10,252 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
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
An Over-Massive Black Hole in a Typical Star-Forming Galaxy, 2 Billion Years After the Big Bang
Supermassive black holes (SMBHs) and their host galaxies are generally
thought to coevolve, so that the SMBH achieves up to about 0.2 to 0.5% of the
host galaxy mass in the present day. The radiation emitted from the growing
SMBH is expected to affect star formation throughout the host galaxy. The
relevance of this scenario at early cosmic epochs is not yet established. We
present spectroscopic observations of a galaxy at redshift z = 3.328, which
hosts an actively accreting, extremely massive BH, in its final stages of
growth. The SMBH mass is roughly one-tenth the mass of the entire host galaxy,
suggesting that it has grown much more efficiently than the host, contrary to
models of synchronized coevolution. The host galaxy is forming stars at an
intense rate, despite the presence of a SMBH-driven gas outflow.Comment: Author's version, including the main paper and the Supplementary
Materials (16+21 pages, 3+3 figures
Recommended from our members
Increasing the intensity and comprehensiveness of aphasia services: identification of key factors influencing implementation across six countries
Background: Aphasia services are currently faced by increasing evidence for therapy of greater intensity and comprehensiveness. Intensive Comprehensive Aphasia Programs (ICAPs) combine these elements in an evidence-based, time-limited group program. The incorporation of new service delivery models in routine clinical practice is, however, likely to pose challenges for both the service provider and administering clinicians. This program of research aims to identify these challenges from the perspective of aphasia clinicians from six countries and will seek to trial potential solutions. Continual advancements in global communication technologies suggest that solutions will be easily shared and accessed across multiple countries.
Aims: To identify the perceived and experienced barriers and facilitators to the implementation of 1) intensive aphasia services, 2) comprehensive aphasia services, and 3) ICAPs, from aphasia clinicians across six countries.
Methods and procedures: A qualitative enquiry approach included data from six focus groups (n = 34 participants) in Australia, New Zealand, Canada, United States of America (USA), United Kingdom (UK), and Ireland. A thematic analysis of focus group data was informed by the Theoretical Domains Framework (TDF).
Outcomes and results: Five prominent theoretical domains from the TDF influenced the implementation of all three aphasia service types across participating countries: environmental context and resources, beliefs about consequences, social/professional role and identity, skills, and knowledge. Four overarching themes assisted the identification and explanation of the key barriers and facilitators: 1. Collaboration, joint initiatives and partnerships, 2. Advocacy, the promotion of aphasia services and evidence-based practice, 3. Innovation, the ability to problem solve challenges, and 4. Culture, the influence of underlying values.
Conclusions: The results of this study will inform the development of a theoretically informed intervention to improve health services’ adherence to aphasia best practice recommendations
Probing Red Giant Atmospheres with Gravitational Microlensing
Gravitational microlensing provides a new technique for studying the surfaces
of distant stars. Microlensing events are detected in real time and can be
followed up with precision photometry and spectroscopy. This method is
particularly adequate for studying red giants in the Galactic bulge. Recently
we developed an efficient method capable of computing the lensing effect for
thousands of frequencies in a high-resolution stellar spectrum. Here we
demonstrate the effects of microlensing on synthesized optical spectra of red
giant model atmospheres. We show that different properties of the stellar
surface can be recovered from time-dependent photometry and spectroscopy of a
point-mass microlensing event with a small impact parameter. In this study we
concentrate on center-to-limb variation of spectral features. Measuring such
variations can reveal the depth structure of the atmosphere of the source star.Comment: 23 pages with 11 Postscript figures, submitted to ApJ; Section 2
expanded, references added, text revise
Fermi-liquid behaviour of the low-density 2D hole gas in GaAs/AlGaAs heterostructure at large values of r_s
We examine the validity of the Fermi-liquid description of the dilute 2D hole
gas in the crossover from 'metallic'-to-'insulating' behaviour of R(T).It has
been established that, at r_s as large as 29, negative magnetoresistance does
exist and is well described by weak localisation. The dephasing time extracted
from the magnetoresistance is dominated by the T^2 -term due to Landau
scattering in the clean limit. The effect of hole-hole interactions, however,
is suppressed when compared with the theory for small r_s.Comment: 4 pages ReVTeX, 4 ps figure
- …