1,721,000 research outputs found
THICK TORI AROUND ACTIVE GALACTIC NUCLEI - A COMPARISON OF MODEL PREDICTIONS WITH OBSERVATIONS OF THE INFRARED CONTINUUM AND SILICATE FEATURES
No full textThe continuum expected from active galactic nuclei (AGN) surrounded by thick tori is compared to the data available for a sample of optically selected Seyfert 1 galaxies. The optical and near-IR nuclear fluxes have previously been derived for these objects, and hence the spectral energy distributions (SEDs) of their active nuclei can be analysed. We perform detailed calculations of the 0.1 to 1000 mum SEDs for AGN surrounded by dust tori with different opening angles. The dust mixture is mimicked by using three silicate and three graphite grains of different sizes. The radiative transfer equation for a cloud having azimuthal symmetry and containing a mixture of dust grains is solved by means of a numerical code that takes absorption, emission and scattering into account. We discuss the optimization of the free parameters by comparing the spectra predicted by the code to available data. The general absence of the silicate emission feature at about 10 mum in the spectra of broad-line AGN and the general presence of the 10-mum silicate absorption feature in narrow-line AGN are addressed. The ensuing constraints are discussed. We show that models of thick tori extending up to a few hundred parsecs in which physical processes such as shocks significantly reduce the silicate grain abundance within the first few tens of parsecs are fully consistent with available broad-band data and high-resolution IR spectra of Seyfert 1 and 2 nuclei
Thick tori around active galactic nuclei: The case for extended tori and consequences for their X-ray and infrared emission
No full textTwo families of models of dusty tori in active galactic nuclei (AGNs; moderately thick and extended versus very thick and compact) are tested against available observations. The confrontation suggests that the former class better explains the infrared (IR) broadband spectra of both broad-and narrow-line AGNs, the anisotropy of the emission deduced by comparing IR properties of Seyfert 1 and 2 nuclei, and the results of IR spectroscopy and those of high spatial resolution observations. There is, however, clear evidence for a broad distribution of optical depths. We also examine the relationship between IR and X-ray emission. The data support a view in which the matter responsible for the X-ray absorption is mostly dust free, lying inside the dust sublimation radius. The consequences of these results for the hard X-ray background as well as IR counts and background are discussed
Dust-enshrouded AGN models for hyperluminous, high-redshift infrared galaxies
No full textWe investigate models for the power supply and broadband spectral energy distribution (SED) of hyperluminous IR galaxies, recently discovered at high redshifts, in terms of the emission from an active nucleus embedded in a torus-like dusty structure. We find consistent solutions in terms of a simple torus model extended several hundreds of parsecs, with A(v) in the equatorial plane of a few hundred and a typical covering factor of over 50%. Objects as different as the prototypical high-z galaxy IRAS F10214, the z = 0.93 IR object IRAS F15307, IRAS 09104, found in a high-z cooling flow, and the optically selected broad absorption line ''Cloverleaf'' quasar, are all fitted by the same solution for decreasing values of the polar angle to The line of sight and proper scaling of the luminosities, We suggest that such luminous, high-z IR objects are heavily buried quasars surrounded by large amounts of dust with high covering factors and large optical depths. Comparison with ultraviolet-excess QSOs suggests that they are observed during a transient phase. Forthcoming observations in the far-IR will soon allow probing of this phase and its relationship with the-possibly concomitant-formation of the nuclear black hole and the host galaxy
The connection between spheroidal galaxies and QSOs
No full textIn view of the extensive evidence of tight inter-relationships between spheroidal galaxies (and galactic bulges) with massive black holes hosted at their centers, a consistent model must deal jointly with the evolution of the two components. We describe one such model, which successfully accounts for the local luminosity function of spheroidal galaxies, for their photometric and chemical properties, for deep galaxy counts in different wavebands, including those in the (sub)-mm region which proved to be critical for current semi-analytic models stemming from the standard hierarchical clustering picture, for clustering properties of SCUBA galaxies, of EROs, and of LBGs, as well as for the local mass function of massive black holes and for quasar evolution. Predictions that can be tested by surveys carried out by SIRTF are presented
Observational tests of the galaxy formation process
No full textThe mutual feedback between star fori nation and nuclear activity in large spheroidal galaxies may be a key ingredient to overcome several difficulties plaguing current semi-analytic models for galaxy formation. We discuss some observational implications of the model by Granato et al. for the co-evolution of galaxies and active nuclei at their centers and stress the potential of the forthcoming surveys of the Sunyaev-Zeldovich effect on arcminute scales, down to muK levels, to investigate the early galaxy-formation phases, difficult to access by other means
A STUDY OF A HOMOGENEOUS SAMPLE OF OPTICALLY SELECTED ACTIVE GALACTIC NUCLEI .3. OPTICAL OBSERVATIONS
No full textWe present CCD observations in BVR optical bands of the homogeneous sample of 42 Seyfert 1 galaxies, that we also observed in near-IR JHK bands (Zitelli et al. 1993; Danese et al. 1992). We have applied to these data the same analysis procedure used for the infrared images, with the aim of separating the galaxian and nuclear fluxes, as well as investigating the main characteristics of the hosts. Nuclear fluxes have been estimated with typical global errors of 0. 1 5 mag, while 0.3 mag errors are on.average associated to host galaxy magnitudes. It is shown that in half of the host galaxies the bulge contribute more than 40% to the total observed fluxes even within small apertures. In the sample galaxies disks are dominating over the bulge in the large majority of the cases, suggesting that a significant fraction of Seyfert galaxies could be late-type spirals. Moreover the galactic morphological parameters are in the ranges of typical spiral galaxies. The statistics of galactic colors show that the host galaxies tend to be bluer than the normal ones. This fact added to redder colors found in the IR-bands strengthens the suggestion that in Seyfert galaxies the star formation is on average enhanced
An evolutionary model for GHz peaked spectrum sources - Predictions for high frequency surveys
No full textWe have explored, in the general framework of the "young source" scenario, evolutionary models for GHz Peaked Spectrum (GPS) galaxies and quasars which reproduce the observed counts, redshift and peak frequency distributions of currently available samples. Substantially different cosmological evolution properties are found for the two populations: the quasar luminosity function must evolve strongly up to z similar to 1, while the data on galaxies may be consistent with no evolution. The models show that GPS sources (mostly quasars) may comprise quite a significant fraction of bright (S > 1 Jy) radio sources at v greater than or equal to 30 GHz if the intrinsic distribution of peak frequencies extends up to similar to 1000 GHz. In any case, however, their fraction decreases rapidly with decreasing flux and their contribution to small scale fluctuations in the frequency range covered by the forthcoming space missions MAP and Planck Surveyor is expected to be minor
Theoretical predictions on the clustering of SCUBA galaxies and implications for small-scale fluctuations at submillimetre wavelengths
No full textThis paper investigates the clustering properties of Submillimetre Common User Bolometric Array (SCUBA) selected galaxies within the framework of a unifying scheme relating the formation of quasi-stellar objects and spheroids. The theoretical angular correlation function is derived for different bias functions, corresponding to different values of the ratio Mh(alo)/M-sph between the mass of the dark halo and the final mass in stars. SCUBA sources are predicted to be strongly clustered, with a clustering strength increasing with mass. We show that the model accounts for the clustering of Lyman-break galaxies, seen as the optical counterpart of low- to intermediate-mass primeval spheroidal galaxies, and is also consistent with the observed angular correlation function of extremely red objects. Best agreement is obtained for M-halo/M-sph = 100. We also consider the implications for small-scale fluctuations observed at submillimetre wavelengths by current or forthcoming experiments aimed at mapping the cosmic microwave background (CMB). The predicted amplitude of the clustering signal in the 350-GHz channel of the Planck mission strongly depends on the halo-to-bulge mass ratio and may be of comparable amplitude to primary CMB anisotropies for multipole numbers 1 greater than or similar to 50
A Physical Model for the Coevolution of QSOs and Their Spheroidal Hosts
No full textWe present a physically motivated model for the early coevolution of massive spheroidal galaxies and active nuclei at their centers. Within dark matter halos, forming at the rate predicted by the canonical hierarchical clustering scenario, the gas evolution is controlled by gravity, radiative cooling, and heating by feedback from supernovae and from the growing active nucleus. Supernova heating is increasingly effective with decreasing binding energy in slowing down the star formation and in driving gas outflows. The more massive protogalaxies virializing at earlier times are thus the sites of the faster star formation. The correspondingly higher radiation drag fastens the angular momentum loss by the gas, resulting in a larger accretion rate onto the central black hole. In turn, the kinetic energy carried by outflows driven by active nuclei can unbind the residual gas, thus halting both the star formation and the black hole growth, in a time again shorter for larger halos. For the most massive galaxies the gas unbinding time is short enough for the bulk of the star formation to be completed before Type Ia supernovae can substantially increase the Fe abundance of the interstellar medium, thus accounting for the α-enhancement seen in the largest galaxies. The feedback from supernovae and from the active nucleus also determines the relationship between the black hole mass and the mass, or the velocity dispersion, of the host galaxy, as well as the black hole mass function. In both cases the model predictions are in excellent agreement with the observational data. Coupling the model with GRASIL (Silva et al. 1998), the code computing in a self-consistent way the chemical and spectrophotometric evolution of galaxies over a very wide wavelength interval, we have obtained predictions in excellent agreement with observations for a number of observables that proved to be extremely challenging for all the current semianalytic models, including the submillimeter counts and the corresponding redshift distributions, and the epoch-dependent K-band luminosity function of spheroidal galaxies
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