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Seyfert Galaxies (seyfert + galaxy)

Distribution by Scientific Domains

Physics and Astronomy	100%

Selected Abstracts

Gas dynamics of the central few parsec region of NGC 1068 fuelled by the evolving nuclear star cluster

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010
M. Schartmann
ABSTRACT Recently, high-resolution observations with the help of the near-infrared adaptive optics integral field spectrograph Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) at the Very Large Telescope proved the existence of massive and young nuclear star clusters in the centres of a sample of Seyfert galaxies. With the help of three-dimensional high-resolution hydrodynamical simulations with the Pluto code, we follow the evolution of such clusters, especially focusing on stellar mass loss feeding gas into the ambient interstellar medium and driving turbulence. This leads to a vertically wide distributed clumpy or filamentary inflow of gas on large scales (tens of parsec), whereas a turbulent and very dense disc builds up on the parsec scale. In order to capture the relevant physics in the inner region, we treat this disc separately by viscously evolving the radial surface density distribution. This enables us to link the tens of parsec-scale region (accessible via SINFONI observations) to the (sub-)parsec-scale region (observable with the mid-infrared interferometer instrument and via water maser emission). Thereby, this procedure provides us with an ideal testbed for data comparison. In this work, we concentrate on the effects of a parametrized turbulent viscosity to generate angular momentum and mass transfer in the disc and additionally take star formation into account. Most of the input parameters are constrained by available observations of the nearby Seyfert 2 galaxy NGC 1068, and we discuss parameter studies for the free parameters. At the current age of its nuclear starburst of 250 Myr, our simulations yield disc sizes of the order of 0.8,0.9 pc, gas masses of 106 M, and mass transfer rates of 0.025 M, yr,1 through the inner rim of the disc. This shows that our large-scale torus model is able to approximately account for the disc size as inferred from interferometric observations in the mid-infrared and compares well to the extent and mass of a rotating disc structure as inferred from water maser observations. Several other observational constraints are discussed as well. [source]

The impact of radio feedback from active galactic nuclei in cosmological simulations: formation of disc galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Takashi Okamoto
ABSTRACT In this paper, we present a new implementation of feedback due to active galactic nuclei (AGN) in cosmological simulations of galaxy formation. We assume that a fraction of jet energy, which is generated by an AGN, is transferred to the surrounding gas as thermal energy. Combining a theoretical model of mass accretion on to black holes with a multiphase description of star-forming gas, we self-consistently follow evolution of both galaxies and their central black holes. The novelty in our model is that we consider two distinct accretion modes: standard radiatively efficient thin accretion discs and radiatively inefficient accretion flows which we will generically refer to as RIAFs; motivated by theoretical models for jet production in accretion discs, we assume that only the RIAF is responsible for the AGN feedback. The focus of this paper is to investigate the interplay between galaxies and their central black holes during the formation of a disc galaxy. We find that, after an initial episode of bursting star formation, the accretion rate on to the central black hole drops so that the accretion disc switches to a RIAF structure. At this point, the feedback from the AGN becomes efficient and slightly suppresses star formation in the galactic disc and almost completely halts star formation in the bulge. This suppression of the star formation regulates mass accretion on to the black hole and associated AGN feedback. As a result, the nucleus becomes a stochastically fuelled low-luminosity AGN (Seyfert galaxy) with recurrent short-lived episodes of activity after the star bursts. During the ,on' events, the AGN produces reasonably powerful jets (radio-loud state) and is less luminous than the host galaxy, while in the ,off' phase, the nucleus is inactive and ,radio quiet'. Our model predicts several properties of the low-luminosity AGN including the bolometric luminosity, jet powers, the effect on kpc scale of the radio jet and the AGN lifetime, which are in broad agreement with observations of Seyfert galaxies and their radio activity. We also find that the ratios between the central black hole mass and the mass of the host spheroid at z= 0 are ,10,3 regardless of the strength of either supernova feedback or AGN feedback because the radiation drag model directly relates the star formation activity in the Galactic Centre and the mass accretion rate on to the central black hole. [source]

Are galaxies with active galactic nuclei a transition population?

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
P. B. Westoby
ABSTRACT We present the results of an analysis of a well-selected sample of galaxies with active and inactive galactic nuclei from the Sloan Digital Sky Survey, in the range 0.01 < z < 0.16. The SDSS galaxy catalogue was split into two classes of active galaxies, Type 2 active galactic nuclei (AGN) and composites, and one set of inactive, star-forming/passive galaxies. For each active galaxy, two inactive control galaxies were selected by matching redshift, absolute magnitude, inclination, and radius. The sample of inactive galaxies naturally divides into a red and a blue sequence, while the vast majority of AGN hosts occur along the red sequence. In terms of H, equivalent width (EW), the population of composite galaxies peaks in the valley between the two modes, suggesting a transition population. However, this effect is not observed in other properties such as the colour,magnitude space or colour,concentration plane. Active galaxies are seen to be generally bulge-dominated systems, but with enhanced H, emission compared to inactive red-sequence galaxies. AGN and composites also occur in less dense environments than inactive red-sequence galaxies, implying that the fuelling of AGN is more restricted in high-density environments. These results are therefore inconsistent with theories in which AGN host galaxies are a ,transition' population. We also introduce a systematic 3D spectroscopic imaging survey, to quantify and compare the gaseous and stellar kinematics of a well-selected, distance-limited sample of up to 20 nearby Seyfert galaxies, and 20 inactive control galaxies with well-matched optical properties. The survey aims to search for dynamical triggers of nuclear activity and address outstanding controversies in optical/infrared imaging surveys. [source]

Central kiloparsec of Seyfert and inactive host galaxies: a comparison of two-dimensional stellar and gaseous kinematics

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
Gaëlle Dumas
ABSTRACT We investigate the properties of the two-dimensional distribution and kinematics of ionized gas and stars in the central kiloparsec of a matched sample of nearby active (Seyfert) and inactive galaxies, using the SAURON integral field unit on the William Herschel Telescope. The ionized gas distributions show a range of low-excitation regions, such as star formation rings in Seyfert and inactive galaxies, and high-excitation regions related to photoionization by the active galactic nucleus (AGN). The stellar kinematics of all galaxies in the sample show regular rotation patterns typical of disc-like systems, with kinematic axes that are well aligned with those derived from the outer photometry and provide a reliable representation of the galactic line of nodes. After removal of the non-gravitational components due to, for example, AGN-driven outflows, the ionized gas kinematics in both the Seyfert and inactive galaxies are also dominated by rotation with global alignment between stars and gas in most galaxies. This result is consistent with previous findings from photometric studies that the large-scale light distribution of Seyfert hosts is similar to that of inactive hosts. However, by fully exploiting the two-dimensional nature of our spectroscopic data, deviations from axisymmetric rotation in the gaseous velocity fields are identified, which suggest that the gaseous kinematics are more disturbed at small radii in the Seyfert galaxies compared with the inactive galaxies. This provides a tentative link between nuclear gaseous streaming and nuclear activity. [source]

OH megamasers, starburst and AGN activity in Markarian 231

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2005
A. M. S. Richards
ABSTRACT We present Multi-Element Radio-Linked Interferometer Network (MERLIN) observations of OH maser and radio continuum emission within a few hundred pc of the core of the ultraluminous infrared galaxy (ULIRG) Markarian 231. This is the only known OH megamaser galaxy classed as a Seyfert 1. Maser emission is identified with the 1665- and 1667-MHz transitions over a velocity extent of 720 km s,1. Both lines show a similar position,velocity structure including a gradient of 1.7 km s,1 pc,1 from NW to SE along the 420-pc major axis. The (unresolved) inner few tens of pc possess a much steeper velocity gradient. The maser distribution is modelled as a torus rotating about an axis inclined at ,45° to the plane of the sky. We estimate the enclosed mass density to be 320 ± 90 M, pc,3 in a flattened distribution. This includes a central unresolved mass of ,8 × 106 M,. All the maser emission is projected against a region with a radio continuum brightness temperature ,105 K, giving a maser gain of ,2.2. The 1667:1665 MHz line ratio is close to 1.8 (the value predicted for thermal emission) consistent with radiatively pumped, unsaturated masers. This behaviour and the kinematics of the torus suggest that the size of individual masing regions is in the range 0.25,4 pc with a covering factor close to unity. There are no very bright compact masers, in contrast to galaxies such as the Seyfert 2 Markarian 273, where the masing torus is viewed nearer edge-on. The comparatively modest maser amplification seen from Markarian 231 is consistent with its classification in the unification scheme for Seyfert galaxies. Most of the radio continuum emission on 50,500 pc scales is probably of starburst origin but the compact peak is 0.4 per cent polarized by a magnetic field running north,south, similar to the jet direction on these scales. There is no close correlation between maser and continuum intensity, suggesting that much of the radio continuum must originate in the foreground and indeed the relative continuum brightness is slightly greater in the direction of the approaching jet. Comparisons with other data show that the jet changes direction close the nucleus and suggest that the sub-kpc disc hosting the masers and starburst activity is severely warped. [source]

A deep Chandra survey of the Groth Strip , I. The X-ray data

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2005
K. Nandra
ABSTRACT We present the results of a 200-ks Chandra observation of part of the Groth Strip region, using the ACIS-I instrument. We present a relatively simple method for the detection of point sources and calculation of limiting sensitivities, which we argue is at least as sensitive and more self-consistent than previous methods presented in the literature. A total of 158 distinct X-ray sources are included in our point-source catalogue in the ACIS-I area. The number counts show a relative dearth of X-ray sources in this region. For example, at a flux limit of 10,15 erg cm,2 s,1, around 20 per cent more soft-band sources are detected in the HDF-N and almost 50 per cent more in the ELAIS-N1 field, which we have analysed by the same method for comparison. We find, however, that these differences are consistent with Poisson variations at <2, significance, and therefore there is no evidence for cosmic variance based on these number counts alone. We determine the average spectra of the objects and find a marked difference between the soft-band-selected sources, which have ,= 1.9 typical of unobscured active galactic nuclei (AGN), and the hard-band-selected sources, which have ,= 1.0. Reassuringly, the sample as a whole has a mean spectrum of ,= 1.4 ± 0.1, the same as the X-ray background. None the less, our results imply that the fraction of sources with significant obscuration is only ,25 per cent, much less than predicted by standard AGN population synthesis models. This is confirmed by direct spectral fitting, with only a handful of objects showing evidence for absorption. After accounting for absorption, all objects are consistent with a mean intrinsic spectrum of ,= 1.76 ± 0.08, very similar to local Seyfert galaxies. The survey area is distinguished by having outstanding multiwaveband coverage. Comparison with these observations and detailed discussion of the X-ray source properties will be presented in future papers. [source]

The nature of X-ray spectral variability in Seyfert galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2003
Richard D. Taylor
ABSTRACT We use a model-independent technique to investigate the nature of the 2,15 keV X-ray spectral variability in four Seyfert galaxies and distinguish between spectral pivoting and the two-component model for spectral variability. Our analysis reveals conclusively that the softening of the X-ray continuum with increasing flux in MCG,6-30-15 and NGC 3516 is a result of summing two spectral components: a soft varying component (SVC) with spectral shape independent of flux and a hard constant component (HCC). In contrast, the spectral variability in NGC 4051 can be well described by simple pivoting of one component, together with an additional hard constant component. The spectral variability model for NGC 5506 is ambiguous, due to the smaller range of fluxes sampled by the data. We investigate the shape of the hard spectral component in MCG,6-30-15 and find that it appears similar to a pure reflection spectrum, but requires a large reflected fraction (R > 3). We briefly discuss physical interpretations of the different modes of spectral variability. [source]

An XMM,Newton observation of Ton S180: constraints on the continuum emission in ultrasoft Seyfert galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002
S. Vaughan
Abstract We present an XMM,Newton observation of the bright, narrow-line, ultrasoft type 1 Seyfert galaxy Ton S180. The 0.3,10 keV X-ray spectrum is steep and curved, showing a steep slope above 2.5 keV (,, 2.3) and a smooth, featureless excess of emission at lower energies. The spectrum can be adequately parametrized using a simple double power-law model. The source is strongly variable over the course of the observation but shows only weak spectral variability, with the fractional variability amplitude remaining approximately constant over more than a decade in energy. The curved continuum shape and weak spectral variability are discussed in terms of various physical models for the soft X-ray excess emission, including reflection off the surface of an ionized accretion disc, inverse Compton scattering of soft disc photons by thermal electrons, and Comptonization by electrons with a hybrid thermal/non-thermal distribution. We emphasize the possibility that the strong soft excess may be produced by dissipation of accretion energy in the hot, upper atmosphere of the putative accretion disc. [source]

On the relation between the coronal line emission and the infrared/X-ray emission in Seyfert galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002
M. Almudena Prieto
The relation between the X-ray, the coronal line and the infrared (IR) emissions in a sample of the brightest known Seyfert galaxies is analysed. A close relationship between the absorption-corrected soft X-ray emission and both the mid-IR and the coronal line emission is found for the Seyfert type 2 objects in the sample. The coronal line and the X-ray emissions are both main tracers of active galactic nuclei (AGN) activity and their relationship with the mid-IR emission points to the heat of the carbuncle dust as the main energetic process associated with the AGN. On the other hand, the above relations do not seem to hold for the type 1 Seyfert discussed in the sample, at least when the comparisons are made in a flux diagram. This is partially because of the reduced number of objects of this type analysed in this work and also the fact that the measured soft X-ray emission in Seyfert 1s is systematically larger, by at least an order of magnitude, than that in the Seyfert 2 counterparts. Finally, the hard X-ray emission in the studied sample appears unrelated to either the mid-IR or the coronal line emission. [source]

Variability in active galactic nuclei: confrontation of models with observations

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002
M.R.S. Hawkins
The variability of active galactic nuclei (AGN) has long held the promise of shedding light on their detailed structure, and possibly other astrophysical phenomena. Different emission mechanisms lead to different patterns of variability in flux, which are in principle easily distinguishable. Recent predictions for the expected spectrum of variations for various models are now in such a form that they can be compared with the observed statistical properties of AGN light curves from large-scale monitoring programmes. In this paper, we use the results of a long-term monitoring programme of a large sample of quasars and Seyfert galaxies, as well as individual light curves from the literature, to distinguish between the various model predictions. The results favour a model based on accretion disc instability over the starburst model, where the variation comes from a succession of supernova bursts, but it also appears that much of the observed variation in quasars is due to gravitational microlensing. [source]

A short introduction to broad and variable iron lines around black holes

ASTRONOMISCHE NACHRICHTEN, Issue 10 2006
A. C. Fabian
Abstract Accreting black holes often show iron line emission in their X-ray spectra. When this line emission is very broad or variable then it is likely to originate from close to the black hole. The theory and observations of such broad and variable iron lines are briefly reviewed here. In order for a clear broad line to be found, one or more of the following have to occur: high iron abundance, dense disk surface and minimal complex absorption. Several excellent examples are found from observations of Seyfert galaxies and Galactic Black Holes. In several cases there is strong evidence that the black hole is rapidly spinning. Further examples are expected as more long observations are made with XMM-Newton, Chandra and Suzaku. Intriguing instances of rapid variability of some narrow iron lines, both emission and absorption, have been reported. These may reflect variations in the irradiation or motion of physical structures on the accretion disk. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Resolving the source populations that contribute to the X-ray background: The 2 Ms Chandra Deep Field-North Survey

ASTRONOMISCHE NACHRICHTEN, Issue 1-2 2003
D. M. Alexander
Abstract With , 2 Ms of exposure, the Chandra Deep Field-North (CDF-N) survey provides the deepest view of the Universe in the 0.5,8.0 keV band. Five hundred and three (503) X-ray sources are detected down to on-axis 0.5,2.0 keV and 2,8 keV flux limits of , 1.5 × 10,17 erg cm,2 s,1 and , 1.0 × 10,16 erg cm,2 s,1, respectively. These flux limits correspond to L0.5,8.0 keV, 3 × 1041 erg s,1 at z = 1 and L0.5,8.0 keV, 2 × 1043 erg s,1 at z = 6; thus this survey is sensitive enough to detect starburst galaxies out to moderate redshift and Seyfert galaxies out to high redshift. We present the X-ray observations, describe the broad diversity of X-ray selected sources, and review the prospects for deeper exposures. [source]

The disc-dominated host galaxy of FR-I radio source B2 0722+30

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
B. H. C. Emonts
ABSTRACT We present new observational results that conclude that the nearby radio galaxy B2 0722+30 is one of the very few known disc galaxies in the low-redshift Universe that host a classical double-lobed radio source. In this paper, we use H i observations, deep optical imaging, stellar population synthesis modelling and emission-line diagnostics to study the host galaxy, classify the active galactic nucleus (AGN) and investigate environmental properties under which a radio-loud AGN can occur in this system. Typical for spiral galaxies, B2 0722+30 has a regularly rotating gaseous disc throughout which star formation occurs. Dust heating by the ongoing star formation is likely responsible for the high infrared luminosity of the system. The optical emission-line properties of the central region identify a Low Ionization Nuclear Emission-line Region (LINER)-type nucleus with a relatively low [O iii] luminosity, in particular when compared with the total power of the Fanaroff & Riley type-I radio source that is present in this system. This classifies B2 0722+30 as a classical radio galaxy rather than a typical Seyfert galaxy. The environment of B2 0722+30 is extremely H i -rich, with several nearby interacting galaxies. We argue that a gas-rich interaction involving B2 0722+30 is a likely cause for the triggering of the radio AGN and/or the fact that the radio source managed to escape the optical boundaries of the host galaxy. [source]