2,746 research outputs found
Elusive AGN in the XMM-Newton bright serendipitous survey
AIMS: We investigate here the nature of all the sources (35 in total) in the
XBS survey (which is 86% optically identified) showing an optical spectrum
dominated by the light from the host galaxy with no evidence (or little
evidence) for the presence of an AGN. METHODS: We use the X-ray spectral
analysis to assess the presence of an AGN in these sources and to characterize
its properties. RESULTS: We detect AGN activity in 33 out of 35 sources. The
remaining 2 sources are the ones with the lowest X-ray luminosity in the sample
(L[2-10keV]<10^41 erg s^-1) and their X-ray emission could be produced within
the host galaxy. We find that the ``recognition problem'' for AGN is very
critical in the low-luminosity regime (at least 60% of the AGN with
L[2-10keV]<10^43 erg s^-1 are elusive) becoming negligible for high X-ray
luminosities (~1.5% of elusive AGN with L[2-10keV]>10^44 erg s^-1). This
problem affects mostly absorbed AGN (~40% of type~2 AGN in the survey are
elusive) but also a significant fraction of unabsorbed AGN (8%). CONCLUSIONS:
We find that the simplest explanations of why these 33 (or most of them) AGNs
are elusive are two: at low X-ray luminosities (<10^43 erg s^-1) the most
important reason is the intrinsically low AGN/galaxy contrast (optical
dilution) while at high luminosities (>10^44 erg s^-1) it is due to the optical
absorption (in the Compton-thin regime, i.e. NH<10^24 cm^-2). Alternative
hypotheses, like the presence of Compton-thick sources, BL Lac objects or
``non-standard'' AGN (e.g. with alpha_OX<1 or with weak/absorbed Narrow Line
Region) are not supported by the data although we cannot exclude the presence
in the sample of a few sources of these types.Comment: accepted for publication in A&A, 17 pages, 9 figure
A new sample of X-ray selected narrow emission-line galaxies. I. The nature of optically elusive AGN
Using the 3XMM catalogue of serendipitous X-ray sources, and the SDSS-DR9
spectroscopic catalogue, we have obtained a new sample of X-ray selected narrow
emission line galaxies. The standard optical diagnostic diagram and selection
by hard X-ray luminosity expose a mismatch between the optically-based and
X-ray-based classifications. The nature of these misclassified elusive AGN can
be understood in terms of their broader X-ray and optical properties and leads
to a division of this sub-sample into two groups. A little more than half are
likely to be narrow-line Seyfert 1s (NLS1s), so misclassified because of the
contribution of the Broad Line Region (BLR) to their optical spectra. The
remainder have some of the properties of Seyfert 2 (Sy2) AGN; their optical
elusiveness can be explained by optical dilution from the host galaxy plus a
star-formation contribution and by their underluminous optical emission due to
low accretion rates. Because some of the Sy2 sources have very low accretion
rates, are unabsorbed, plus the fact that they lack broad optical emission
lines, they are good candidates to be True Sy2 AGN.Comment: 13 pages, 13 figues, accepted for publication in A&
Collision-induced conformational changes in glycine
We present quantum dynamical calculations on the conformational changes of glycine in collisions with the He, Ne, and Ar rare-gas atoms. For two conformer interconversion processes (III-->I and IV-->I), we find that the probability of interconversion is dependent on several factors, including the energy of the collision, the angle at which the colliding atom approaches the glycine molecule, and the strength of the glycine-atom interaction. Furthermore, we show that attractive interactions between the colliding atom and the glycine molecule catalyze conformer interconversion at low collision energies. In previous infrared spectroscopy studies of glycine trapped in rare-gas matrices and helium clusters, conformer III has been consistently observed, but conformer IV has yet to be conclusively detected. Because of the calculated thermodynamic stability of conformer IV, its elusiveness has been attributed to the IV-->I conformer interconversion process. However, our calculations present little indication that IV-->I interconversion occurs more readily than III-->I interconversion. Although we cannot determine whether conformer IV interconverts during experimental Ne- and Ar-matrix depositions, our evidence suggests that the conformer should be present in helium droplets. Anharmonic vibrational frequency calculations illustrate that previous efforts to detect conformer IV may have been hindered by the overlap of its IR-absorption bands with those of other conformers. We propose that the redshifted symmetric –CH2 stretch of conformer IV provides a means for its conclusive experimental detection
A simple fixed parameter tractable algorithm for computing the hybridization number of two (not necessarily binary) trees
Here we present a new fixed parameter tractable algorithm to compute the
hybridization number r of two rooted, not necessarily binary phylogenetic trees
on taxon set X in time (6^r.r!).poly(n)$, where n=|X|. The novelty of this
approach is its use of terminals, which are maximal elements of a natural
partial order on X, and several insights from the softwired clusters
literature. This yields a surprisingly simple and practical bounded-search
algorithm and offers an alternative perspective on the underlying combinatorial
structure of the hybridization number problem
Origin of life in a digital microcosm
While all organisms on Earth descend from a common ancestor, there is no
consensus on whether the origin of this ancestral self-replicator was a one-off
event or whether it was only the final survivor of multiple origins. Here we
use the digital evolution system Avida to study the origin of self-replicating
computer programs. By using a computational system, we avoid many of the
uncertainties inherent in any biochemical system of self-replicators (while
running the risk of ignoring a fundamental aspect of biochemistry). We
generated the exhaustive set of minimal-genome self-replicators and analyzed
the network structure of this fitness landscape. We further examined the
evolvability of these self-replicators and found that the evolvability of a
self-replicator is dependent on its genomic architecture. We studied the
differential ability of replicators to take over the population when competed
against each other (akin to a primordial-soup model of biogenesis) and found
that the probability of a self-replicator out-competing the others is not
uniform. Instead, progenitor (most-recent common ancestor) genotypes are
clustered in a small region of the replicator space. Our results demonstrate
how computational systems can be used as test systems for hypotheses concerning
the origin of life.Comment: 20 pages, 7 figures. To appear in special issue of Philosophical
Transactions of the Royal Society A: Re-Conceptualizing the Origins of Life
from a Physical Sciences Perspectiv
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