3,790 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
Major Galaxy Mergers and the Growth of Supermassive Black Holes in Quasars
Despite observed strong correlations between central supermassive black holes
(SMBHs) and star-formation in galactic nuclei, uncertainties exist in our
understanding of their coupling. We present observations of the ratio of
heavily-obscured to unobscured quasars as a function of cosmic epoch up to z~3,
and show that a simple physical model describing mergers of massive, gas-rich
galaxies matches these observations. In the context of this model, every
obscured and unobscured quasar represent two distinct phases that result from a
massive galaxy merger event. Much of the mass growth of the SMBH occurs during
the heavily-obscured phase. These observations provide additional evidence for
a causal link between gas-rich galaxy mergers, accretion onto the nuclear SMBH
and coeval star formation.Comment: Accepted for publication in Science. Published by Science Express on
March 25th. 17 pages, 5 figures, including supplemental online materia
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
Learning the rules of the ‘student game’: Transforming the ‘student habitus’ through [im]mobility
In recent years, a growing body of literature has emerged concerning the mobilities of students, specifically relating to the interactions between local and non-local students, which can accentuate unequal access to education; social interactions and learner outcomes. Central to much of this literature is a sense that being mobile in institutional choice is the most appropriate and expected approach to successful university life. Conversely, local students, disadvantaged by their age, history, external commitments and immobility, are thought to be unlikely to share the same ‘student experiences’ as their traditional counterparts, leading to feelings of alienation within the student community. This paper will seek to problematise this binary by examining the experiences of a group of local and non-local students studying at the University of Portsmouth using Bourdieu’s reading of habitus and capital. This is useful as it provides a more critical insight into how students’ [dis]advantaged learner identities are [re]produced through their everyday sociability. Moreover, these findings extend previous discussions of first year transitions by questioning the influence of accommodation upon the formation of identities and the initial experiences of ‘being’, or ‘becoming’ students. This paper also seeks to extend previous theoretical tendencies that privilege identity formation through mobility rather than stasis
Urban encounters: juxtapositions of difference and the communicative interface of global cities
This article explores the communicative interface of global cities, especially as it is shaped in the juxtapositions of difference in culturally diverse urban neighbourhoods. These urban zones present powerful examples, where different groups live cheek by jowl, in close proximity and in intimate interaction — desired or unavoidable. In these urban locations, the need to manage difference is synonymous to making them liveable and one's own. In seeking (and sometimes finding) a location in the city and a location in the world, urban dwellers shape their communication practices as forms of everyday, mundane and bottom-up tactics for the management of diversity. The article looks at three particular areas where cultural diversity and urban communication practices come together into meaningful political and cultural relations for a sustainable cosmopolitan life: citizenship, imagination and identity
Replica theory for learning curves for Gaussian processes on random graphs
Statistical physics approaches can be used to derive accurate predictions for
the performance of inference methods learning from potentially noisy data, as
quantified by the learning curve defined as the average error versus number of
training examples. We analyse a challenging problem in the area of
non-parametric inference where an effectively infinite number of parameters has
to be learned, specifically Gaussian process regression. When the inputs are
vertices on a random graph and the outputs noisy function values, we show that
replica techniques can be used to obtain exact performance predictions in the
limit of large graphs. The covariance of the Gaussian process prior is defined
by a random walk kernel, the discrete analogue of squared exponential kernels
on continuous spaces. Conventionally this kernel is normalised only globally,
so that the prior variance can differ between vertices; as a more principled
alternative we consider local normalisation, where the prior variance is
uniform
Variability Time Scales of TeV Blazars Observed in the ASCA Continuous Long-Look X-ray Monitoring
Three uninterrupted, long (lasting respectively 7, 10, and 10 days) ASCA
observations of the well-studied TeV-bright blazars Mrk 421, Mrk 501 and PKS
2155-304 all show continuous strong X-ray flaring. Despite the relatively faint
intensity states in 2 of the 3 sources, there was no identifiable quiescent
period in any of the observations. Structure function analysis shows that all
blazars have a characteristic time scale of ~ a day, comparable to the
recurrence time and to the time scale of the stronger flares. On the other
hand, examination of these flares in more detail reveals that each of the
strong flares is not a smooth increase and decrease, but exhibits substructures
of shorter flares having time scales of ~10 ks. We verify via simulations that
in order to explain the observed structure function, these shorter flares
("shots") are unlikely to be fully random, but in some way are correlated with
each other. The energy dependent cross-correlation analysis shows that
inter-band lags are not universal in TeV blazars. This is important since in
the past, only positive detections of lags were reported. In this work, we
determine that the sign of a lag may differ from flare to flare; significant
lags of both signs were detected from several flares, while no significant lag
was detected from others. However, we also argue that the nature of the
underlying component can affect these values. The facts that all flares are
nearly symmetric and that fast variability shorter than the characteristic time
scale is strongly suppressed, support the scenario where the light crossing
time dominates the variability time scales of the day-scale flares.Comment: 29 pages, 12 figures, accepted for publication in Ap
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