Fueling Of Active Galactic Nuclei Research Articles

Gas Dynamics in AGN Galaxies: First Results of the HI-NUGA Survey

AbstractThe process of fueling Active Galactic Nuclei (AGN) in galaxies is commonly associated with gas aggregation onto a central supermassive black hole. Up to now a general picture of the AGN fueling is missing: What mechanisms are responsible for the various kinds of AGNs? How to remove angular momentum from the material far away from the gravitational influence of the central black hole? Over the last decade observations have been pointing towards a hierarchy of mechanisms that all combine to transport the gas from the large kpc scales down to the inner pc scales. In addition, fueling mechanisms excited by the galactic environment, e.g. through minor mergers and tidal interactions, have been suggested.In order to study the gas transport from the outskirts of galaxies to the central AGN, we have embarked on a key project, NUGA (Nuclei of Galaxies): a high spectral and angular resolution CO and HI survey of low luminosity AGN in nearby galaxies (i.e. Seyferts, LINERs and transition objects).For our detailed HI study, 16 nearby AGN galaxies were observed in their 21 cm line emission of neutral HI gas using the NRAO Very Large Array (VLA) interferometer in its C- and D-array configuration (~20'' angular and ~5.2 km s−1 spectral resolution). The spiral galaxies in our sample range in Hubble type from Sa to Sbc. For all galaxies high quality mm-interferometric CO data as well as optical/NIR imaging data are available.Here, we present first results on the atomic gas distribution and kinematics in these galaxies: The HI gas properties are analyzed and then compared to the optical properties. Their HI morphology reveals a wide variety: rings, spiral arms, and central concentrated, where ring structures are mostly found in LINER hosts. The integrated surface brightness profiles show some significant differences between the HI gas and stellar light as well as their radial extent. Our analysis of the HI environment yields that 4 galaxies are surrounded by satellites and 8 galaxies have disturbed outer HI disks, probably due to ram pressure stripping and/or minor mergers. Our analysis of the HI kinematics provides rotation curves, dynamical masses and evidence for streaming motions for each galaxy.The inclusion of recently obtained VLA B-array data will provide ~7″ resolution for more than half of the sample. We will utilize gravity torques based on a definite phase shift between the gravitational potential and the HI gas distribution to determine the gas inflow, and thus the efficiency for feeding the central black hole. Further, the combined CO+HI+NIR/opt data will allow us to develop dynamical models describing the gas flow in galactic disks from the large kpc scales down to the inner pc scales for selected galaxies in our sample.

Molecular gas in NUclei of GAlaxies (NUGA)

We discuss the efficiency of stellar gravity torques as a mechanism to account for the feeding of the central engines of four low luminosity Active Galactic Nuclei (AGN): NGC 4321 (HII nucleus/LINER), NGC 4826 (HII nucleus/LINER), NGC 4579 (LINER 1.9/Seyfert 1.9) and NGC 6951 (Seyfert 2). These galaxies have been observed as part of the NUclei of GAlaxies–(NUGA) CO project, aimed at the study of AGN fueling mechanisms. Our calculations allow us to derive the characteristic time-scales for gas flows and discuss whether torques from the stellar potentials are efficient enough to drain the gas angular momentum in the inner 1 kpc of these galaxies. The stellar potentials are derived using high-resolution near infrared (NIR) images and the averaged effective torques on the gas are estimated using the high-resolution (~0.5´´–2´´) CO maps of the galaxies. Results indicate paradoxically that feeding should be thwarted close to the AGNs: in the four cases analyzed, gravity torques are mostly positive inside pc, resulting in no inflow on these scales. As a possible solution for the paradox, we speculate that the agent responsible for driving inflow to still smaller radii is transient and thus presently absent in the stellar potential. Alternatively, the gravity torque barrier associated with the Inner Lindblad Resonance of the bars in these galaxies could be overcome by other mechanisms that become competitive in due time against gravity torques. In particular, we estimate on a case-by-case basis the efficiency of viscosity versus gravity torques to drive AGN fueling. We find that viscosity can counteract moderate-to-low gravity torques on the gas if it acts on a nuclear ring of high gas surface density contrast and ~a few 100 pc size. We propose an evolutionary scenario in which gravity torques and viscosity act in concert to produce recurrent episodes of activity during the typical lifetime of any galaxy. In this scenario the recurrence of activity in galaxies is indirectly related to that of the bar instabilities although the active phases are not necessarily coincident with the maximum strength of a single bar episode. The general implications of these results for the current understanding of fueling of low-luminosity AGN are discussed.

Radiation Pressure–supported Starburst Disks and Active Galactic Nucleus Fueling

We consider the structure of marginally Toomre-stable starburst disks under the assumption that radiation pressure on dust grains provides the dominant vertical support against gravity. This is particularly appropriate when the disk is optically thick to its own IR radiation, as in the central regions of ULIRGs. Because the disk radiates at its Eddington limit, the Schmidt-law for star formation changes in the optically-thick limit, with the star formation rate per unit area scaling as Sigma_g/kappa, where Sigma_g is the gas surface density and kappa is the mean opacity. We show that optically thick starburst disks have a characteristic flux and dust effective temperature of F ~ 10^{13} L_sun/kpc^2 and T_eff ~ 90K, respectively. We compare our predictions with observations and find good agreement. We extend our model from many-hundred parsec scales to sub-parsec scales and address the problem of fueling AGN. We assume that angular momentum transport proceeds via global torques rather than a local viscosity. We account for the radial depletion of gas due to star formation and find a strong bifurcation between two classes of disk models: (1) solutions with a starburst on large scales that consumes all of the gas with little fueling of a central AGN and (2) models with an outer large-scale starburst accompanied by a more compact starburst on 1-10 pc scales and a bright central AGN. The luminosity of the latter models is in many cases dominated by the AGN. We show that the vertical thickness of the starburst disk on pc scales can approach h ~ r, perhaps accounting for the nuclear obscuration in some Type 2 AGN. We also argue that the disk of young stars in the Galactic Center may be the remnant of such a compact nuclear starburst.

AChandraSnapshot Survey of Infrared‐bright LINERs: A Possible Link Between Star Formation, Active Galactic Nucleus Fueling, and Mass Accretion

We present results from a high-resolution X-ray imaging study of nearby LINERs observed by ACIS on board Chandra. This study complements and extends previous X-ray studies of LINERs, focusing on the underexplored population of nearby dust-enshrouded infrared-bright LINERs. The sample consists of 15 IR-bright LINERs (LFIR/LB > 3), with distances that range from 11 to 26 Mpc. Combining our sample with previous Chandra studies, we find that ~51% (28/55) of the LINERs display compact hard X-ray cores. The nuclear 2-10 keV luminosities of the galaxies in this expanded sample range from ~2 × 1038 to ~2 × 1044 ergs s-1. We find that the most extreme IR-faint LINERs are exclusively active galactic nuclei (AGNs). The fraction of LINERs containing AGNs appears to decrease with IR brightness and increase again at the highest values of LFIR/LB. We find that of the 24 LINERs showing compact nuclear hard X-ray cores in the expanded sample that were observed at Hα wavelengths, only eight actually show evidence of a broad line. Similarly, of the 14 LINERs showing compact nuclear hard X-ray cores with corresponding radio observations, only eight display a compact flat spectrum radio core. These findings emphasize the need for high-resolution X-ray imaging observations in the study of IR-bright LINERs. Finally, we find an intriguing trend in the Eddington ratio versus LFIR and LFIR/LB for the AGN-LINERs in the expanded sample that extends over 7 orders of magnitude in L/LEdd. This correlation may imply a link between black hole growth, as measured by the Eddington ratio, and the star formation rate, as measured by the far-IR luminosity and IR-brightness ratio. If the far-IR luminosity is an indicator of the molecular gas content in our sample of LINERs, our results may further indicate that the mass accretion rate scales with the host galaxy's fuel supply. We discuss the potential implications of our results in the framework of black hole growth and AGN fueling in low-luminosity AGNs.

Near‐Infrared Spectroscopy of High‐Redshift Active Galactic Nuclei. I. A Metallicity–Accretion Rate Relationship

We present new near infrared spectroscopic measurements of the H_beta region for a sample of 29 luminous high redshift quasars. We have measured the width of H_beta in those sources, and added archival H_beta width measurements, to create a sample of 92 active galactic nuclei (AGNs) for which H_beta width and rest-frame UV measurements of N V \lambda 1240 and C IV \lambda 1549 emission-lines are available. Our sample spans six orders of magnitude in luminosity and includes 31 radio-loud AGNs. It also includes 10 narrow-line Seyfert 1 galaxies and one broad absorption-line quasar. We find that metallicity, indicated by the N V/C IV line ratio, is primarily correlated with accretion rate, which is a function of luminosity and H_beta line-width. This may imply an intimate relation between starburst, responsible for the metal enrichment of the nuclear gas, and AGN fueling, represented by the accretion rate. The correlation of metallicity with luminosity, or black hole (BH) mass, is weaker in contrast with recent results which were based on measurements of the width of C IV. We argue that using C IV as a proxy to H_beta in estimating M_BH might be problematic and lead to spurious BH mass and accretion rate estimates in individual sources. We discuss the potential implications of our new result in the framework of the starburst-AGN connection and theories of BH growth.

On Bars, Bulges, and the Fueling of Active Galactic Nuclei

Both the fueling of a central black hole and the buildup of bulges are—at least theoretically—facilitated by nonaxisymmetric potentials. It is argued here that the recently established surface brightness division between bulge-dominated galaxies that dominate the local type 2 active galactic nucleus (AGN) luminosity function and disk-dominated galaxies that host (at best) weak AGNs can be understood in terms of a simple bar-instability criterion.

Circumnuclear Dust in Nearby Active and Inactive Galaxies. II. Bars, Nuclear Spirals, and the Fueling of Active Galactic Nuclei

(Abridged) We present a detailed study of the relation between circumnuclear dust morphology, host galaxy properties, and nuclear activity in nearby galaxies. We use our sample of 123 nearby galaxies with visible--near-infrared colormaps from the Hubble Space Telescope to create well-matched, paired samples of 28 active and 28 inactive galaxies, as well as 19 barred and 19 unbarred galaxies, that have the same host galaxy properties. Comparison of the barred and unbarred galaxies shows that grand design nuclear dust spirals are only found in galaxies with a large-scale bar. Tightly wound nuclear dust spirals, in contrast, show a strong tendency to avoid galaxies with large-scale bars. Comparison of the AGN and inactive samples shows that nuclear dust spirals, which may trace shocks and angular momentum dissipation in the ISM, occur with comparable frequency in both active and inactive galaxies. The only difference between the active and inactive galaxies is that several inactive galaxies appear to completely lack dust structure in their circumnuclear region, while none of the AGN lack this structure. The comparable frequency of nuclear spirals in active and inactive galaxies, combined with previous work that finds no significant differences in the frequency of bars or interactions between well-matched active and inactive galaxies, suggests that no universal fueling mechanism for low-luminosity AGN operates at spatial scales greater than ~100 pc radius from the galactic nuclei. The similarities instead suggest that the lifetime of nuclear activity is less than the characteristic inflow time from these spatial scales. An order-of-magnitude estimate of this inflow time is the dynamical timescale. This sets an upper limit of several million years to the lifetime of an individual episode of nuclear activity.

The Formation of Nuclear Rings in Barred Spiral Galaxies

Although nuclear rings of gas and star formation are common in barred spiral galaxies, current theories of why and how they form do not provide the level of detail needed to quantify the effect that these rings can have on the fueling of active galactic nuclei and on the evolution of their host galaxy. In this paper we use detailed modeling to show that existence of nuclear rings is directly related to the existence of the orbit family whose major axis is perpendicular to the major axis of the bar (x_2). We explore a large range of barred galaxy potentials and for each potential we use a two-dimensional hydrodynamic simulation to determine whether and at what radius a nuclear ring forms. We compare the results of the hydrodynamic simulations to numerical integrations of periodic orbits in a barred potential and show that the rings only form when a minimum amount of x_2 orbits exists. Because the rings migrate inwards with time as they accumulate gas, the radius at which a nuclear ring is seen does not give direct information on the shape of the rotation curve. We also show that the common assumption that nuclear rings are related to an inner Lindblad resonance is incorrect. In fact, we show that there is no RESONANCE at the inner Lindblad resonance in barred galaxies. We also compare the predictions of this theory to HST observations and show that it correctly predicts the observed gas and star formation morphology of nuclear rings.

Hubble Space TelescopeImaging of the Circumnuclear Environments of the CfA Seyfert Galaxies: Nuclear Spirals and Fueling

We present archival Hubble Space Telescope (HST) images of the nuclear regions of 43 of the 46 Seyfert galaxies found in the volume limited,spectroscopically complete CfA Redshift Survey sample. Using an improved method of image contrast enhancement, we created detailed high-quality structure maps that allow us to study the distributions of dust, star clusters, and emission-line gas in the circumnuclear regions (100-1000 pc scales) and in the associated host galaxy. Essentially all of these Seyfert galaxies have circumnuclear dust structures with morphologies ranging from grand-design two-armed spirals to chaotic dusty disks. In most Seyfert galaxies there is a clear physical connection between the nuclear dust spirals on hundreds of parsec scales and large-scale bars and spiral arms in the host galaxies proper. These connections are particularly striking in the interacting and barred galaxies. Such structures are predicted by numerical simulations of gas flows in barred and interacting galaxies and may be related to the fueling of active galactic nuclei by matter inflow from the host galaxy disks. We see no significant differences in the circumnuclear dust morphologies of Seyfert 1s and 2s, and very few Seyfert 2 nuclei are obscured by large-scale dust structures in the host galaxies. If Sevfert 2s are obscured Sevfert Is, then the obscuration must occur on smaller scales than those probed by HST.

Hubble Space TelescopeObservations of the CfA Seyfert 2 Galaxies: Near‐Infrared Surface Photometry and Nuclear Bars

We present near-infrared J and H surface photometry of 24 of the nearby Seyfert 1.8, 1.9, and 2 galaxies from the CfA Seyfert sample. The excellent angular resolution of the Hubble Space Telescope probes spatial scales as small as tens of parsecs in most of these active galactic nuclei (AGNs) and is sensitive to the presence of nuclear bars and other potential signatures of the AGN fueling process that channels host galaxy gas and dust to the nuclear region. We have used elliptical isophote techniques to search for nuclear bars in all of these galaxies and have employed a two-dimensional fitting technique to model the nuclear point source and surface brightness distribution of a bright subsample of these galaxies in an attempt to alleviate the impact of the nuclear point source on our sensitivity to nuclear bars. We find stellar nuclear bar candidates in four of these galaxies: Mrk 471, Mrk 270, Mrk 573, and NGC 5929, nearly 20% of the total sample. The percentage rises to ~30% when systems with disturbed morphologies or high inclinations are excluded. The nuclear bars in Mrk 573 and Mrk 270 exhibit some evidence for dust lanes along their leading edges, analogous to the structures seen in host galaxy bars, while the dust lanes in Mrk 471 and NGC 5929 exhibit a more complex morphology. The fact that most of these AGNs do not appear to contain stellar nuclear bars suggests that they are not the fueling mechanism for most low-luminosity AGNs.

Bar‐driven Spiral Density Waves and Accretion in Gaseous Disks

The nonlinear response to a periodic bar potential in a differentially rotating gaseous disk is studied. A comprehensive theory, based on the work of Shu, Yuan, & Lissauer and Yuan & Cheng, is developed further. This approach allows us to include the effects of viscosity and self-gravitation explicitly in the formulation, and calculates directly the streamlines of the gas flow distorted by the rotating bar potential. Using it, we show that a major morphological difference exists among the spiral waves excited at the three types of Lindblad resonance. Spirals associated with the outer Lindblad resonance are tightly wound, while those associated with the inner Lindblad resonances are relatively open. In general, spirals are trailing. However, those excited at the inner inner Lindblad resonance are leading. These results help us understand the underlying physics of the spiral structure revealed in the recent radio interferometric observations of the central regions of disk galaxies. The transport of angular momentum between the bar and the disk is related directly to these waves. The spiral waves excited at the inner Lindblad resonances would induce inflows of disk material toward the center, while those excited at the outer Lindblad resonance induce outflows. Formulae to estimate the rates of the inflows and outflows are derived, and they are used to examine the process of fueling of active galactic nuclei and starburst rings.

Stellar Mass Loss in Elliptical Galaxies and the Fueling of Active Galactic Nuclei

The connection between some observational properties of active galactic nuclei and their host galaxies is studied using a self-consistent model of galactic evolution which reproduces the main features of elliptical galaxies. It is shown that stellar mass loss, calculated in detail from a mixture of stellar generations, can explain the bolometric luminosities and active nucleus-to-host galaxy luminosity ratios of local quasars residing in ellipticals (i.e., the radio-loud ones), under the hypothesis that the power supply is primarily gravitational. The time dependence of the mass-loss rate, however, seems to be too weak to explain quasar evolution as inferred from observations

Dynamical heating of stars as a fueling mechanism for active galactic nuclei

view Abstract Citations (6) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamical Heating of Stars as a Fueling Mechanism for Active Galactic Nuclei Allen, A. J. ; Hughes, P. A. Abstract A theoretical model is developed for the fueling of active galactic nuclei by the capture of stars by massive black holes. Although the stars may be outside the Roche lobe, a small accretion flow would be sufficient to move stars to lower orbits. Disruption would occur from heating during multiple passages through pericenter on timescales shorter than those of the transport of energy from the stellar interior. The calculations cover the orbital evolution in terms of two-body relaxation, orbital decay and dissipative processes. Attention is also given to ablation and heating of stars due to encounters with the accretion torus, X-ray heating and tidal heating, and permit describing conditions under which a star is disrupted. Publication: The Astrophysical Journal Pub Date: February 1987 DOI: 10.1086/164957 Bibcode: 1987ApJ...313..152A Keywords: Active Galactic Nuclei; Black Holes (Astronomy); Kinetic Heating; Stellar Orbits; Stellar Temperature; Orbit Decay; Pressure Effects; Radiative Heat Transfer; Stellar Interiors; Tides; Astrophysics; BLACK HOLES; GALAXIES: NUCLEI; QUASARS; STARS: STELLAR DYNAMICS full text sources ADS |