548 research outputs found
Intergalactic Photon Spectra from the Far IR to the UV Lyman Limit for and the Optical Depth of the Universe to High Energy Gamma-Rays
We calculate the intergalactic photon density as a function of both energy
and redshift for 0 < z < 6 for photon energies from .003 eV to the Lyman limit
cutoff at 13.6 eV in a Lambda-CDM universe with and
. Our galaxy evolution model gives results which are
consistent with Spitzer deep number counts and the spectral energy distribution
of the extragalactic background radiation. We use our photon density results to
extend previous work on the absorption of high energy gamma-rays in
intergalactic space owing to interactions with low energy photons and the 2.7 K
cosmic background radiation. We calculate the optical depth of the universe,
tau, for gamma-rays having energies from 4 GeV to 100 TeV emitted by sources at
redshifts from ~0 to 5. We also give an analytic fit with numerical
coefficients for approximating . As an example of the
application of our results, we calculate the absorbed spectrum of the blazar
PKS 2155-304 at z = 0.117 and compare it with the spectrum observed by the
H.E.S.S. air Cherenkov gamma-ray telescope array.Comment: final version to be published in Ap
Infrared-ultraviolet spectra of active galactic nuclei
Data from IRAS and IUE were combined with ground based optical and infrared spectrophotometry to derive emission line free spectral energy distributions (SEDs) for 29 active galactic nuclei (AGNs) between 0.1 and 100 microns. The IRAS data were scaled down to account for extended emission. These correction factors, determined by comparing small aperture ground based 10.6 micron data with large aperture IRAS 12 micron fluxes, were usually less than 25%. These corrected SEDs are shown
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The soft-x-ray spectral shape of x-ray-weak seyferts
(I) We observed eight Seyfert~2s and two X--ray--weak Seyfert~1/QSOs with the ROSAT PSPC, and one Seyfert~2 with the ROSAT HRI. These targets were selected from the Extended 12\um\ Galaxy Sample. (II) Both Seyfert~1/QSOs vary by factors of 1.5---2. The photon indices steepen in the more luminous state, consistent with the variability being mainly due to the softest X--rays, which are confined to a size of less than a parsec. (III) Both the Seyfert~2s and Seyfert~1/QSOs are best fit with a photon index of \Gamma\sim3, which is steeper than the canonical value of \Gamma\sim1.7 measured for X--ray--strong Seyferts by ROSAT and at higher energies. Several physical explanations are suggested for the steeper slopes of X--ray--weak objects. (IV) We observed one Seyfert~2, NGC~5005, with the ROSAT HRI, finding about 13\% of the soft X--rays to come from an extended component. This and other observations suggest that different components to the soft X--ray spectrum of some, if not all, X--ray--weak Seyferts may come from spatially distinct regions
The central molecular gas structure in LINERs with low luminosity AGN: evidence for gradual disappearance of the torus
We present observations of the molecular gas in the nuclear environment of
three prototypical low luminosity AGN (LLAGN), based on VLT/SINFONI AO-assisted
integral-field spectroscopy of H2 1-0 S(1) emission at angular resolutions of
~0.17". On scales of 50-150 pc the spatial distribution and kinematics of the
molecular gas are consistent with a rotating thin disk, where the ratio of
rotation (V) to dispersion (sigma) exceeds unity. However, in the central 50
pc, the observations reveal a geometrically and optically thick structure of
molecular gas (V/sigma10^{23} cm^{-2}) that is likely to be
associated with the outer extent of any smaller scale obscuring structure. In
contrast to Seyfert galaxies, the molecular gas in LLAGN has a V/sigma<1 over
an area that is ~9 times smaller and column densities that are in average ~3
times smaller. We interpret these results as evidence for a gradual
disappearance of the nuclear obscuring structure. While a disk wind may not be
able to maintain a thick rotating structure at these luminosities, inflow of
material into the nuclear region could provide sufficient energy to sustain it.
In this context, LLAGN may represent the final phase of accretion in current
theories of torus evolution. While the inflow rate is considerable during the
Seyfert phase, it is slowly decreasing, and the collisional disk is gradually
transitioning to become geometrically thin. Furthermore, the nuclear region of
these LLAGN is dominated by intermediate-age/old stellar populations (with
little or no on-going star formation), consistent with a late stage of
evolution.Comment: 15 pages, including 4 figures and 1 table, Accepted for publication
in ApJ Letter
An Empirically Based Model for Predicting Infrared Luminosity Functions, Deep Infrared Galaxy Counts and the Diffuse Infrared Background
We predict luminosity functions and number counts for extragalactic infrared sources at various wavelengths using our empirically based model. This is the same model which we used successfully to predict the spectral energy distribution of the diffuse infrared background. Comparisons of galaxy count results with existing data indicate that either galaxy luminosity evolution is not stronger that Q=3.1 (where L is proportional to (1+z)^{Q}) or that this evolution does not continue beyond a redshift of 2. However, measurements of the far infrared background from COBE-DIRBE seem to suggest a stronger evolution for far infrared emission with Q > 4 in the redshift range beteen 0 and 1. We discuss several interpretations of these results and also discuss how future observations can reconcile this apparent conflict. We also make predictions of the redshift distributions of extragalactic infrared sources at selected flux levels which can be tested by planned detectors. Finally, we predict the fluxes at which various future surveys will become confusion limited
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Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei; 1, Comparing the Photoionization and Reverberation Techniques
The masses and emission-line region sizes of Active Galactic Nuclei (AGNs) can be measured by ``reverberation-mapping'' (measuring the lag of the emission-line luminosity after changes in the continuum). We use tis technique to calibrate similar size and mass estimates made by photoionization models of the AGN line-emitting regions. We compile a sample of 19 AGNs with reliable reverberation and spectroscopy data, twice the number available previously. The data provide strong evidence that the BLR size and the emission-line width measure directly the central mass. Two methods are used to estimate the distance of the broad emission-line region (BLR) from the ionizing source: the photoionization method (available for many AGNs but has large intrinsic uncertainties), and the reverberation method (gives very reliable distances, but available for only a few objects). The distance estimate is combined with the velocity dispersion, derived from the broad Hb line profile, to estimate the virial mass. Comparing the central masses calculated with the reverberation method to those calculated using a photoionization model, we find a highly significant, nearly linear correlation. This provides a calibration of the photoionization method on the objects with presently available reverberation data, which should enable mass estimates for all AGNs with measured Hb line width. Comparing the BLR sizes given by the two methods also enables us to estimate the ionizing EUV luminosity which is directly unobservable. We find it to be typically ten times the visible (monochromatic luminosity at 5100A). The inferred Eddington ratio of the individual objects in our sample are 0.001-0.03 (visible luminosity) and 0.01-0.3 (ionizing luminosity)
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