11,776 research outputs found
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
Combinatorial Bounds and Characterizations of Splitting Authentication Codes
We present several generalizations of results for splitting authentication
codes by studying the aspect of multi-fold security. As the two primary
results, we prove a combinatorial lower bound on the number of encoding rules
and a combinatorial characterization of optimal splitting authentication codes
that are multi-fold secure against spoofing attacks. The characterization is
based on a new type of combinatorial designs, which we introduce and for which
basic necessary conditions are given regarding their existence.Comment: 13 pages; to appear in "Cryptography and Communications
Axigluons cannot explain the observed top quark forward-backward asymmetry
We study an SU(3)^2 axigluon model introduced by Frampton, Shu, and Wang to
explain the recent Fermilab Tevatron observation of a significant positive
enhancement in the top quark forward-backward asymmetry relative to standard
model predictions. First, we demonstrate that data on neutral B_d-meson mixing
excludes the region of model parameter space where the top asymmetry is
predicted to be the largest. Keeping the gauge couplings below the critical
value that would lead to fermion condensation imposes further limits at large
axigluon mass, while precision electroweak constraints on the model are
relatively mild. Furthermore, by considering an extension to an SU(3)^3 color
group, we demonstrate that embedding the model in an extra-dimensional
framework can only dilute the axigluon effect on the forward-backward
asymmetry. We conclude that axigluon models are unlikely to be the source of
the observed top quark asymmetry.Comment: 12 pages, 7 eps figures included. Minor changes to conform with
published versio
Spin-dependent transport in molecular tunnel junctions
We present measurements of magnetic tunnel junctions made using a
self-assembled-monolayer molecular barrier. Ni/octanethiol/Ni samples were
fabricated in a nanopore geometry. The devices exhibit significant changes in
resistance as the angle between the magnetic moments in the two electrodes is
varied, demonstrating that low-energy electrons can traverse the molecular
barrier while maintaining spin coherence. An analysis of the voltage and
temperature dependence of the data suggests that the spin-coherent transport
signals can be degraded by localized states in the molecular barriers.Comment: 4 pages, 5 color figure
Don't feedback in anger : enhancing student experience of feedback
This research explores three iterations of the delivery of audio feedback in relation to formative assessments at the School of Law, University of Sheffield. The evidence base includes similar practice at Edge Hill University and collaboration on good practice between the two institutions.
This paper will set out the context for the implementation of audio feedback, namely to help address the difficult issues experienced with feedback from non-engagement by the student in the whole feedback process, to a lack of utilization of formative feedback for 'feedforward' purposes. Qualitative comments from both students and staff experiencing this model of feedback will be drawn upon, which include references to the perceived benefits and challenges of this mode of feedback by both sets of stakeholders.
This paper will then take participants through the methods addressed to engage student with feedback on formative assessments, in order to create and encourage proper 'feedforward' to summative assessments, and to provide effective, focused, consistent and constructive feedback.
This paper in particular aims to show how the provision of audio feedback has the potential to greatly enhance the student learning experience, and can provide a more positive attitude generally to the giving, and receiving of feedback from both staff and students alike
Effects of accidental microconstriction on the quantized conductance in long wires
We have investigated the conductance of long quantum wires formed in
GaAs/AlGaAs heterostructures. Using realistic fluctuation potentials from donor
layers we have simulated numerically the conductance of four different kinds of
wires. While ideal wires show perfect quantization, potential fluctuations from
random donors may give rise to strong conductance oscillations and degradation
of the quantization plateaux. Statistically there is always the possibility of
having large fluctuations in a sample that may effectively act as a
microconstriction. We therefore introduce microconstrictions in the wires by
occasional clustering of donors. These microconstrictions are found to restore
the quantized plateaux. A similar effect is found for accidental lithographic
inaccuracies.Comment: 4 pages, 2 figures, paper for NANO2002 symposium, will appear in SPIE
proceeding
Transformation of stimulus correlations by the retina
Redundancies and correlations in the responses of sensory neurons seem to
waste neural resources but can carry cues about structured stimuli and may help
the brain to correct for response errors. To assess how the retina negotiates
this tradeoff, we measured simultaneous responses from populations of ganglion
cells presented with natural and artificial stimuli that varied greatly in
correlation structure. We found that pairwise correlations in the retinal
output remained similar across stimuli with widely different spatio-temporal
correlations including white noise and natural movies. Meanwhile, purely
spatial correlations tended to increase correlations in the retinal response.
Responding to more correlated stimuli, ganglion cells had faster temporal
kernels and tended to have stronger surrounds. These properties of individual
cells, along with gain changes that opposed changes in effective contrast at
the ganglion cell input, largely explained the similarity of pairwise
correlations across stimuli where receptive field measurements were possible.Comment: author list corrected in metadat
Energy-level pinning and the 0.7 spin state in one dimension: GaAs quantum wires studied using finite-bias spectroscopy
We study the effects of electron-electron interactions on the energy levels
of GaAs quantum wires (QWs) using finite-bias spectroscopy. We probe the energy
spectrum at zero magnetic field, and at crossings of opposite-spin-levels in
high in-plane magnetic field B. Our results constitute direct evidence that
spin-up (higher energy) levels pin to the chemical potential as they populate.
We also show that spin-up and spin-down levels abruptly rearrange at the
crossing in a manner resembling the magnetic phase transitions predicted to
occur at crossings of Landau levels. This rearranging and pinning of subbands
provides a phenomenological explanation for the 0.7 structure, a
one-dimensional (1D) nanomagnetic state, and its high-B variants.Comment: 6 pages, 4 figure
Conicoid Mirrors
The first order equation relating object and image location for a mirror of
arbitrary conic-sectional shape is derived. It is also shown that the parabolic
reflecting surface is the only one free of aberration and only in the limiting
case of distant sources.Comment: 9 page
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