Gas In Active Galactic Nuclei Research Articles

Bridging relativistic jets from black hole scales to long-term electromagnetic radiation distances: A moving-mesh general relativistic hydrodynamics code with the HLLC Riemann solver

Relativistic jets accompany the collapse of massive stars, the merger of compact objects, or the accretion of gas in active galactic nuclei. They carry information about the central engine and generate electromagnetic radiation. No self-consistent simulations have been able to follow these jets from their birth at the black hole scale to the Newtonian dissipation phase, making the inference of central engine property through astronomical observations undetermined. We present the general relativistic moving-mesh framework to achieve the continuity of jet simulations throughout space and time. We implement the general relativistic extension for the moving-mesh relativistic hydrodynamic code, , and develop a tetrad formulation to utilize the Harten-Lax-van Leer Contact Riemann solver in the general relativistic moving-mesh code. The new framework is able to trace the radial movement of relativistic jets from central regions where strong gravity holds all the way to distances of jet dissipation. Published by the American Physical Society 2024

Modeling complex AGN media in X‐rays with skirt

AbstractWe present the new X‐ray functionalities of the skirt radiative transfer code, forming an advanced tool to study cold dusty gas in active galactic nuclei (AGN), with a particular focus on complex three‐dimensional transfer media. Focusing on the X‐ray broadband spectra of obscured AGN, skirt incorporates scattering on free electrons, absorption and fluorescence by cold gas, scattering on bound electrons, as well as absorption and scattering by dust. This goes beyond the X‐ray physical processes that are currently implemented in most spectral models for fitting distant reflection in AGN. With the introduction of these X‐ray physics, we obtain a high‐performance radiative transfer code that can model intricate multi‐phase AGN media with complex geometries in X‐rays, which furthermore supports advanced kinematics and allows for self‐consistent model predictions from infrared to X‐ray energies.

Properties of the continuum and broad line emission gas in active galactic nuclei with moderate to strong Feii emission

AbstractWe study a sample of 67 active galactic nuclei (AGN) with moderate to strong Feii emission to investigate the relation between the observed AGN continuum and the lines produced by the broad line region (BLR), in particular, Feii. We found that the continuum in the optical/near‐infrared can be described in terms of two components: a power‐law, characterized by the spectral index α, and hot dust emission, represented by a Plank function of temperature T. These two parameters seem not to be related to the strength of Feii emission. In addition to Feii, we study other low‐ionization BLR lines such as Oi and Caii (hereafter CaT). We found that the full‐width at half maximum of these three lines are similar, suggesting that they are produced in the same spatial region of the BLR. We measure the Feii strength in the optical and the near‐infrared region and found that they are strongly correlated. This result evidences the presence of a common excitation mechanism for the Feii emission in the wavelength interval 4000–24000 Å.

Radiation pressure confinement – V. The predicted free–free absorption and emission in active galactic nuclei

ABSTRACT The effect of radiation pressure compression (RPC) on ionized gas in active galactic nuclei (AGNs) likely sets the photoionized gas density structure. The photoionized gas free–free absorption and emission are therefore uniquely set by the incident ionizing flux. We use the photoionization code cloudy RPC model results to derive the expected relations between the free–free emission and absorption properties and the distance from the AGN centre, for a given AGN luminosity. The free–free absorption frequency of RPC gas is predicted to increase from ∼100 MHz on the kpc scale to ∼100 GHz on the sub-pc scale, consistent with observations of spatially resolved free–free absorption. The free–free emission at 5 GHz is predicted to yield a radio loudness (R) of ∼0.03, below the typical observed values of R ∼ 0.1–1 in radio-quiet AGNs. However, the flat free–free radio continuum may become dominant above 100 GHz. The suggested detection of optically thin free–free emission in NGC 1068, on the sub-pc torus scale, is excluded as the brightness temperature is too high for optically thin free–free emission. However, excess emission observed with Atacama Large Millimeter/submillimeter Array (ALMA) above 150 GHz in NGC 1068 is consistent with the predicted free–free emission from gas just outside the broad-line region, a region that overlaps the hot dust disc resolved with GRAVITY. Extended ∼100 pc-scale free–free emission is also likely present in NGC 1068. Future sub-mm observation of radio-quiet AGNs with ALMA may allow to image the free–free emission of warm photoionized gas in AGNs down to the 30 mas scale, including highly absorbed AGNs.

X-ray signatures of the polar dusty gas in AGN

ABSTRACT Recent mid-infrared interferometry observations of nearby active galactic nuclei (AGN) revealed that a significant part of the dust emission extends in the polar direction, rather than the equatorial torus/disc direction as expected by the traditional unification model. We study the X-ray signatures of this polar dusty gas with ray-tracing simulations. Different from those from the ionized gas, the scattered emission from the polar dusty gas produces self-absorption and neutral-like fluorescence lines, which are potentially a unique probe of the kinematics of the polar dusty gas. The anomalously small Fe Kα/Si Kα ratios of type 2 AGN observed previously can be naturally explained by the polar dusty gas, because the polar emission does not suffer from heavy absorption by the dense equatorial gas. The observed Si Kα lines of the Circinus galaxy and NGC 1068 show blueshifts with respect to the systemic velocities of the host galaxies, consistent with an outflowing scenario of the Si Kα-emitting gas. The 2.5–3 keV image of the Circinus galaxy is elongated along the polar direction, consistent with an origin of the polar gas. These results show that the polar-gas-scattered X-ray emission of type 2 AGN is an ideal objective for future X-ray missions, such as Athena.

Black hole masses in active galactic nuclei

The masses of supermassive black holes, key to many cosmological studies, are highly uncertain beyond our local Universe. The main challenge is to establish the spatial and kinematic structure of the broad-line emitting gas in active galactic nuclei.

Ultraviolet/Optical Emission of the Ionized Gas in AGN: Diagnostics of the Ionizing Source and Gas Properties

Spectroscopic studies of active galactic nuclei (AGN) are powerful means of probing the physical properties of the ionized gas within them. In particular, near future observational facilities, such as the {\itshape James Webb Space Telescope} (JWST), will allow detailed statistical studies of rest-frame ultraviolet and optical spectral features of the very distant AGN with unprecedented accuracy. In this proceedings, we discuss the various ways of exploiting new dedicated photoionization models of the narrow-line emitting regions (NLR) of AGN for the interpretation of forthcoming revolutionary datasets.

Near infrared hydrogen emission line ratios as diagnostics of the broad emission line region

Broad emission line flux ratios are a powerful diagnostic of the physical conditions of the broad-line region gas in Active Galactic Nuclei. With recent advances in infrared spectroscopy, previously unstudied emission lines provide a new means to investigate the physical nature of the BELR gas. The hydrogen emission lines are particularly sensitive to the upper limits of both the radius from the central ionising source and the number density of the gas. Using an existing subset of near-infrared quasar spectra from the Glikman et al. (2006) sample [1] together with Cloudy photoionization simulations, we confirm the Locally Optimally emitting Cloud (LOC) model's ability to reproduce observed emission line flux ratios. The model is then used to constrain physical conditions for individual sources. The photoionization models show that high number density, low incident flux gas is required to reproduce observed near-infrared hydrogen emission line ratios. We also find that comparison to individual sources, rather than composites, is vital.

Turbulent and fast motions of H2gas in active galactic nuclei

Querying the Spitzer archive for optically-selected active galactic nuclei (AGN) observed in high-resolution mode spectroscopy, we identified radio and/or interacting galaxies with highly turbulent motions of the H2 gas at a temperature of a few hundred Kelvin. Unlike all other AGN that have unresolved H2 line profiles at a spectral resolution of ~600, 3C236, 3C293, IRAS09039+0503, MCG-2-58-22, and Mrk463E have intrinsic velocity dispersions exceeding 200 km/s for at least two of the rotational S0, S1, S2, and S3 lines. In a sixth source, 4C12.50, a blue wing was detected in the S1 and S2 line profiles, indicating the presence of a warm molecular gas component moving at -640 km/s with respect to the bulk of the gas at systemic velocity. Its mass is 5.2*10^7 M_sun, accounting for more than one fourth of the H2 gas at 374K, but less than 1% of the cold H2 gas computed from CO observations. Because no diffuse gas component of 4C12.50 has been observed to date to be moving at more than 250 km/s from systemic velocity, the H2 line wings are unlikely to be tracing gas in shock regions along the tidal tails of this merging system. They can instead be tracing gas driven by a jet or entrained by a nuclear outflow, which is known to emerge from the west nucleus of 4C12.50. It is improbable that such an outflow, with an estimated mass loss rate of 130 M_sun/yr, entirely quenches the star formation around this nucleus.

THE STRUCTURE AND DYNAMICS OF AN ACTIVE GALACTIC NUCLEUS TORUS: CO LINE PREDICTIONS FOR ALMA FROM THREE-DIMENSIONAL HYDRODYNAMICAL SIMULATIONS WITH X-RAY-DRIVEN CHEMISTRY

Many efforts have been made to model the mass distribution and dynamical evolution of the circumnuclear gas in active galactic nuclei (AGNs). However, chemical evolution is not included in detail in three-dimensional (3-D) hydrodynamic simulations. The X-ray radiation from the AGN can drive the gas chemistry and affect the thermodynamics, as well as the excitation of the interstellar medium (ISM). Therefore, we estimate the effects (on chemical abundances and excitation) of X-ray irradiation by the AGN, for atomic and molecular gas in a 3-D hydrodynamic model of an AGN torus. We obtain the abundances of various species from an X-ray chemical model. A 3-D radiative transfer code estimates the level populations, which result in line intensity maps. Predictions for the CO J=1-0 to J=9-8 lines indicate that mid-J CO lines are excellent probes of density and dynamics in the central (<60 pc) region of the AGN, in contrast to the low-J CO lines. Analysis of the X_CO/\alpha conversion factors shows that only the higher-J CO lines can be used for gas mass determination in AGN tori. The [C II] 158 um emission traces mostly the hot (T_k>1000m K) central (<60 pc) region of the AGN torus. The [C II] 158 um line will be useful for ALMA observations of high redshift (z>1) AGNs. The spatial scales (>0.25 pc) probed with our simulations match the size of the structures that ALMA will resolve in nearby (<45 Mpc at 0.01") galaxies.

DENSE MOLECULAR GAS EXCITATION AT HIGH REDSHIFT: DETECTION OF HCO+(J= 4 → 3) EMISSION IN THE CLOVERLEAF QUASAR

We report the detection of HCO+(J = 4 → 3) emission in the Cloverleaf Quasar at z = 2.56, using the IRAM Plateau de Bure Interferometer. HCO+ emission is a star formation indicator similar to HCN, tracing dense molecular hydrogen gas (n(H2) ≃ 105 cm−3) within star-forming molecular clouds. We derive a lensing-corrected HCO+(J = 4 → 3) line luminosity of K km s−1 pc2, which corresponds to only 48% of the HCO+(J = 1 → 0) luminosity, and ≲4% of the CO(J = 3 → 2) luminosity. The HCO+ excitation thus is clearly subthermal in the J = 4 → 3 transition. Modeling of the HCO+ line radiative transfer suggests that the HCO+ emission emerges from a region with physical properties comparable to that exhibiting the CO line emission, but 2× higher gas density. This suggests that both HCO+ and CO lines trace the warm, dense molecular gas where star formation actively takes place. The HCO+ lines have only ∼2/3 the width of the CO lines, which may suggest that the densest gas is more spatially concentrated. In contrast to the z = 3.91 quasar APM 08279+5255, the dense gas excitation in the Cloverleaf is consistent with being purely collisional, rather than being enhanced by radiative processes. Thus, the physical properties of the dense gas component in the Cloverleaf are consistent with those in the nuclei of nearby starburst galaxies. This suggests that the conditions in the dense, star-forming gas in active galactic nucleus–starburst systems at early cosmic times like the Cloverleaf are primarily affected by the starburst itself, rather than the central active black hole.

Ionized gas in active galactic nuclei

This review gives an introduction to the physics of ionized gas in active galactic nuclei (AGN). The main topics are: Definition of the AGN phenomenon, methods for discovering AGN, basic AGN observations, photoionization and recombination of low density plasma, energy balance and gas temperature, the spectrum of ionized gas, dust and reddening, the motion of ionized gas under the influence of strong radiation fields, the broad line region (BLR), the narrow line region (NLR), and the highly ionized outflowing gas in AGN.

Dielectronic recombination and stability of warm gas in active galactic nuclei

Abstract Understanding the thermal equilibrium (stability) curve may offer insights into the nature of the warm absorbers often found in active galactic nuclei. Its shape is determined by factors such as the spectrum of the ionizing continuum and the chemical composition of the gas. We find that the stability curves obtained under the same set of the above-mentioned physical factors, but using recently derived dielectronic recombination rates, give significantly different results, especially in the regions corresponding to warm absorbers, leading to different physical predictions. Using the current rates we find a larger probability of having a thermally stable warm absorber at 105 K than previous predictions and also a greater possibility for its multiphase nature. The results obtained with the current dielectronic recombination rate coefficients are more reliable because the warm absorber models along the stability curve have computed coefficient values, whereas previous calculations relied on guessed averages for these because of a lack of available data.

The Case for Optically Thick High‐Velocity Broad‐Line Region Gas in Active Galactic Nuclei

A combined analysis of the profiles of the main broad quasar emission lines in both Hubble Space Telescope and optical spectra shows that while the profiles of the strong UV lines are quite similar, there is frequently a strong increase in the Ly-alpha/H-alpha ratio in the high-velocity gas. We show that the suggestion that the high velocity gas is optically-thin presents many problems. We show that the relative strengths of the high velocity wings arise naturally in an optically-thick BLR component. An optically-thick model successfully explains the equivalent widths of the lines, the Ly-alpha/H-alpha ratios and flatter Balmer decrements in the line wings, the strengths of CIII] and the lambda 1400 blend, and the strong variability of high-velocity, high-ionization lines (especially HeII and HeI).

Molecular hydrogen and [Fe II] in active galactic nuclei -- II. Results for Seyfert 2 galaxies

Near-infrared spectroscopy is used to study the kinematics and excitation mechanisms of H2 and [Fe II] lines in a sample dominated by Seyfert 2 galaxies. The spectra simultaneously cover the JHK bands, allowing us to compare line fluxes emitted in the interval 0.8‐2.4 µm and avoiding aperture and seeing effects. The H2 lines are systematically narrower than the narrow-line region lines, suggesting that, very likely, the H2 does not originate from the same parcel of gas that forms the narrow-line region. Emission-line ratios between H2 lines favour thermal excitation mechanisms for the molecular gas in active galactic nuclei. It was found that non-thermal excitation contributes, at most, 30 per cent of the observed H2. Thermal excitation is also confirmed by the rather similar vibrational and rotational temperatures in the objects (∼2000 K). The mass of hot H2 ranges from 10 2 to 10 3 M� , with nearly half of objects showing values of <500 M� .I tshows that the fraction of molecular mass present in the nuclear region and emitting in the near-infrared is a very small fraction of the warm molecular mass present in the centre. A diagnostic diagram composed of the line ratios H2/Brγ and [Fe II]/Paβ proves to be a useful tool in the near-infrared for separating emission-line objects by their degree of nuclear activity. We found that active galactic nuclei are characterized by H2 2.121 µm/Brγ and [Fe II] 1.257 µm/Paβ flux ratios between 0.6 and 2. Starburst/H II galaxies display line ratios <0.6 while low-ionization nuclear emission-line regions are characterized by values larger than 2 in either ratio. Ke yw ords: line: formation ‐ molecular processes ‐ galaxies: Seyfert ‐ infrared: galaxies.

Kinematic Linkage between the Broad- and Narrow-Line–emitting Gas in Active Galactic Nuclei

We investigate the radial velocity difference between the [O III] λλ4959, 5007 and Hβ lines for a sample of ≈200 low-redshift active galactic nuclei. We identify seven objects showing an [O III] λ5007 blueshift relative to Hβ with an amplitude larger than 250 km s-1 (blue outliers). These line shifts are found in sources where the broad high-ionization lines (e.g., C IV λ1549) also show a large systematic blueshift. Such blueshifts occur only in the population A region of the Eigenvector 1 parameter domain (that also contains narrow-line Seyfert 1 galaxies). We suggest that [O III] λλ4959, 5007 blueshifts are also associated with the high-ionization outflow originating in these highly accreting sources. This is a direct kinematic linkage between narrow- and broad-line-emitting gas.

5.6. Mapping the nucleus of NGC1068 in CO(2-1)

Molecular gas in active galactic nuclei (AGN) may play multiple roles: it can fuel a massive black hole, collimate jets, and/or obscure a broad-line region from most viewing angles. In order to understand the physical state and kinematics of molecular gas in AGN, we are using the Owens Valley Millimeter Array to map several nearby active galxies at high resolution in the CO(2–1) line. Our first target is NGC 1068, which hosts an obscured broad-line Seyfert nucleus as well as an extended starburst. We acquired data from 35 baselines out to 242 m, with 10.4 km s−1 resolution. After calibration with MMA (Scoville et al. 1992), we subtracted a 23 mJy nuclear continuum source from the visibility data and made CLEAN maps with AIPS. The resulting sensitivity in each line channel is 32 mJy per 1.8″ × 1.5″ robustly-weighted beam (1″ = 70 pc at 14.4 Mpc).

Physical Conditions of the Coronal Line Region in Seyfert Galaxies

The launch of the Infrared Space Observatory and new atomic data have opened a window for the study of high-ionization gas in active galactic nuclei (AGNs). We present the results of a large number of photoionization simulations of the region in AGNs, employing new atomic data from the Opacity and Iron Projects. Our grid of line emission spans 8 orders of magnitude in gas density and 14 orders of magnitude in ionizing flux in an effort to identify the optimal conditions in which these lines form. We show that coronal lines form at distances from just outside the broad-line region to ~400L1/243.5 pc, in gas with ionization parameter -2.0 log U(H) 0.75, corresponding to gas densities of 102-108.5 cm-3, with electron temperatures ~12,000-150,000 K. A large range of distances from the central source implies significant line width variation among the coronal lines. We identify several line ratios that could be used to measure relative abundances, and we use these to show that the coronal line gas is likely to be dust free.

High-Energy Lines in Active Galactic Nuclei

AbstractThe recent observations of a highly ionized, X-ray absorbing gas in active galactic nuclei (AGN) suggest a new nuclear component, the so-called ‘warm absorber’. This gas is likely to be at a temperature of ~ 1–2 × 105 K and is most easily detected in the 0.5–10keV range, where several oxygen absorption edges are often observed.This review describes the properties of warm absorbers and the relation to other nuclear components, such as the broad-line emitting gas. The stability of the gas is a key issue and analysis shows that it is likely to be thermally stable, at the above mentioned temperature. When successful models are compared to the data, they can be used to infer the column density, composition and level of ionization of the X-ray absorbing gas. They also show that, on top of the strong continuum absorption, the gas must emit X-ray lines that are at the limit of detection by present day X-ray instruments.New calculations of X-ray emission lines emitted by ionized X-ray absorbers are shown and discussed. Various line equivalent widths are defined and examples are shown over a large range of column density and ionization parameter. The equivalent width of the strongest 0.5-5 keV lines is only a few tens of eV, but in cases of obscured X-ray source, like in Seyfert 2s, the lines are measured against the scattered and diffuse radiation with much larger equivalent widths. The X-ray absorbing and emitting gas is responsible also for a fraction of the observed flux of some UV emission lines. It is also the cause of the detection of several UV absorption lines. The calculations predict that some of those absorption lines, in particular 0 VI λ1035, are very sensitive warm-absorber tracers. Thus, analysis of the combined X-ray and UV properties is the best way to identify the location and properties of this gas.Understanding the origin and properties of warm X-ray absorbers is a major challenge of AGN research. Several new ideas are briefly discussed, trying to relate the location, mass, and motion of this gas to what is known about other observed nuclear components.

X‐Ray Lines in Active Galactic Nuclei and Photoionized Gases

This paper addresses the observed 0.1-10 keV spectrum in active galactic nuclei (AGNs) and other photoionized gases. Detailed calculations, encompassing a large range in density (10-1014 cm–3), column density (1021-1023.5 cm–2), and level of ionization (3 orders of magnitude in ionization parameter), are described and discussed. The results are presented as line intensity and three types of equivalent width over the entire range of physical conditions.We identify the most likely conditions in the X-ray-absorbing gas in AGNs and argue that its temperature is below 2 × 105 K and that it is thermally stable. The strongest 0.1-10 keV lines, when observed against the central source continuum, have typical equivalent widths of 5-50 eV and are thus at the limit of present-day detection. The equivalent width can be significantly stronger when the line is observed against the absorbed continuum. Much larger equivalent widths, of 100-1000 eV, are expected when the central continuum is obscured, as in Seyfert 2 galaxies. The importance of line-continuum fluorescence diminishes with line optical depth, and we do not find this process to be important in warm AGN absorbers. The X-ray line intensities depend on the gas density, especially for very low (N 1012 cm–3) densities. They are also sensitive to the geometry and covering fraction.We discuss the ultraviolet and extreme-ultraviolet spectrum and argue that the highly ionized gas contributes significantly to the observed intensity of Ne VIII λ774 and, in some cases, also to O VI λ1035. We confirm that observed ultraviolet absorption lines, in several AGNs, originate from gas with conditions similar to the X-ray-absorbing gas and show the expected column density and absorption equivalent width for the strongest species. If indeed the same physical component, wherever C IV λ1549 absorption is visible, we predict substantially greater absorption in the line O VI λ1035. None of the observed Extreme Ultraviolet Explorer lines in the spectrum of NGC 5548 are consistent with the ionized gas component, and there is no satisfactory model to explain these observations.A new model is presented, in which evaporating seeds (high-density clouds or bloated stars) in the broad-line region are filling the intercloud medium with material that is streaming outward. The material's location, density, column density, covering fraction, and perhaps also dynamical state are consistent with observed properties of X-ray and ultraviolet absorbers.