN). Methods. We fitted the emission spectrum and performed spectral analysis of the identified absorption features to derive the corresponding ionic column densities and covering fractions of the associated outflows. We employed photoionisation modelling to constrain the total column density and the ionisation parameter of four detected kinematic components. By investigating the implications of the results together with the observed kinematic properties of both emission and absorption features, we derived constraints on the structure and geometry of the absorbing medium in the AGN environment. Results. We find and characterise absorption line systems from outflowing ionised gas in four distinct kinematic components, located at −60, −280, −1950, and −2900 km s−1 with respect to the source rest frame. While the two slower outflows are consistent with a full covering of the underlying radiation source, the well-constrained doublet line ratios of the faster two, higher column density, outflows suggest partial covering, with a covering fraction of Cf ∼ 0.4. The faster outflows show also line-locking in the N V doublet, a signature of acceleration via line absorption. This makes I Zw 1 possibly the closest object that shows evidence for hosting line-driven winds. The observed −1950 km s−1 absorption is likely due to the same gas as an X-ray warm absorber. Furthermore, the behaviour in UV and X-ray bands implies that this outflow has a clumpy structure. We find that the highly asymmetric broad emission lines in I Zw 1, indicative of a collimated, outflowing broad line region, are covered by the absorbing gas. Finally, the strongest UV–X-ray absorber may be connected to some of the blueshifted line emission, indicative of a more spatially extended structure of this ionised medium.

A Self-consistent Treatment of the Line-driving Radiation Force for Active Galactic Nuclei Outflows: New Prescriptions for Simulations

Flows driven by photons have been studied for almost a century, and a quantitative description of the radiative forces on atoms and ions is important for understanding a wide variety of systems with outflows and accretion disks, such as active galactic nuclei (AGN). Quantifying the associated forces is crucial to determining how these outflows enable interactive mechanisms within these environments, such as AGN feedback. The total number of spectral lines in any given ion of the outflow material must be tabulated in order to give a complete characterization of this force. Here, we provide calculations of the dimensionless line force multiplier for AGN environments. For a wide array of representative AGN sources, we explicitly calculate the photoionization balance at the proposed wind-launching region above the accretion disk, compute the strength of the line-driving force on the gas, and revisit and formalize the role of the commonly used ionization parameter ξ in ultimately determining the line-driving force. We perform these computations and analyses for a variety of AGN central source properties, such as black hole mass, initial wind velocity, and number density. We find that, while useful, the ionization parameter provides an incomplete description of the overall ionization state of the outflow material. We use these findings to provide an updated method for calculating the strength of the radiative line-driving using both the X-ray spectral index Γ X and the ionization parameter.

Fast Radio Bursts in the disks of Active Galactic Nuclei

Abstract Fast radio bursts (FRBs) are luminous millisecond-duration radio pulses with extragalactic origin, which were discovered more than a decade ago. Despite the numerous samples, the physical origin of FRBs remains poorly understood. FRBs have been thought to originate from young magnetars or accreting compact objects (COs). Massive stars or COs are predicted to be embedded in the accretion disks of active galactic nuclei (AGNs). The dense disk absorbs FRBs severely, making them difficult to observe. However, progenitors’ ejecta or outflow feedback from the accreting COs interact with the disk material to form a cavity. The existence of the cavity can reduce the absorption by the dense disk materials, making FRBs escape. Here we investigate the production and propagation of FRBs in AGN disks and find that the AGN environments lead to the following unique observational properties, which can be verified in future observation. First, the dense material in the disk can cause large dispersion measure (DM) and rotation measure (RM). Second, the toroidal magnetic field in the AGN disk can cause Faraday conversion. Third, during the shock breakout, DM and RM show non-power-law evolution patterns over time. Fourth, for accreting-powered models, higher accretion rates lead to more bright bursts in AGN disks, accounting for up to 1 % of total bright repeating FRBs.

An Extended Lyα Outflow from a Radio Galaxy at z = 3.7?

Spatially resolved observations of active galactic nuclei (AGN) host galaxies undergoing feedback processes are one of the most relevant avenues through which galactic evolution can be studied, given the long-lasting effects AGN feedback has on gas reservoirs, star formation, and AGN environments at all scales. Within this context, we report results from Very Large Telescope/MUSE integral field optical spectroscopy of TN J1049-1258, one of the most powerful radio sources known, at a redshift of 3.7. We detected extended (∼18 kpc) Lyα emission, spatially aligned with the radio axis, redshifted by 2250 ± 60 km s−1 with respect to the host galaxy systemic velocity, and cospatial with UV continuum emission. This Lyα emission could arise from a companion galaxy, although there are arguments against this interpretation. Alternatively, it might correspond to an outflow of ionized gas stemming from the radio galaxy. The outflow would be the highest redshift spatially resolved ionized outflow to date. The enormous amount of energy injected, however, appears to be unable to quench the host galaxy’s prodigious star formation, occurring at a rate of ∼4500 M ⊙yr−1, estimated using its far-infrared luminosity. Within the field, we also found two companion galaxies at projected distances of ∼25 and ∼60 kpc from the host, which suggests the host galaxy is harbored within a protocluster.

A colourful analysis: Probing the eclipse of the black hole and central engine in NGC 6814 using X-ray colour–colour grids

ABSTRACT Eclipsing of the X-ray emitting region in active galactic nuclei (AGN) is a potentially powerful probe to examine the AGN environment and absorber properties. Here, we study the eclipse data from the 2016 XMM–Newton observation of NGC 6814 using a colour–colour analysis. Colours (i.e. hardness ratios) can provide the advantage of better time resolution over spectral analysis alone. Colour–colour grids are constructed to examine the effects of different parameters on the observed spectral variability during the eclipse. Consistent with previous spectral analysis, the variations are dominated by changes in the column density and covering fraction of the absorber. However, during maximum eclipse the behaviour of the absorber changes. Just after ingress, the eclipse is described by changes in column density and covering fraction, but prior to egress, the variations are dominated by changes in column density alone. Simulations are carried out to consider possible absorber geometries that might produce this behaviour. The behaviour is inconsistent with a single homogeneous cloud, but simulations suggest that multiple clouds, perhaps embedded in a highly ionized halo, could reproduce the results. In addition, we determine the orbital covering factor (fraction of orbital path-length) based on evidence of several eclipses in the 2016; 64-d Swift light curve. We estimate that ∼2–4 per cent of the orbit is covered by obscuring clouds and that the distribution of clouds is not isotropic.

Probing Accretion Physics with Gravitational Waves

Gravitational-wave observations of extreme mass ratio inspirals (EMRIs) offer the opportunity to probe the environments of active galactic nuclei (AGN) through the torques that accretion disks induce on the binary. Within a Bayesian framework, we study how well such environmental effects can be measured using gravitational wave observations from the Laser Interferometer Space Antenna (LISA). We focus on the torque induced by planetary-type migration on quasicircular inspirals, and use different prescriptions for geometrically thin and radiatively efficient disks. We find that LISA could detect migration for a wide range of disk viscosities and accretion rates, for both $\alpha$ and $\beta$ disk prescriptions. For a typical EMRI with masses $50M_\odot+10^6M_\odot$, we find that LISA could distinguish between migration in $\alpha$ and $\beta$ disks and measure the torque amplitude with $\sim 20\%$ relative precision. Provided an accurate torque model, we also show how to turn gravitational-wave measurements of the torque into constraints on the disk properties. Furthermore, we show that, if an electromagnetic counterpart is identified, the multimessenger observations of the AGN EMRI system will yield direct measurements of the disk viscosity. Finally, we investigate the impact of neglecting environmental effects in the analysis of the gravitational-wave signal, finding 3$\sigma$ biases in the primary mass and spin, and showing that ignoring such effects can lead to false detection of a deviation from general relativity. This work demonstrates the scientific potential of gravitational observations as probes of accretion-disk physics, accessible so far through electromagnetic observations only.

Quasars and the Intergalactic Medium at Cosmic Dawn

Quasars at cosmic dawn provide powerful probes of the formation and growth of the earliest supermassive black holes (SMBHs) in the Universe, their connections to galaxy and structure formation, and the evolution of the intergalactic medium (IGM) at the epoch of reionization (EoR). Hundreds of quasars have been discovered in the first billion years of cosmic history, with the quasar redshift frontier extended to z ∼ 7.6. Observations of quasars at cosmic dawn show the following: ▪The number density of luminous quasars declines exponentially at z > 5, suggesting that the earliest quasars emerge at z ∼ 10; the lack of strong evolution in their average spectral energy distribution indicates a rapid buildup of the active galactic nucleus environment.▪Billion-solar-mass black holes (BHs) already exist at z > 7.5; they must form and grow in less than 700 Myr, by a combination of massive early BH seeds with highly efficient and sustained accretion.▪The rapid quasar growth is accompanied by strong star formation and feedback activity in their host galaxies, which show diverse morphological and kinetic properties, with typical dynamical mass lower than that implied by the local BH/galaxy scaling relations.▪Hi absorption in quasar spectra probes the tail end of cosmic reionization at z ∼ 5.3–6 and indicates the EoR midpoint at 6.9 < z < 7.6, with large spatial fluctuations in IGM ionization. Observations of heavy element absorption lines suggest that the circumgalactic medium also experiences evolution in its ionization structure and metal enrichment during the EoR.

Discovery of a Radio-loud Narrow-line Seyfert 1 Galaxy at z = 0.9 Reddened with a Strong 2175 Å Bump

We present a broadband spectrophotometric analysis of an unusual quasar, SDSS J160558.86+474300.1, at z = 0.9, found in a systematic search for reddened radio-loud narrow-line Seyfert 1 galaxies (NLS1s). J1605+4743 qualifies for the conventional definition of an NLS1 in its permitted emission line widths and Hβ/[O iii] flux ratio. It also shows strong Fe ii emissions. J1605+4743 has a steep radio spectrum. With a radio loudness of ∼150 (70) before (after) reddening correction, it is among the most radio-loud NLS1s discovered. In addition, the blueshifted emission lines are also found to be weak, suggesting that it can be an exotic “wind-dominated” Seyfert similar to weak-line quasars, especially the ones with strong reddening. As an extreme hybrid central engine, its environment and host galaxy become rather intriguing. We discover in it a proximate 2175 Å absorber with a bump strength similar to the average value of 2175 Å absorbers in the Milky Way, which we suspect can be intrinsic to the quasar. In addition, the quasar seems to have enhanced instead of suppressed star formation at a rate of ≳94 M ⊙ yr−1. The combination of these unusual properties, i.e., the unique extinction, strong Fe ii and [O ii] lines, weak [O iii], Mg ii, and Balmer lines, and large radio loudness, might hold important clues to accretion physics, the formation/destruction of dust grains in the environment of active galactic nuclei, and possibly also the coevolution of supermassive black holes and their host galaxies in general.

Quenching in the Right Place at the Right Time: Tracing the Shared History of Starbursts, Active Galactic Nuclei, and Poststarburst Galaxies Using Their Structures and Multiscale Environments

This work uses multiscale environments and the structures of galaxies in the Sloan Digital Sky Survey as consistency checks on the evolution from starburst to quiescence at redshift z < 0.2. The environmental indicators include fixed-aperture mass overdensities (δ xMpc, x ∈ {0.5, 1, 2, 4, 8} h −1 Mpc), k-nearest neighbor distances, the tidal parameter, halo mass (M h ), and satellite/central classification. The residuals of the specific star formation rates (Δ SSFRs) are used to select starbursts (ΔSSFR > 0.6 dex, N ≈ 8600). Quenched poststarbursts (QPSBs) are selected using Hα < 3 Å emission and Hδ A > 4 Å absorption (N ≈ 750). The environments of starbursts and QPSBs are compared with those of active galactic nuclei (AGNs) and inactive galaxies of varying ΔSSFR. The environments of starbursts, AGNs, and QPSBs are unlike the environments of most quiescent galaxies (QGs). About 70%–90% of starbursts, AGNs with Hδ A > 4 Å, and QPSBs are centrals; ∼80%–90% have M h < 1013 M ⊙, and only ∼2%–4% have M h > 1014 M ⊙ or live in clusters. Their M h and satellite fractions are also different from those of QGs. All QPSBs are matched to some SFGs, starbursts, AGNs, and QGs of similar M ⋆, environments, concentration indices, and velocity dispersions. A significant fraction (∼20%–30%) of starbursts cannot be matched to QPSBs or QGs. The implications are that: (1) some starbursts do not quench rapidly; (2) the satellite-quenching mechanisms operating in high-density environments cannot account for most QPSBs; (3) the evolution from starbursts to QPSBs to QGs is not the dominant path at z < 0.2; and (4) starbursts are not mainly triggered by tidal interactions.

On the relation of host properties and environment of AGN galaxies across the standard optical diagnostic diagram

ABSTRACT We study the host properties and environment of active galactic nuclei (AGNs) galaxies, taken from SDSS-DR12, across the [O iii]/H β versus [N ii]/H α diagnostic diagram. We select AGN subsamples defined as parallel and perpendicular to the star-forming locus on the BPT diagram based on the Kauffmann et al. and Schawinski et al. criteria. For parallel subsamples we find that AGN host properties exhibit a morphological evolution as they become more distant to the star-forming sequence. The local density environment shows a more evident morphology–density relationship for subsamples mainly formed by Composite and Spiral galaxies than those containing LINERs and Seyferts, where the AGN emission is the dominant source. We also analyse the properties of the five closest AGN neighbours observing no significant differences in the environment, although the AGN host properties of every subsample have noticeable variations. The AGNs belonging to perpendicular subsamples show clear differences on their host properties from left top to right bottom on the diagram. However, the analysis of the local density environment do not reflect strong dependency with the host AGN properties. This result is reinforced by the characteristics of the AGN neighbouring galaxies. These findings suggest that mixed AGN/star-forming galaxies present environmental features more similar to that of non-active galaxies. However, as AGNs at the centre of the more evolved galaxies become the dominant source, the environment tends to provide suitable conditions for the central black hole feeding with an increasing content of gas and likelihood of a higher merger rate.

An Infrared View of the Obscured AGN Environment in NGC 4945

NGC 4945 harbors one of the nearest active galactic nuclei (AGNs), which allows us to reach high spatial resolution with current observational facilities. The Seyfert 2 nucleus is deeply obscured by an edge-on disk with A V ∼ 14, requiring infrared observations to study circumnuclear structures and the interstellar medium. In this work, we present an imaging and long-slit spectroscopic study of the nuclear region with a spatial resolution of 6.5 pc, based on Flamingos-2 (F2) and T-ReCS data taken at the Gemini South observatory. We report subarcsecond photometric measurements of the nucleus in J, H, and K s filters, and at larger apertures. We do not detect nuclear variability. The nuclear spectra confirm that even in the K band the AGN emission-line features are completely obscured by dust. We detect a circumnuclear disk in the K band as well as in the mid-infrared N and Q abands, with a radial scale length of ∼120 pc. The disk shows knots mostly in a ring-like arrangement that has been previously detected with Paα observations from the Hubble Space Telescope, indicating that these are deeply embedded, massive young star clusters. We present here the spectrum of one of the brightest unresolved objects (R < 5 pc), which we identify as a super star cluster candidate with . For the circumnuclear region, a detailed rotation curve allows us to measure a nuclear mass of M = (4.4 ± 3) × 106 M ⊙ within a radius of ∼6.5 pc. We also report the detection of hot dust (∼1000 K) out to a nuclear distance of 80 pc measured along the semimajor axis.

Multiply charged naphthalene and its C10H8 isomers: bonding, spectroscopy, and implications in AGN environments

ABSTRACT Naphthalene (C10H8) is the simplest polycyclic aromatic hydrocarbon (PAH) and an important component in a series of astrochemical reactions involving hydrocarbons. Its molecular charge state affects the stability of its isomeric structures, which is specially relevant in ionized astrophysical environments. We thus perform an extensive computational search for low-energy molecular structures of neutral, singly, and multiply charged naphthalene and its isomers with charge states +q = 0–4 and investigate their geometric properties and bonding situations. We find that isomerization reactions should be frequent for higher charged states and that open chains dominate their low-energy structures. We compute both the scaled-harmonic and anharmonic infrared (IR) spectra of selected low-energy species and provide the calculated scaling factors for the naphthalene neutral, cation, and dication global minima. All simulated spectra reproduce satisfactorily the experimental data and, thus, are adequate for aiding observations. Moreover, the potential presence of these species in the emission spectra of the circumnuclear regions of active galactic nuclei (AGNs), with high energetic X-ray photon fluxes, is explored using the experimental value of the naphthalene photodissociation cross-section, σph − d, to determine its half-life, t1/2, at a photon energy of 2.5 keV in a set of relevant sources. Finally, we show that the computed IR bands of the triply and quadruply charged species are able to reproduce some features of the selected AGN sources.

Redshift evolution of the H I detection rate in radio-loud active galactic nuclei

We present a search for associated H I 21 cm absorption in a sample of 29 radio-loud active galactic nuclei (AGNs) at 0.7 &lt; z &lt; 1, carried out with the upgraded Giant Metrewave Radio Telescope. We detect H I 21 cm absorption against none of our target AGNs, obtaining 3σ upper limits to the optical depth of ≲1% per 50 km s−1 channel. The radio luminosity of our sources is lower than that of most AGNs searched for H I 21 cm absorption at similar redshifts in the literature, and, for all targets except two, the UV luminosity is below the threshold 1023 W Hz−1, above which the H I in the AGN environment has been suggested to be completely ionised. We stacked the H I 21 cm spectra to obtain a more stringent limit of ≈0.17% per 50 km s−1 channel on the average H I 21 cm optical depth of the sample. The sample is dominated by extended radio sources, 24 of which are extended on scales of tens of kiloparsecs. Including similar extended sources at 0.7 &lt; z &lt; 1.0 from the literature, and comparing with a low-z sample of extended radio sources, we find statistically significant (≈3σ) evidence that the strength of H I 21 cm absorption towards extended radio sources is weaker at 0.7 &lt; z &lt; 1.0 than at z &lt; 0.25, with a lower detection rate of H I 21 cm absorption at 0.7 &lt; z &lt; 1.0. Redshift evolution in the physical conditions of H I is the likely cause of the weaker associated H I 21 cm absorption at high redshifts, due to either a low H I column density or a high spin temperature in high-z AGN environments.

Spectral-timing of AGN ionized outflows with Athena

ABSTRACT Spectral-timing techniques have proven valuable in studying the interplay between the X-ray corona and the accretion disc in variable active galactic nuclei (AGNs). Under certain conditions, photoionized outflows emerging from central AGN regions also play a role in the observable spectral-timing properties of the nuclear components. The variable ionizing flux causes the intervening gas to ionize or recombine, resulting in a time-dependent absorption spectrum. Understanding the spectral-timing properties of these outflows is critical not only for the determination of their role in the AGN environment but also for the correct interpretation of timing signatures of other AGN components. In this paper, we test the capabilities of the Athena X-IFU instrument in studying the spectral and spectral-timing properties of a black hole system displaying a variable outflow. We take the narrow-line Seyfert 1 IRAS 13224−3809 as a test case. Our findings show that while the non-linear response of the absorbing medium can result in complex behaviour of time lags, the resulting decrease in the coherence can be used to constrain gas density and distance to the central source. Ultimately, modelling the coherence spectra of AGN outflows may constitute a valuable tool in studying the physical properties of the outflowing gas.

Searching for High-Energy Neutrinos from Ultra-Luminous Infrared Galaxies with IceCube

This work presents an IceCube search for high-energy neutrinos from Ultra-Luminous Infrared Galaxies (ULIRGs). ULIRGs are the most luminous infrared objects in the sky, with infrared luminosities exceeding 1012 solar luminosities. They are mainly powered by starbursts that exhibit star-formation rates larger than 100 solar masses per year. In addition, an active galactic nucleus (AGN) can also contribute significantly to the ULIRG luminosity output. The acceleration of hadrons, and consequently the production of neutrinos, can occur both in starburst and AGN environments. As such, ULIRGs form a source population that could be responsible for a significant fraction of the diffuse neutrino flux observed by IceCube. In this study we perform a stacking analysis on a representative sample of 75 ULIRGs with redshift z ≤ 0.13 using 7.5 years of IceCube data. We find no evidence for astrophysical neutrinos correlated with our selection of ULIRGs. We therefore compute upper limits on the contribution of the ULIRG source population to the diffuse neutrino observations, and also use these limits to constrain model predictions.

Merger Histories and Environments of Dwarf AGN in IllustrisTNG

Abstract The relationship between active galactic nuclei (AGN) activity and environment has been long discussed, but it is unclear if these relations extend into the dwarf galaxy mass regime—in part due to the limits in both observations and simulations. We aim to investigate if the merger histories and environments are significantly different between AGN and non-AGN dwarf galaxies in cosmological simulations, which may be indicative of the importance of these for AGN activity in dwarf galaxies, and whether these results are in line with observations. Using the IllustrisTNG flagship TNG100-1 run, 6771 dwarf galaxies are found with 3863 (∼57%) having some level of AGN activity. In order to quantify environment, two measures are used: (1) the distance to a galaxy’s 10th nearest neighbor at six redshifts and (2) the time since last merger for three different minimum merger mass ratios. A similar analysis is run on TNG50-1 and Illustris-1 to test for the robustness of the findings. Both measures yield significantly different distributions between AGN and non-AGN galaxies; more non-AGN than AGN galaxies have long term residence in dense environments, while recent (≤4 Gyr) minor mergers are more common for intermediate AGN activity. While no statements are made about the micro or macrophysics from these results, it is nevertheless indicative of a non-negligible role of mergers and environments.

Modeling Grain Rotational Disruption by Radiative Torques and Extinction of Active Galactic Nuclei

Abstract Extinction curves observed toward individual Active Galactic Nuclei (AGN) usually show a steep rise toward far-ultraviolet (FUV) wavelengths and can be described by the Small Magellanic Cloud (SMC)-like dust model. This feature suggests the dominance of small dust grains of size a ≤ 0.1 μm in the local environment of AGN, but the origin of such small grains is unclear. In this paper, we aim to explain this observed feature by applying the RAdiative Torque Disruption (RATD) to model the extinction of AGN radiation from FUV to mid-infrared (MIR) wavelengths. We find that in the intense radiation field of AGN, large composite grains of size a ≥ 0.1 μm are significantly disrupted to smaller sizes by RATD up to d RATD &gt; 100 pc in the polar direction and d RATD ∼ 10 pc in the torus region. Consequently, optical–MIR extinction decreases, whereas FUV-near-ultraviolet extinction increases, producing a steep far-UV rise extinction curve. The resulting total-to-selective visual extinction ratio thus significantly drops to R V &lt; 3.1 with decreasing distances to AGN center due to the enhancement of small grains. The dependence of R V with the efficiency of RATD will help us to study the dust properties in the AGN environment via photometric observations. In addition, we suggest that the combination of the strength between RATD and other dust destruction mechanisms that are responsible for destroying very small grains of a ≤ 0.05 μm is the key for explaining the dichotomy observed “SMC” and “gray” extinction curve toward many AGN.

Accretion environments of active galactic nuclei

Abstract We study accretion environments of active galactic nuclei when a supermassive black hole wanders in a circumnuclear region and passes through an interstellar medium there. It is expected that Bondi–Hoyle–Lyttleton-type accretion of the interstellar matter takes place and an accretion stream of matter trapped by the black hole gravitational field appears from a tail shock region. Since the trapped matter is likely to have a certain amount of specific angular momentum, the accretion stream eventually forms an accretion ring around the black hole. According to recent studies, the accretion ring consists of a thick envelope and a thin core, and angular momenta are transferred from the inner side facing the black hole to the opposite side in the envelope and the core respectively. As a result, a thick accretion flow and a thick excretion flow extend from the envelope, and a thin accretion disk and a thin excretion disk extend from the core. The thin excretion disk is predicted to terminate at some distance, forming an excretion ring, while the thick excretion flow is considered to become a supersonic wind flowing to infinity. The thick excretion flow from the accretion ring is expected to interact with the accretion stream toward the accretion ring and to be collimated to bipolar cones. These pictures provide a likely guideline to interpreting the overall accretion environments suggested from observations.