7,432 research outputs found
Chandra Observation of the Radio Source / X-ray Gas Interaction in the Cooling Flow Cluster Abell 2052
We present a Chandra observation of Abell 2052, a cooling flow cluster with a
central cD that hosts the complex radio source 3C 317. The data reveal
``holes'' in the X-ray emission that are coincident with the radio lobes. The
holes are surrounded by bright ``shells'' of X-ray emission. The data are
consistent with the radio source displacing and compressing, and at the same
time being confined by, the X-ray gas. The compression of the X-ray shells
appears to have been relatively gentle and, at most, slightly transonic. The
pressure in the X-ray gas (the shells and surrounding cooler gas) is
approximately an order of magnitude higher than the minimum pressure derived
for the radio source, suggesting that an additional source of pressure is
needed to support the radio plasma. The compression of the X-ray shells has
speeded up the cooling of the shells, and optical emission line filaments are
found coincident with the brightest regions of the shells.Comment: accepted for publication in ApJ Letters; for high-resolution color
figures, see http://www.astro.virginia.edu/~elb6n/abell2052.htm
The powerful outburst in Hercules A
The radio source Hercules A resides at the center of a cooling flow cluster
of galaxies at redshift z = 0.154. A Chandra X-ray image reveals a shock front
in the intracluster medium (ICM) surrounding the radio source, about 160 kpc
from the active galactic nucleus (AGN) that hosts it. The shock has a Mach
number of 1.65, making it the strongest of the cluster-scale shocks driven by
an AGN outburst found so far. The age of the outburst ~5.9e7 y, its energy
about 3e61 erg and its mean power ~1.6e46 erg/s. As for the other large AGN
outbursts in cooling flow clusters, this outburst overwhelms radiative losses
from the ICM of the Hercules A cluster by a factor of ~100. It adds to the case
that AGN outbursts are a significant source of preheating for the ICM. Unless
the mechanical efficiency of the AGN in Hercules A exceeds 10%, the central
black hole must have grown by more than 1.7e8 Msun to power this one outburst.Comment: 4 pages, 5 figures, accepted by ApJ
The Detectability of AGN Cavities in Cooling-Flow Clusters
Chandra X-ray Observatory has revealed X-ray cavities in many nearby cooling
flow clusters. The cavities trace feedback from the central active galactic
nulceus (AGN) on the intracluster medium (ICM), an important ingredient in
stabilizing cooling flows and in the process of galaxy formation and evolution.
But, the prevalence and duty cycle of such AGN outbursts is not well
understood. To this end, we study how the cooling is balanced by the cavity
heating for a complete sample of clusters (the Brightest 55 clusters of
galaxies, hereafter B55). In the B55, we found 33 cooling flow clusters, 20 of
which have detected X-ray bubbles in their ICM. Among the remaining 13, all
except Ophiuchus could have significant cavity power yet remain undetected in
existing images. This implies that the duty cycle of AGN outbursts with
significant heating potential in cooling flow clusters is at least 60 % and
could approach 100 %, but deeper data is required to constrain this further.Comment: 4 pages, 2 figures; to appear in the proceedings of "The Monsters'
Fiery Breath", Madison, Wisconsin 1-5 June 2009, Eds. Sebastian Heinz & Eric
Wilcots; added annotation to the figur
Towards a High Energy Theory for the Higgs Phase of Gravity
Spontaneous Lorentz violation due to a time-dependent expectation value for a
massless scalar has been suggested as a method for dynamically generating dark
energy. A natural candidate for the scalar is a Goldstone boson arising from
the spontaneous breaking of a U(1) symmetry. We investigate the low-energy
effective action for such a Goldstone boson in a general class of models
involving only scalars, proving that if the scalars have standard kinetic terms
then at the {\em classical} level the effective action does not have the
required features for spontaneous Lorentz violation to occur asymptotically in an expanding FRW universe. Then we study the large limit of
a renormalizable field theory with a complex scalar coupled to massive
fermions. In this model an effective action for the Goldstone boson with the
properties required for spontaneous Lorentz violation can be generated.
Although the model has shortcomings, we feel it represents progress towards
finding a high energy completion for the Higgs phase of gravity.Comment: 20 pages, 5 figures;fixed typos and added reference
Gravity from a fermionic condensate of a gauge theory
The most prominent realization of gravity as a gauge theory similar to the
gauge theories of the standard model comes from enlarging the gauge group from
the Lorentz group to the de Sitter group. To regain ordinary Einstein-Cartan
gravity the symmetry must be broken, which can be accomplished by known
quasi-dynamic mechanisms. Motivated by symmetry breaking models in particle
physics and condensed matter systems, we propose that the symmetry can
naturally be broken by a homogenous and isotropic fermionic condensate of
ordinary spinors. We demonstrate that the condensate is compatible with the
Einstein-Cartan equations and can be imposed in a fully de Sitter invariant
manner. This lends support, and provides a physically realistic mechanism for
understanding gravity as a gauge theory with a spontaneously broken local de
Sitter symmetry.Comment: 16 page
Ryegrass toxicity organism found on other grasses
Yellow slime disease caused by Corynebacterium sp. has recently been found in three grass species.
The diseased grasses were found in the field growing with each other and with affected toxic annual ryegrass
Jet Interactions with the Hot Halos of Clusters and Galaxies
X-ray observations of cavities and shock fronts produced by jets streaming
through hot halos have significantly advanced our understanding of the
energetics and dynamics of extragalactic radio sources. Radio sources at the
centers of clusters have dynamical ages between ten and several hundred million
years. They liberate between 1E58-1E62 erg per outburst, which is enough energy
to regulate cooling of hot halos from galaxies to the richest clusters. Jet
power scales approximately with the radio synchrotron luminosity to the one
half power. However, the synchrotron efficiency varies widely from nearly unity
to one part in 10,000, such that relatively feeble radio source can have
quasar-like mechanical power. The synchrotron ages of cluster radio sources are
decoupled from their dynamical ages, which tend to be factors of several to
orders of magnitude older. Magnetic fields and particles in the lobes tend to
be out of equipartition. The lobes may be maintained by heavy particles (e.g.,
protons), low energy electrons, a hot, diffuse thermal gas, or possibly
magnetic (Poynting) stresses. Sensitive X-ray images of shock fronts and
cavities can be used to study the dynamics of extragalactic radio sources.Comment: 10 pages, 3 figures, invited review, "Extragalactic Jets: Theory and
Observation from Radio to Gamma Ray, held in Girdwood, Alaska, U.S.A. 21-24
May, 2007, minor text changes; one added referenc
Infinite-Dimensional Representations of 2-Groups
A "2-group" is a category equipped with a multiplication satisfying laws like those of a group. Just as groups have representations on vector spaces, 2-groups have representations on "2-vector spaces", which are categories analogous to vector spaces. Unfortunately, Lie 2-groups typically have few representations on the finite-dimensional 2-vector spaces introduced by Kapranov and Voevodsky. For this reason, Crane, Sheppeard and Yetter introduced certain infinite-dimensional 2-vector spaces called "measurable categories" (since they are closely related to measurable fields of Hilbert spaces), and used these to study infinite-dimensional representations of certain Lie 2-groups. Here we continue this work. We begin with a detailed study of measurable categories. Then we give a geometrical description of the measurable representations, intertwiners and 2-intertwiners for any skeletal measurable 2-group. We study tensor products and direct sums for representations, and various concepts of subrepresentation. We describe direct sums of intertwiners, and sub-intertwiners--features not seen in ordinary group representation theory. We classify irreducible and indecomposable representations and intertwiners. We also classify "irretractable" representations--another feature not seen in ordinary group representation theory. Finally, we argue that measurable categories equipped with some extra structure deserve to be considered "separable 2-Hilbert spaces", and compare this idea to a tentative definition of 2-Hilbert spaces as representation categories of commutative von Neumann algebras
Varenicline and suicidal behaviour: a cohort study based on data from the General Practice Research Database
Objective To determine whether varenicline, a recently licensed smoking cessation product, is associated with an increased risk of suicide and suicidal behaviour compared with alternative treatments bupropion and nicotine replacement therapy
- …