643 research outputs found
Mass Distributions of HST Galaxy Clusters from Gravitational Arcs
Although N-body simulations of cosmic structure formation suggest that dark
matter halos have density profiles shallower than isothermal at small radii and
steeper at large radii, whether observed galaxy clusters follow this profile is
still ambiguous. We use one such density profile, the asymmetric NFW profile,
to model the mass distributions of 11 galaxy clusters with gravitational arcs
observed by HST. We characterize the galaxy lenses in each cluster as NFW
ellipsoids, each defined by an unknown scale convergence, scale radius,
ellipticity, and position angle. For a given set of values of these parameters,
we compute the arcs that would be produced by such a lens system. To define the
goodness of fit to the observed arc system, we define a chi^2 function
encompassing the overlap between the observed and reproduced arcs as well as
the agreement between the predicted arc sources and the observational
constraints on the source system. We minimize this chi^2 to find the values of
the lens parameters that best reproduce the observed arc system in a given
cluster. Here we report our best-fit lens parameters and corresponding mass
estimates for each of the 11 lensing clusters. We find that cluster mass models
based on lensing galaxies defined as NFW ellipsoids can accurately reproduce
the observed arcs, and that the best-fit parameters to such a model fall within
the reasonable ranges defined by simulations. These results assert NFW profiles
as an effective model for the mass distributions of observed clusters.Comment: Submitted to ApJ, 14 figures include
A Search for Binary Active Galactic Nuclei: Double-Peaked [OIII] AGN in the Sloan Digital Sky Survey
We present AGN from the Sloan Digital Sky Survey (SDSS) having double-peaked
profiles of [OIII] 5007,4959 and other narrow emission-lines, motivated by the
prospect of finding candidate binary AGN. These objects were identified by
means of a visual examination of 21,592 quasars at z < 0.7 in SDSS Data Release
7 (DR7). Of the spectra with adequate signal-to-noise, 148 spectra exhibit a
double-peaked [OIII] profile. Of these, 86 are Type 1 AGN and 62 are Type 2
AGN. Only two give the appearance of possibly being optically resolved double
AGN in the SDSS images, but many show close companions or signs of recent
interaction. Radio-detected quasars are three times more likely to exhibit a
double-peaked [OIII] profile than quasars with no detected radio flux,
suggesting a role for jet interactions in producing the double-peaked profiles.
Of the 66 broad line (Type 1) AGN that are undetected in the FIRST survey, 0.9%
show double peaked [OIII] profiles. We discuss statistical tests of the nature
of the double-peaked objects. Further study is needed to determine which of
them are binary AGN rather than disturbed narrow line regions, and how many
additional binaries may remain undetected because of insufficient line-of-sight
velocity splitting. Previous studies indicate that 0.1% of SDSS quasars are
spatially resolved binaries, with typical spacings of ~10 to 100 kpc. If a
substantial fraction of the double-peaked objects are indeed binaries, then our
results imply that binaries occur more frequently at smaller separations (< 10
kpc). This suggests that simultaneous fueling of both black holes is more
common as the binary orbit decays through these spacings.Comment: 33 pages, 5 figures, LaTeX. Major revisions. Accepted for publication
in ApJ
Bondi-Hoyle-Lyttleton accretion by binary stars
Binary stars often move through an ambient medium from which they accrete
material and angular momentum, as in triple-star systems, star-forming clouds,
young globular clusters and in the centres of galaxies. A binary form of
Bondi-Hoyle-Lyttleton accretion results whereby the accretion rate depends on
the binary properties: the stellar masses and separation, and the relative wind
speed. We present the results of simulations performed with the hydrodynamic
code GANDALF, to determine the mass accretion rates over a range of binary
separations, inclinations and mass ratios. When the binary separation is short,
the binary system accretes like a single star, while accretion onto stars in
wide binaries is barely affected by their companion. We investigate
intermediate-separation systems in some detail, finding that as the binary
separation is increased, accretion rates smoothly decrease from the rate equal
to that of a single star to the rate expected from two isolated stars. The form
of this decrease depends on the relative centre-of-mass velocity of the binary
and the gas, with faster-moving binaries showing a shallower decrease.
Accretion rates vary little with orbital inclination, except when the orbit is
side-on and the stars pass through each others' wakes. The specific angular
momentum accretion rate also depends on the inclination but is never sufficient
to prevent the binary orbit from contracting. Our results may be applied to
accretion onto protostars, pollution of stars in globular and nuclear clusters,
and wind mass-transfer in multiple stellar systems.Comment: 17 pages, 10 figures, accepted by MNRA
Controls on geothermal heat recovery from a hot sedimentary aquifer in Guardbridge, Scotland:Field measurements, modelling and long term sustainability
Geothermal heat from Hot Sedimentary Aquifers represents a promising intermediate (30°C) resource for district heating systems. A key control on the geothermal productivity of these aquifers is the architecture of faults, which can significantly enhance or reduce the natural permeability of these systems. We present the first three-dimensional coupled groundwater flow and heat transport numerical model, combining multiple data from field mapping and fracture surveys, of two intersecting major fault systems in Central-East Scotland. This includes fault-zone fracture permeability modelling, depth-dependent permeability modelling, geo-mechanical facies assessment and heat productivity estimates for single well and multiple well extraction scenarios. Simulations indicate that with careful location of extraction wells within permeable fault systems, production is sustainable for over 50 years for multiple-well extraction scenarios in this region
Quadratic equations over free groups are NP-complete
We prove that the problems of deciding whether a quadratic equation over a
free group has a solution is NP-complete
Recommended from our members
The big green lab project
YesBeverley Lucas and her colleagues give us a big green welcome to the Ecoversity of Bradford
In 2005, the Higher Education Funding Council for England (Hefce) stated that ‘the greatest contribution a university can make to sustainable development is through the education of their graduates’. The University of Bradford took up the gauntlet, embedding sustainable development in all areas of its campus whilst also transforming the curriculum across the university to educate for sustainable development. This led to them coining themselves an ecoversity.The authors would like to thank the National HE STEM Programme for funding this project
Investigating the evolution of the dual AGN system ESO~509-IG066
We analyze the evolution of the dual AGN in ESO 509-IG066, a galaxy pair
located at z=0.034 whose nuclei are separated by 11 kpc. Previous observations
with XMM-Newton on this dual AGN found evidence for two moderately obscured
( cm) X-ray luminous ( erg/s) nuclear
sources. We present an analysis of subsequent Chandra, NuSTAR and Swift/XRT
observations that show one source has dropped in flux by a factor of 10 between
2004 and 2011, which could be explained by either an increase in the absorbing
column or an intrinsic fading of the central engine possibly due to a decrease
in mass accretion. Both of these scenarios are predicted by galaxy merger
simulations. The source which has dropped in flux is not detected by NuSTAR,
which argues against absorption, unless it is extreme. However, new Keck/LRIS
optical spectroscopy reveals a previously unreported broad H-alpha line which
is highly unlikely to be visible under the extreme absorption scenario. We
therefore conclude that the black hole in this nucleus has undergone a dramatic
drop in accretion rate. From AO-assisted near-infrared integral-field
spectroscopy of the other nucleus, we find evidence that the galaxy merger is
having a direct effect on the kinematics of the gas close to the nucleus of the
galaxy, providing a direct observational link between the galaxy merger and the
mass accretion rate on to the black hole.Comment: Accepted for publication in Ap
GTP and Ca2+ Modulate the Inositol 1,4,5-Trisphosphate-Dependent Ca2+ Release in Streptolysin O-Permeabilized Bovine Adrenal Chromaffin Cells
The inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release was studied using streptolysin O-permeabilized bovine adrenal chromaffin cells. The IP3-induced Ca2+ release was followed by Ca2+ reuptake into intracellular compartments. The IP3-induced Ca2+ release diminished after sequential applications of the same amount of IP3. Addition of 20 μM GTP fully restored the sensitivity to IP3. Guanosine 5'-O-(3-thio)triphosphate (GTPγS) could not replace GTP but prevented the action of GTP. The effects of GTP and GTPγS were reversible. Neither GTP nor GTPγS induced release of Ca2+ in the absence of IP3. The amount of Ca2+ whose release was induced by IP3 depended on the free Ca2+ concentration of the medium. At 0.3 μM free Ca2+, a half-maximal Ca2+ release was elicited with ∼0.1 μM IP3. At 1 μM free Ca2+, no Ca2+ release was observed with 0.1 μM IP3; at this Ca2+ concentration, higher concentrations of IP3 (0.25 μM) were required to evoke Ca2+ release. At 8 μM free Ca2+, even 0.25 μM IP3 failed to induce release of Ca2+ from the store. The IP3-induced Ca2+ release at constant low (0.2 μM) free Ca2+ concentrations correlated directly with the amount of stored Ca2+. Depending on the filling state of the intracellular compartment, 1 mol of IP3 induced release of between 5 and 30 mol of Ca2+
The mechanism of action of GTP on Ca2+ efflux from rat liver microsomal vesicles. Measurement of vesicle fusion by fluorescence energy transfer
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