Active Galactic Nucleus Radiation Research Articles

THE RESOLVED OUTFLOW FROM 3C 48

We investigate the properties of the high-velocity outflow driven by the young radio jet of 3C 48, a compact-steep-spectrum source. We use the Space Telescope Imaging Spectrograph on board the Hubble Space Telecope to obtain (1) low-resolution UV and optical spectra and (2) multi-slit medium-resolution spectra of the ionized outflow. With supporting data from ground-based spectrographs, we are able to accurately measure the ratios of diagnostic emission lines such as [O III] λ5007, [O III] λ3727, [N II] λ6548, Hα, Hβ, [Ne V] λ3425, and [Ne III] λ3869. We fit the observed emission-line ratios using a range of ionization models, powered by active galactic nucleus (AGN) radiation and shocks, produced by the MAPPINGS code. We have determined that AGN radiation is likely the dominant ionization source. The outflow's density is estimated to be in the range n = 10{sup 3}-10{sup 4} cm{sup –3}, the mass is ∼6 × 10{sup 6} M {sub ☉}, and the metallicity is likely equal to or higher than solar. Compared with the typical outflows associated with more evolved radio jets, this young outflow is denser, less massive, and more metal rich. Multi-slit observations allow us to construct a two-dimensional velocity map of the outflow thatmore » shows a wide range of velocities with distinct velocity components, suggesting a wide-angle clumpy outflow.« less

Nuclear 11.3 μm PAH emission in local active galactic nuclei

We present Gran Telescopio CANARIAS CanariCam 8.7$\mu$m imaging and 7.5-13$\mu$m spectroscopy of six local systems known to host an active galactic nucleus (AGN) and have nuclear star formation. Our main goal is to investigate whether the molecules responsible for the 11.3$\mu$m polyclyclic aromatic hydrocarbon (PAH) feature are destroyed in the close vicinity of an AGN. We detect 11.3$\mu$m PAH feature emission in the nuclear regions of the galaxies as well as extended PAH emission over a few hundred parsecs. The equivalent width (EW) of the feature shows a minimum at the nucleus but increases with increasing radial distances, reaching typical star-forming values a few hundred parsecs away from the nucleus. The reduced nuclear EW are interpreted as due to increased dilution from the AGN continuum rather than destruction of the PAH molecules. We conclude that at least those molecules responsible for the 11.3$\mu$m PAH feature survive in the nuclear environments as close as 10pc from the AGN and for Seyfert-like AGN luminosities. We propose that material in the dusty tori, nuclear gas disks, and/or host galaxies of AGN is likely to provide the column densities necessary to protect the PAH molecules from the AGN radiation field.

On the evolution of gas clouds exposed to AGN radiation – I. Three-dimensional radiation hydrodynamic simulations

We perform three-dimensional radiation hydrodynamic simulations of uniform dusty gas clouds irradiated by an active galactic nucleus (AGN) to investigate the dependence of evolution of clouds on the ionization parameter $\mathcal{U}$ and the Str{\"o}mgren number $\mathcal{N}_{S}$. We find that the evolution can be classified into two cases depending on $\mathcal{U}$. In low $\mathcal{U}$ cases ($\mathcal{U}\approx 10^{-2}$), the evolution is mainly driven by photo-evaporation. A approximately spherically-symmetric evaporation flow with velocity of $100\operatorname{-}150\;\mathrm{km\;s^{-1}}$ is launched from the irradiated face. The cloud is compressed by a D-type shock with losing its mass due to photo-evaporation and is finally turned into a dense filament by $t\lesssim 1.5t_{\mathrm{sc}}$. In high $\mathcal{U}$ cases ($\mathcal{U}\approx 5\times 10^{-2}$), radiation pressure suppresses photo-evaporation from the central part of the irradiated face, reducing photo-evaporation rate. A evaporation flow from the outskirts of the irradiated face is turned into a high velocity ($\lesssim 500\;\mathrm{km\;s^{-1}}$) gas wind because of radiation pressure on dust. The cloud is swept by a radiation pressure-driven shock and becomes a dense gas disk by $t\approx t_{\mathrm{sweep}}$. Star formation is expected in these dense regions for both cases of $\mathcal{U}$. We discuss the influences of the AGN radiation on the clumpy torus. A simple estimate suggests that the clumps are destroyed in timescales shorter than their orbital periods. For the clumpy structure to be maintained over long period, the incident radiation field needs to be sufficiently weaken for most of the clumps, or, some mechanism that creates the clumps continuously is needed.

DETERMINING INCLINATIONS OF ACTIVE GALACTIC NUCLEI VIA THEIR NARROW-LINE REGION KINEMATICS. II. CORRELATION WITH OBSERVED PROPERTIES

Active Galactic Nuclei (AGN) are axisymmetric systems to first order; their observed properties are likely strong functions of inclination with respect to our line of sight, yet the specific inclinations of all but a few AGN are generally unknown. By determining the inclinations and geometries of nearby Seyfert galaxies using the kinematics of their narrow-line regions (NLRs), and comparing them with observed properties, we find strong correlations between inclination and total hydrogen column density, infrared color, and H-beta full-width at half maximum (FWHM). These correlations provide evidence that the orientation of AGN with respect to our line of sight affects how we perceive them, beyond the Seyfert type dichotomy. They can also be used to constrain 3D models of AGN components such as the broad-line region and torus. Additionally, we find weak correlations between AGN luminosity and several modeled NLR parameters, which suggests that the NLR geometry and kinematics are dependent to some degree on the AGN's radiation field.

Possible mechanisms for the polarization of the radiation of Active Galactic Nuclei and quasars

Two mechanisms for the formation of polarized radiation in Active Galactic Nuclei and quasars are considered-synchrotron radiation from a jet and the emergence of radiation from a magnetized, optically thick accretion disk (the Milne problem in a magnetized atmosphere). These mechanisms are applied to the polarization of objects studied in [1]. We have shown that the polarization of some sources cannot be explained as synchrotron radiation. The second mechanism is more general and, in principle, can be applied to the polarized radiation of all the sources in [1].

AGN proximity zone fossils and the delayed recombination of metal lines

We model the time-dependent evolution of metal-enriched intergalactic and circumgalactic gas exposed to the fluctuating radiation field from an active galactic nucleus (AGN). We consider diffuse gas densities (n_H=10^-5-10^-3 cm^-3) exposed to the extra-galactic background (EGB) and initially in thermal equilibrium (T \sim 10^4-10^4.5 K). Once the proximate AGN field turns on, additional photo-ionisation rapidly ionises the HI and metals. The enhanced AGN radiation field turns off after a typical AGN lifetime (tau_AGN=1-20 Myr) and the field returns to the EGB intensity, but the metals remain out of ionisation equilibrium for timescales that can significantly exceed tau_AGN. We define this phase as the AGN proximity zone "fossil" phase and show that high ionisation stages (e.g. OVI, NeVIII, MgX) are in general enhanced, while the abundances of low ions are reduced. In contrast, HI re-equilibrates rapidly (<<tau_AGN) owing to its low neutral fraction at diffuse densities. We demonstrate that metal column densities of intervening gas observed in absorption in quasar sight lines are significantly affected by delayed recombination for a wide range of densities, metallicities, and AGN strengths, lifetimes, and duty cycles. We model the exceptionally strong z=0.9 NeVIII absorbers observed by Tripp et al. (2011) as arising in a possible fossil zone or near a recently turned-on AGN and we demonstrate that at low redshift even moderate strength AGN could significantly enhance the high-ion metal columns in the circumgalactic media of galaxies observed without active AGN. Fossil proximity zones may be very important during the quasar era, z \sim 2-5. AGN proximity zone fossils allow a whole new class of non-equilibrium solutions that may be applicable to a large fraction of observed metal absorbers and which could potentially change the inferred physical conditions and masses of diffuse gases.

AGN OBSCURATION THROUGH DUSTY, INFRARED-DOMINATED FLOWS. II. MULTIDIMENSIONAL, RADIATION-HYDRODYNAMICS MODELING

We explore a detailed model in which the active galactic nucleus (AGN) obscuration results from the extinction of AGN radiation in a global flow driven by the pressure of infrared radiation on dust grains. We assume that external illumination by UV and soft X-rays of the dusty gas located at approximately 1pc away from the supermassive black hole is followed by a conversion of such radiation into IR. Using 2.5D, time-dependent radiation hydrodynamics simulations in a flux-limited diffusion approximation we find that the external illumination can support a geometrically thick obscuration via outflows driven by infrared radiation pressure in AGN with luminosities greater than $0.05\, L_{\rm edd}$ and Compton optical depth, $\tau_{\rm T}\gtrsim 1$.

Are supermassive black holes shrouded by ‘super-Oort’ clouds of comets and asteroids?

The last decade has seen a dramatic confirmation that in situ star formation is possible inside the inner parsec of the Milky Way. Here we suggest that giant planets, solid terrestrial-like planets, comets and asteroids may also form in these environments, and that this may have observational implications for active galactic nuclei (AGN). Like in debris discs around main-sequence stars, collisions of large solid objects should initiate strong fragmentation cascades. The smallest particles in such a cascade – the microscopic dust – may provide a significant opacity. We put a number of observational and physical constraints on AGN obscuring tori resulting from such fragmentation cascades. We find that tori fed by fragmenting asteroids disappear at both low and high AGN luminosities. At high luminosities, L∼LEdd, where LEdd is the Eddington limit, the AGN radiation pressure blows out the microscopic dust too rapidly. At low luminosities, on the other hand, the AGN discs may avoid gravitational fragmentation into stars and solids. We also note that these fragmentation cascades may be responsible for astrophysically ‘large’ dust particles of m sizes that were postulated by some authors to explain unusual absorption properties of the AGN tori.

Radiation pressure, absorption and AGN feedback in the Chandra Deep Fields

The presence of absorbing gas around the central engine of Active Galactic Nuclei (AGN) is a common feature of these objects. Recent work has looked at the effect of the dust component of the gas, and how it enhances radiation pressure such that dusty gas can have a lower effective Eddington limit than ionised gas. In this work, we use multi-wavelength data and X-ray spectra from the 2 Ms exposures of the Chandra Deep Field North and Chandra Deep Field South surveys, to characterise the AGN in terms of their Eddington ratio and hydrogen column density. Their distributions are then compared with what is predicted when considering the coupling between dust and gas. Our final sample consists of 234 objects from both fields, the largest and deepest sample of AGN for which this comparison has been made up to date. We find that most of the AGN in our sample tend to be found at low Eddington ratios (typically between 1e-4 and 1e-1) and high column density (>1e22 cm^-2), with black hole masses between ~1e8 and 1e9 solar masses. Their distribution is in agreement with that expected from the enhanced radiation pressure model, avoiding the area where we would predict the presence of outflows. We also investigate how the balance between AGN radiation pressure and gravitational potential influences the behaviour of clouds in the galactic bulge, and describe a scenario where an enhanced radiation pressure can lead to the fundamental plane of black hole/galaxy scaling relations.

THE ROLE OF MERGERS IN EARLY-TYPE GALAXY EVOLUTION AND BLACK HOLE GROWTH

Models of galaxy formation invoke the major merger of gas-rich progenitor galaxies as the trigger for significant phases of black hole growth and the associated feedback that suppresses star formation to create red spheroidal remnants. However, the observational evidence for the connection between mergers and active galactic nucleus (AGN) phases is not clear. We analyze a sample of low-mass early-type galaxies known to be in the process of migrating from the blue cloud to the red sequence via an AGN phase in the green valley. Using deeper imaging from SDSS Stripe 82, we show that the fraction of objects with major morphological disturbances is high during the early starburst phase, but declines rapidly to the background level seen in quiescent early-type galaxies by the time of substantial AGN radiation several hundred Myr after the starburst. This observation empirically links the AGN activity in low-redshift early-type galaxies to a significant merger event in the recent past. The large time delay between the merger-driven starburst and the peak of AGN activity allows for the merger features to decay to the background and hence may explain the weak link between merger features and AGN activity in the literature.

ISOTROPIC MID-INFRARED EMISSION FROM THE CENTRAL 100 pc OF ACTIVE GALAXIES

Dust reprocesses the intrinsic radiation of active galactic nuclei (AGNs) to emerge at longer wavelengths. The observed mid-infrared (MIR) luminosity depends fundamentally on the luminosity of the central engine, but in detail it also depends on the geometric distribution of the surrounding dust. To quantify this relationship, we observe nearby normal AGNs in the MIR to achieve spatial resolution better than 100 pc, and we use absorption-corrected X-ray luminosity as a proxy for the intrinsic AGN emission. We find no significant difference between optically classified Seyfert 1 and 2 galaxies. Spectroscopic differences, both at optical and IR wavelengths, indicate that the immediate surroundings of AGNs is not spherically symmetric, as in standard unified AGN models. A quantitative analysis of clumpy torus radiative transfer models shows that a clumpy local environment can account for this dependence on viewing geometry while producing MIR continuum emission that remains nearly isotropic, as we observe, although the material is not optically thin at these wavelengths. We find some luminosity dependence on the X-ray/MIR correlation in the smallest scale measurements, which may indicate enhanced dust emission associated with star formation, even on these sub-100 pc scales.

The cosmic history of the X–ray background

The Chandra and XMM-Newton X-ray observatories have detected obscured active galactic nuclei (AGN) and almost fully resolved the X-ray background into discrete sources. Ground-based observations of the X-ray sources enable the reconstruction of the history of supermassive black hole accretion from the earliest times to the present. A dramatic cosmic downsizing of AGN luminosities is seen at recent times. Correspondingly, the production rate of the AGN radiation drops rapidly, and the dominant period of supermassive black hole production is seen to be at redshifts near z = 1.

Comparison of Nuclear Starburst Luminosities between Seyfert 1 and 2 Galaxies Based on Near‐Infrared Spectroscopy

We report on infrared K- (2-2.5 ?m) and L-band (2.8-4.1 ?m) slit spectroscopy of 23 Seyfert 1 galaxies in the CfA and 12 ?m samples. A polycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 ?m in the L band is primarily used to investigate nuclear star-forming activity in these galaxies. The 3.3 ?m PAH emission is detected in 10 sources (=43%), demonstrating that detection of nuclear star formation in a significant fraction of Seyfert 1 galaxies is now feasible. For the PAH-detected nuclei, the surface brightness values of the PAH emission are as high as those of typical starbursts, suggesting that the PAH emission probes the putative nuclear starbursts in the dusty tori around the central active galactic nuclei (AGNs). The magnitudes of the nuclear starbursts are quantitatively estimated from the observed 3.3 ?m PAH emission luminosities. The estimated starburst luminosities relative to some indicators of AGN powers in these Seyfert 1 galaxies are compared with 32 Seyfert 2 galaxies in the same samples that we have previously observed. We find that there is no significant difference in nuclear starburst to AGN luminosity ratios of Seyfert 1 and 2 galaxies and that nuclear starburst luminosity positively correlates with AGN power in both types. Our results favor a slightly modified AGN unification model, which predicts that nuclear starbursts occurring in the dusty tori of Seyfert galaxies are physically connected to the central AGNs, rather than the classical unification paradigm, in which the dusty tori simply hide the central AGNs of Seyfert 2 galaxies and reprocess AGN radiation as infrared dust emission in Seyfert galaxies. No significant differences in nuclear star formation properties are recognizable between Seyfert 1 galaxies in the CfA and 12 ?m samples.

A New Mid-Infrared Diagnostic between AGN and Starbursts

We present a new set of diagnostics which allow us to trace and classify in a statistical manner the mid-IR emission produced by active galactic nuclei (AGN) and star-forming regions. We construct a diagram based on the strength of the unidentified infrared band (UIB)at 6.2 μm, and the intensity of the continuum at short (6 μm) and long wavelengths (15 μm). We interpret the integrated mid-IR emission in late-type galaxies as resulting from three individual contributions coming from HII regions, diffuse/photodissociation regions (PDRs), and AGN. Based on this assumption, our diagnostic diagram provides a quantitative estimate of the AGN and starburst contribution to an observed mid-IR spectrum. We show that UIB emission is very faint or absent in regions harbouring intense and hard radiation fields as in the case of AGN or ‘pure’ HII starburst regions where UIB carriers can be destroyed by photodissociation. However, contrary to starburst spectra, typical AGN spectra present a strong hot continuum below 9 μm originating from hot dust heated by the AGN radiation field. An extrapolation of this diagnostic towards other mid-IR observations should improve our knowledge of the AGN/starburst connection.

The spectral properties of BL Lac objects

A model of plasma reactor is examined for a unified interpretation of the radiation of active galactic nuclei and, particularly, of BL Lac objects. The physical scenario is as follows: a large number of relativistic electrons are ejected by the central engine into the plasma reactor which surrounds the central core, and rapidly lose their energy by synchrotron radiation. Meanwhile, these electrons are accelerated by absorbing synchrotron radiation which is not transparent in the plasma reactor and set up a steady and isotropic power law distribution with spectral index γ=3. Then they emerge through surface diffusion or outburst of the plasma reactor and generate low-frequency synchrotron radiation. The higher frequency synchrotron radiation produced by the plasma reactor shows a rapid spectral steepening as the flux decreases. The properties of synchrotron radiation source with radiation loss and injection of relativistic electrons are studied in detail.

ITAL]Infrared Space Observatory[/ITAL] Measurements of [C [CSC]ii[/CSC]] Line Variations in Galaxies

We report measurements of the [C II] fine-structure line at 157.714 ?m in 30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared (FIR) luminosity, LC II/LFIR, measures the ratio of the cooling of gas to that of dust, and thus the efficiency of the grain photoelectric heating process. This ratio varies by more than a factor of 40 in the current sample. About two-thirds of the galaxies have LC II/LFIR ratios in the narrow range of (2-7) ? 10 -->?3. The other one-third show trends of decreasing LC II/LFIR with increasing dust temperature, as measured by the flux ratio of infrared emission at 60 and 100 ?m, F?(60 ?m)/F?(100 ?m), and with increasing star formation activity, measured by the ratio of FIR and blue-band luminosity, LFIR/L -->B. We also find three FIR-bright galaxies that are deficient in the [C II] line, which is undetected with 3 ? upper limits of LC II/LFIR ?4. The trend in the LC II/LFIR ratio with the temperature of dust and with star formation activity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed photodissociation region lines have among the highest LFIR/L -->B and F?(60 ?m)/F?(100 ?m) ratios. Their lack of [C II] lines may be due to a continuing trend of decreasing LC II/LFIR with increasing star formation activity and dust temperature seen in one-third of the sample with warm IRAS colors. In that case, the upper limits on LC II/LFIR imply a ratio of UV flux to gas density of G -->0/n>10 cm -->3 (where G -->0 is in units of the local average interstellar field). The low LC II/LFIR ratio could also be due to either weak [C II], owing to self-absorption, or a strong FIR continuum from regions weak in [C II], such as dense H II regions or plasma ionized by hard radiation of active galactic nuclei. The mid-infrared and radio images of these galaxies show that most of the emission comes from a compact nucleus. CO and H I are detected in these galaxies, with H I seen in absorption toward the nucleus.

Self-gravitating accretion disks in active galactic nuclei

Accretion onto supermassive black holes (SBHs) is widely believed to be responsible for the phenomena of active galactic nuclei (AGN). While there has been considerable theoretical work on the inner regions of such accretion flows, little is known about the outer boundaries of AGN. Except for the broad and narrow emission lines, the observational constraints are weak, and evidence linking the emission-line regions to the accretion flow is ambiguous at best. It is not even known whether the accretion flow is fuelled by mass loss from a dense cluster of stars surrounding the SBH, or enters the nucleus from the galactic interstellar medium (ISM). Here we adopt the latter view, assuming that most of the incoming fuel forms a thin accretion disk at distances of ∼ 10–103 pc from the SBH. Such a disk must be vertically self-gravitating. We analyse some of its thermal and dynamical properties, and conclude that its energetics is likely to be dominated by backscattered AGN radiation. We also discuss the conditions under which Jeans fragmentation of such a disk into weakly interacting cloudlets can be avoided, and give a necessary condition for disk fragmentation to occur.

A nonthermal model for the XUV, soft X-ray emission of AGN and QSOs

XUV and soft X-ray radiation of active galactic nuclei may be explained by synchrotron emission of secondary electrons resulting from pp collisions in the vicinity of a supermassive compact object. The spectrum would be the natural extension of the ultraviolet emission responsible for photoionization of the emission-line gas. 28 references.

The Electron-Positron Cauldron and the Formation of the Hard Radiation of Quasars and AGN

Active Galactic Nuclei (AGN) and quasars have unique physical parameters among all the objects in the Universe. Undoubtedly it is the uniqueness of the physical conditions in these systems that gives rise to the peculiar physical processes in them.