3,190 research outputs found
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
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Failure to regulate: counterproductive recruitment of top-down prefrontal-subcortical circuitry in major depression
Although depressed mood is a normal occurrence in response to adversity in all individuals, what distinguishes those who are vulnerable to major depressive disorder (MDD) is their inability to effectively regulate negative mood when it arises. Investigating the neural underpinnings of adaptive emotion regulation and the extent to which such processes are compromised in MDD may be helpful in understanding the pathophysiology of depression. We report results from a functional magnetic resonance imaging study demonstrating left-lateralized activation in the prefrontal cortex (PFC) when downregulating negative affect in nondepressed individuals, whereas depressed individuals showed bilateral PFC activation. Furthermore, during an effortful affective reappraisal task, nondepressed individuals showed an inverse relationship between activation in left ventrolateral PFC and the amygdala that is mediated by the ventromedial PFC (VMPFC). No such relationship was found for depressed individuals, who instead show a positive association between VMPFC and amygdala. Pupil dilation data suggest that those depressed patients who expend more effort to reappraise negative stimuli are characterized by accentuated activation in the amygdala, insula, and thalamus, whereas nondepressed individuals exhibit the opposite pattern. These findings indicate that a key feature underlying the pathophysiology of major depression is the counterproductive engagement of right prefrontal cortex and the lack of engagement of left lateral-ventromedial prefrontal circuitry important for the downregulation of amygdala responses to negative stimuli
Ultraviolet and X-ray observations of the BL Lac object PKS 05 8-322
Spectrophotometric observations of the BL Lac object PKS 0548-322 were made with ultraviolet and X-ray instruments on the IUE and HEAO-2 satellites. Two observations in each spectral region, one set of which was obtained simultaneously are presented. A power law of energy index approximately 1.0 gives a good description of the data from ultraviolet through X-ray frequencies. This source is reported to be variable on short timescales (Gilmore 1980); the implications of the spectral properties and apparent variability are discussed in light of a relativistic jet model. Findings support the jet picture of BL Lac objects
X-rays from Radio-Galaxies: BeppoSAX Observations
We briefly review BeppoSAX observations of X-ray bright radio-galaxies. Their
X-ray spectra are quite varied, and perhaps surprisingly, any similarity
between radio-loud AGN and Seyfert galaxies is the exception rather than the
rule. When detected, reprocessing features (iron line and reflection) are
generally weak, suggesting two possible scenarios: either: (1) non-thermal
(jet?) radiation dilutes the X-ray emission from the disk in radio-loud
objects, or (2) the solid angle subtended by the X-ray reprocessing material is
smaller in radio-loud than in radio-quiet AGN due to different characteristics
of the accretion disk itself.Comment: 6 pages, to appear in `Life Cycles of Radio Galaxies', ed. J. Biretta
et al., New Astronomy Review
Jets from Sub-Parsec to Kiloparsec Scales: A Physical Connection
The Chandra discovery of bright X-ray emission from kpc-scale jets allows
insight into the physical parameters of the jet flow at large scale. At the
opposite extreme, extensive studies of the inner relativistic jets in Blazars
with multiwavelength observations, yield comparable information on sub-parsec
scales. In the framework of simple radiation models for the emission regions we
compare the physical parameters of jets on these two very different scales in
the only two well studied Blazars for which large-scale emission has been
resolved by Chandra. Notably, we find that the relativistic Doppler factors and
powers derived independently at the two scales are consistent, suggesting that
the jet does not suffer severe deceleration or dissipation. Moreover the
internal equipartition pressures in the inner jet and in the external X-ray
bright knots scale inversely with the jet cross section as expected in the
simple picture of a freely expanding jet in equipartition.Comment: 4 figures, accepted by Ap
The last Gift of BeppoSAX: PDS Observations of the two Blazars 1ES 0507-040 and PKS 1229-021
Towards the end of the BeppoSAX mission, the only operated Narrow Field
Instrument was the PDS, which covers the energy range 13-300 keV. Two blazars,
1ES 0507-040 (z=0.304) and PKS 1229-021 (z= 1.045), suitably located in the sky
for BeppoSAX pointing, were observed for about 2 days each in Spring 2002 with
the PDS and detected up to 50 keV. 1ES0507-040 had been already observed by the
BeppoSAX NFIs at an earlier epoch for a much shorter time. We have re-analysed
those data as well, and identified a possible contamination problem in the PDS
spectrum. We present our recent PDS data on both sources, combined with the
previous BeppoSAX data and with non-simultaneous observations at other
frequencies. The derived Spectral Energy Distributions allow us to discuss the
origin of the high energy component.Comment: 4 pages, 6 figures, poster presented at the symposium: The Restless
High-Energy Universe, Amsterdam, May 200
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