3,144 research outputs found
Coordinated observations of X-ray bright BL Lacertae objects
No new International Ultraviolet Explorer (IUE) observations were scheduled. The work on the spectral extraction routine was continued. The computer code was adapted from the RDAF PDP 11/44 computer to a VAX 11/750, in order that it run more quickly. A chapter on ultraviolet observations of blazars for a book celebrating the scientific accomplishment of IUE was written in colloboration. A copy of the final draft is attached
Luminosity enhancement in relativistic jets and altered luminosity functions for beamed objects
Due to relativistic effects, the observed emission from relativistic jets is quite different from the rest frame emission. Systematic differences between the observed and intrinsic intensities of sources in which jet phenomena are occurring are discussed. Assuming that jets have a power law luminosity function of a slope B, the observed luminosity distribution as a function of the velocity of the jet, the spectral index of the rest frame emission, and the range of angles of the jets relative to our line of sight are calculated. The results is well-approximated by two power laws, the higher luminosity end having the original power law index X and the lower luminosity end having a flattened exponent independent of B and only slightly greater than 1. A model consisting of beamed emission from a jet and unbeamed emission from a stationary central component is investigated. The luminosity functions for these two-component sources are calculated for two ranges of angles. For sources in which beaming is important, the luminosity function is much flatter. Because of this, the relative numbers of ""beamed'' and ""unbeamed'' sources detected on the sky depend strongly on the luminosity at which the comparison is made
LHEA contributions to the Future of Ultraviolet Astronomy Based on Six Years of IUE Research
Astronomical models of galactic nuclei emission spectra are reassessed in light of ultraviolet and X-ray spectroscopic observations. Spectral analysis of BL Lacertae objects using data collected by the International Ultraviolet Explorer (IUE) and other astronomical observatories is presented
PKS 2155-304 relativistically beamed synchrotron radiation from BL LAC object
The newly discovered BL Lacertae object, PKS 2155-304, was observed with the medium and high intensity energy detectors of the HEAO-1 A2 experiment. The variability by a factor of two in less than a day reported by Snyder, et al (1979) is confirmed. Two spectra, obtained a year apart, while the satellite was in scanning mode, are well fit by simple power laws with energy spectral index alpha sub 1 equals approximately 1.4. A third spectrum, of higher statistical quality, obtained while the satellite was pointed at its source, has has two components. An acceptable fit was obtained using a two power law model, with indices alpha sub 1 equals 2.0 (+1.2, -0.6) and alpha sub 2 equals -1.5 (+1.5, -2.3). An interpretation of the overall spectrum from radio through X-rays in terms of a synchrotron self-Compton model gives a good description of the data if allowance is made for relativistic beaming. Thus, from a consideration of the spectrum, combined with an estimate of the size of the source, the presence of jets is inferred without their observation
Major Galaxy Mergers Only Trigger the Most Luminous AGN
Using multiwavelength surveys of active galactic nuclei across a wide range
of bolometric luminosities (10^{43}<L_{bol}(erg/s<5x10^{46}) and redshifts
(0<z<3), we find a strong, redshift-independent correlation between the AGN
luminosity and the fraction of host galaxies undergoing a major merger. That
is, only the most luminous AGN phases are connected to major mergers, while
less luminous AGN appear to be driven by secular processes. Combining this
trend with AGN luminosity functions to assess the overall cosmic growth of
black holes, we find that ~50% by mass is associated with major mergers, while
only 10% of AGN by number, the most luminous, are connected to these violent
events. Our results suggest that to reach the highest AGN luminosities -where
the most massive black holes accreted the bulk of their mass - a major merger
appears to be required. The luminosity dependence of the fraction of AGN
triggered by major mergers can successfully explain why the observed scatter in
the M-\sigma relation for elliptical galaxies is significantly lower than in
spirals. The lack of a significant redshift dependence of the
L_{bol}-f_{merger} relation suggests that downsizing, i.e., the general decline
in AGN and star formation activity with decreasing redshift, is driven by a
decline in the frequency of major mergers combined with a decrease in the
availability of gas at lower redshifts.Comment: Accepted for publication by Astrophysical Journal Letters, 6 pages in
emulateapj format, 3 figure
The Accuracy of Morphological Decomposition of Active Galactic Nucleus Host Galaxies
In order to assess the accuracy with which we can determine the morphologies
of AGN host galaxies, we have simulated more than 50,000 ACS images of galaxies
with z < 1.25, using image and noise properties appropriate for the GOODS
survey. We test the effect of central point-source brightness on host galaxy
parameter recovery with a set of simulated AGN host galaxies made by adding
point sources to the centers of normal galaxies. We extend this analysis and
also quantify the recovery of intrinsic morphological parameters of AGN host
galaxies with a set of fully simulated inactive and AGN host galaxies.
We can reliably separate good from poor fit results using a combination of
reasonable error cuts, in the regime where L_{host}:L_{PS} > 1:4. We give
quantitative estimates of parameter errors as a function of
host-to-point-source ratio. In general, we separate host and point-source
magnitudes reliably at all redshifts; point sources are well recovered more
than 90% of the time, although spurious detection of central point sources can
be as high as 25% for bulge-dominated sources. We find a general correlation
between Sersic index and intrinsic bulge-to-total ratio, such that a host
galaxy with Sersic n < 1.5 generally has at least 80% of its light from a disk
component. Likewise, "bulge-dominated" galaxies with n > 4 typically derive at
least 70% of their total host galaxy light from a bulge, but this number can be
as low as 55%. Single-component Sersic fits to an AGN host galaxy are
statistically very reliable to z < 1.25 (for ACS survey data like ours). In
contrast, two-component fits involving separate bulge and disk components tend
to over-estimate the bulge fraction by ~10%, with uncertainty of order 50%.Comment: 45 pages, 20 figures, submitted to ApJ ; Accepted Version --
additions to introduction and conclusions; title changed, was "Simulations of
AGN Host Galaxy Morphologies
Coordinated observations of X-ray bright BL lacertae objects
Simultaneous multifrequency observations of the BL Lac object Mkn421 covering radio through X-ray wavelengths were performed. Composite multifrequency spectra of the central nonthermal component were obtained at the two epochs after subtracting the optical and infrared light of the underlying galaxy. Physical parameters of Mkn421 are discussed in terms of the synchrotron self-Compton model. Taking the spectral turnover between infrared and radio for synchrotrom self absorption, the radio emmision originates in a more extended region than the infrared to X-ray emission, the source size of which should be less than .01 milliarcseconds. Relativistic beaming is required if the angular size is smaller than a few times .001 milliarcseconds. A possible explanation of the spectral change during the two epochs is also discussed
The Space Density of Compton-thick AGN
We constrain the number density and evolution of Compton-thick Active
Galactic Nuclei (AGN), and their contribution to the extragalactic X-ray
background. In the local Universe we use the wide area surveys from the Swift
and INTEGRAL satellites, while for high redshifts we explore candidate
selections based on mid-IR parameters. We present the properties of a sample of
211 heavily-obscured AGN candidates in the Extended Chandra Deep Field-South
(ECDF-S) selecting objects with f24/fR>1000 and R-K>4.5. The X-ray to mid-IR
ratios for these sources are significantly larger than that of star-forming
galaxies and ~2 orders of magnitude smaller than for the general AGN
population, suggesting column densities of NH>5x10^24 cm^-2. The space density
of CT AGN at z~2 derived from these observations is ~10^-5 Mpc^{-3}, finding a
strong evolution in the number of LX>10^44 erg/s sources from z=1.5 to 2.5.Comment: 4 pages, 3 figures, to appear in proceedings for 'X-ray Astronomy
2009', Bologna 09/2009, AIP Conference Series, Eds. A. Comastri, M. Cappi, L.
Angelini; author list fixe
- …