1,413 research outputs found
X-ray line tomography of AGN-induced motion in clusters of galaxies
The thermal broadening of emission lines of heavy ions is small enough such
that Doppler shifts due to bulk motions may be detected with the next
generation of X-ray observatories. This opens up the possibility to study gas
velocities in the intra-cluster medium. Here we study the effect of bulk
motions induced by a central active galactic nucleus (AGN) on the emission
lines around the FeXXV complex. We have modelled the evolution of AGN-induced
bubbles in a realistic cosmological framework and studied the resulting FeXXV
line profiles. We found that in clusters with AGN feedback, motions induced by
the inflation of bubbles and their buoyant rise lead to distinct features in
the iron emission lines that are detectable with a spectral resolution of about
10 eV. These observations will help to determine the mechanical energy that
resides in the bubbles and thereby the kinetic luminosity of the AGN.Comment: to appear in Ap
Radio relics in cosmological simulations
Radio relics have been discovered in many galaxy clusters. They are believed
to trace shock fronts induced by cluster mergers. Cosmological simulations
allow us to study merger shocks in detail since the intra-cluster medium is
heated by shock dissipation. Using high resolution cosmological simulations,
identifying shock fronts and applying a parametric model for the radio emission
allows us to simulate the formation of radio relics. We analyze a simulated
shock front in detail. We find a rather broad Mach number distribution. The
Mach number affects strongly the number density of relativistic electrons in
the downstream area, hence, the radio luminosity varies significantly across
the shock surface. The abundance of radio relics can be modeled with the help
of the radio power probability distribution which aims at predicting radio
relic number counts. Since the actual electron acceleration efficiency is not
known, predictions for the number counts need to be normalized by the observed
number of radio relics. For the characteristics of upcoming low frequency
surveys we find that about thousand relics are awaiting discovery.Comment: 10 pages, 4 figures, Invited talk at the conference "Diffuse
Relativistic Plasmas", Bangalore, 1-4 March 2011; in press in special issue
of Journal of Astrophysics and Astronom
Feedback in AGN heating of galaxy clusters
One of the challenges that models of AGN heating of the intracluster medium
(ICM) face, is the question how the mechanical luminosity of the AGN is tuned
to the radiative losses of the ICM. Here we implement a simple 1D model of a
feedback mechanism that links the luminosity of the AGN to the accretion rate.
We demonstrate how this simple feedback mechanism leads to a quasi-steady state
for a broad range of parameters. Moreover, within this feedback model, we
investigate the effect of thermal conduction and find that its relative
importance depends strongly on the cluster mass.Comment: 12 pages, 6 figures, submitted to Ap
The Fossil Phase in the Life of a Galaxy Group
We investigate the origin and evolution of fossil groups in a concordance
LCDM cosmological simulation. We consider haloes with masses between
(1-5)\times10^{13} \hMsun and study the physical mechanisms that lead to the
formation of the large gap in magnitude between the brightest and the second
most bright group member, which is typical for these fossil systems. Fossil
groups are found to have high dark matter concentrations, which we can relate
to their early formation time. The large magnitude-gaps arise after the groups
have build up half of their final mass, due to merging of massive group
members. We show that the existence of fossil systems is primarily driven by
the relatively early infall of massive satellites, and that we do not find a
strong environmental dependence for these systems. In addition, we find
tentative evidence for fossil group satellites falling in on orbits with
typically lower angular momentum, which might lead to a more efficient merger
onto the host. We find a population of groups at higher redshifts that go
through a ``fossil phase'': a stage where they show a large magnitude-gap,
which is terminated by renewed infall from their environment.Comment: 9 pages and 8 figures, submitted to MNRA
The local adsorption structure of benzene on Si(001)-(2 × 1): a photoelectron diffraction investigation
Scanned-energy mode C 1s photoelectron diffraction has been used to investigate the local adsorption geometry of benzene on Si(001) at saturation coverage and room temperature. The results show that two different local bonding geometries coexist, namely the 'standard butterfly' (SB) and 'tilted bridge' (TB) forms, with a composition of 58 ± 29% of the SB species. Detailed structural parameter values are presented for both species including Si–C bond lengths. On the basis of published measurements of the rate of conversion of the SB to the TB form on this surface, we estimate that the timescale of our experiment is sufficient for achieving equilibrium, and in this case our results indicate that the difference in the Gibbs free energy of adsorption, ΔG(TB)−ΔG(SB), is in the range −0.023 to +0.049 eV. We suggest, however, that the relative concentration of the two species may also be influenced by a combination of steric effects influencing the kinetics, and a sensitivity of the adsorption energies of the adsorbed SB and TB forms to the nature of the surrounding benzene molecules
Signatures from a merging galaxy cluster and its AGN population : LOFAR observations of Abell 1682
Reproduced with permission from Astronomy & Astrophysics. © 2019 ESOWe present LOFAR data from 110-180 MHz of the merging galaxy cluster Abell 1682, alongside archival optical, radio, and X-ray data. Our images of 6 arcsec in resolution at low frequencies reveal new structures associated with numerous radio galaxies in the cluster. At a resolution of 20 arcsec we see diffuse emission throughout the cluster over hundreds of kiloparsecs, indicating particle acceleration mechanisms are in play as a result of the cluster merger event and powerful active galactic nuclei. We show that a significant part of the cluster emission is from an old radio galaxy with very steep spectrum emission (having a spectral index of α < -2.5). Furthermore, we identify a new region of diffuse steep-spectrum emission (α < -1.1) as a candidate for a radio halo which is co-spatial with the centre of the cluster merger. We suggest its origin as a population of old and mildly relativistic electrons left over from radio galaxies throughout the cluster which have been re-accelerated to higher energies by shocks and turbulence induced by the cluster merger event. We also note the discovery of six new giant radio galaxies in the vicinity of Abell 1682.Peer reviewedFinal Accepted Versio
Performance of a new pulse contour method for continuous cardiac output monitoring: validation in critically ill patients
Background A new calibrated pulse wave analysis method (VolumeView™/EV1000™, Edwards Lifesciences, Irvine, CA, USA) has been developed to continuously monitor cardiac output (CO). The aim of this study was to compare the performance of the VolumeView method, and of the PiCCO2™ pulse contour method (Pulsion Medical Systems, Munich, Germany), with reference transpulmonary thermodilution (TPTD) CO measurements. Methods This was a prospective, multicentre observational study performed in the surgical and interdisciplinary intensive care units of four tertiary hospitals. Seventy-two critically ill patients were monitored with a central venous catheter, and a thermistor-tipped femoral arterial VolumeView™ catheter connected to the EV1000™ monitor. After initial calibration by TPTD CO was continuously assessed using the VolumeView-CCO software (CCOVolumeView) during a 72 h period. TPTD was performed in order to obtain reference CO values (COREF). TPTD and arterial wave signals were transmitted to a PiCCO2™ monitor in order to obtain CCOPiCCO values. CCOVolumeView and CCOPiCCO were recorded over a 5 min interval before assessment of COTPTD. Bland-Altman analysis, %errors, and concordance (trend analysis) were calculated. Results A total of 338 matched sets of data were available for comparison. Bias for CCOVolumeView−COREF was −0.07 litre min−1 and for CCOPiCCO-COREF +0.03 litre min−1. Corresponding limits of agreement were 2.00 and 2.48 litre min−1 (P<0.01), %errors 29 and 37%, respectively. Trending capabilities were comparable for both techniques. Conclusions The performance of the new VolumeView™-CCO method is as reliable as the PiCCO2™-CCO pulse wave analysis in critically ill patients. However, an improved precision was observed with the VolumeView™ technique. Clinicaltrials.gov identifier NCT0140504
Quantification of depth of anesthesia by nonlinear time series analysis of brain electrical activity
We investigate several quantifiers of the electroencephalogram (EEG) signal
with respect to their ability to indicate depth of anesthesia. For 17 patients
anesthetized with Sevoflurane, three established measures (two spectral and one
based on the bispectrum), as well as a phase space based nonlinear correlation
index were computed from consecutive EEG epochs. In absence of an independent
way to determine anesthesia depth, the standard was derived from measured blood
plasma concentrations of the anesthetic via a pharmacokinetic/pharmacodynamic
model for the estimated effective brain concentration of Sevoflurane. In most
patients, the highest correlation is observed for the nonlinear correlation
index D*. In contrast to spectral measures, D* is found to decrease
monotonically with increasing (estimated) depth of anesthesia, even when a
"burst-suppression" pattern occurs in the EEG. The findings show the potential
for applications of concepts derived from the theory of nonlinear dynamics,
even if little can be assumed about the process under investigation.Comment: 7 pages, 5 figure
Deep LOFAR observations of the merging galaxy cluster CIZA J2242.8+5301
Previous studies have shown that CIZA J2242.8+5301 (the 'Sausage' cluster, z = 0.192) is a massive merging galaxy cluster that hosts a radio halo and multiple relics. In this paper, we present deep, high-fidelity, low-frequency images made with the LOw-Frequency Array (LOFAR) between 115.5 and 179 MHz. These images, with a noise of 140 μJy beam- 1 and a resolution of θbeam = 7.3 arcsec × 5.3 arcsec, are an order of magnitude more sensitive and five times higher resolution than previous low-frequency images of this cluster. We combined the LOFAR data with the existing Giant Metrewave Radio Telescope (GMRT) (153, 323, 608 MHz) and Westerbork Synthesis Radio Telescope (WSRT) (1.2, 1.4, 1.7, 2.3 GHz) data to study the spectral properties of the radio emission from the cluster. Assuming diffusive shock acceleration (DSA), we found Mach numbers of Mn=2.7{}_{-0.3}^{+0.6} and Ms=1.9_{-0.2}^{+0.3} for the northern and southern shocks. The derived Mach number for the northern shock requires an acceleration efficiency of several percent to accelerate electrons from the thermal pool, which is challenging for DSA. Using the radio data, we characterized the eastern relic as a shock wave propagating outwards with a Mach number of Me=2.4_{-0.3}^{+0.5}, which is in agreement with MeX=2.5{}_{-0.2}^{+0.6} that we derived from Suzaku data. The eastern shock is likely to be associated with the major cluster merger. The radio halo was measured with a flux of 346 ± 64 mJy at 145 MHz. Across the halo, we observed a spectral index that remains approximately constant (α ^{145 MHz-2.3 GHz}_{{across ˜ 1 Mpc}^2}=-1.01± 0.10) after the steepening in the post-shock region of the northern relic. This suggests a generation of post-shock turbulence that re-energies aged electrons
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