178 research outputs found
Probing the dynamical state of galaxy clusters
We show how hydrostatic equilibrium in galaxy clusters can be quantitatively
probed combining X-ray, SZ, and gravitational-lensing data. Our previously
published method for recovering three-dimensional cluster gas distributions
avoids the assumption of hydrostatic equilibrium. Independent reconstructions
of cumulative total-mass profiles can then be obtained from the gas
distribution, assuming hydrostatic equilibrium, and from gravitational lensing,
neglecting it. Hydrostatic equilibrium can then be quantified comparing the
two. We describe this procedure in detail and show that it performs well on
progressively realistic synthetic data. An application to a cluster merger
demonstrates how hydrostatic equilibrium is violated and restored as the merger
proceeds.Comment: 10 pages, 6 figures, submitted to A&
SHARP: A Spatially Higher-order, Relativistic Particle-in-Cell Code
Numerical heating in particle-in-cell (PIC) codes currently precludes the
accurate simulation of cold, relativistic plasma over long periods, severely
limiting their applications in astrophysical environments. We present a
spatially higher-order accurate relativistic PIC algorithm in one spatial
dimension, which conserves charge and momentum exactly. We utilize the
smoothness implied by the usage of higher-order interpolation functions to
achieve a spatially higher-order accurate algorithm (up to fifth order). We
validate our algorithm against several test problems -- thermal stability of
stationary plasma, stability of linear plasma waves, and two-stream instability
in the relativistic and non-relativistic regimes. Comparing our simulations to
exact solutions of the dispersion relations, we demonstrate that SHARP can
quantitatively reproduce important kinetic features of the linear regime. Our
simulations have a superior ability to control energy non-conservation and
avoid numerical heating in comparison to common second-order schemes. We
provide a natural definition for convergence of a general PIC algorithm: the
complement of physical modes captured by the simulation, i.e., those that lie
above the Poisson noise, must grow commensurately with the resolution. This
implies that it is necessary to simultaneously increase the number of particles
per cell and decrease the cell size. We demonstrate that traditional ways for
testing for convergence fail, leading to plateauing of the energy error. This
new PIC code enables us to faithfully study the long-term evolution of plasma
problems that require absolute control of the energy and momentum conservation.Comment: 26 pages, 19 figures, discussion about performance is added,
published in Ap
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Zoomed cosmological simulations of Milky Way-sized haloes in f(R) gravity
We investigate the impact of f(R) modified gravity on the internal properties
of Milky Way sized dark matter halos in a set of cosmological zoom simulations
of seven halos from the Aquarius suite, carried out with our code MG-GADGET in
the Hu & Sawicki f(R) model. Also, we calculate the fifth forces in ideal
NFW-halos as well as in our cosmological simulations and compare them against
analytic model predictions for the fifth force inside spherical objects. We
find that these theoretical predictions match the forces in the ideal halos
very well, whereas their applicability is somewhat limited for realistic
cosmological halos. Our simulations show that f(R) gravity significantly
affects the dark matter density profile of Milky Way sized objects as well as
their circular velocities. In unscreened regions, the velocity dispersions are
increased by up to 40% with respect to LCDM for viable f(R) models. This
difference is larger than reported in previous works. The Solar circle is fully
screened in models for Milky Way sized halos, while this
location is unscreened for slightly less massive objects. Within the scope of
our limited halo sample size, we do not find a clear dependence of the
concentration parameter of dark matter halos on .VS and CA would like to thank the Klaus Tschira foundation and acknowledge support from the Deutsche Forschungsgemeinschaft (DFG) through Transregio 33, ‘The Dark Universe’. EP gratefully acknowledges support by the Kavli Foundation and the FP7 European Research Council Advanced Grant Emergence-320596. The authors acknowledge CPU-time from the Juelich Supercomputer Centre on the JURECA system.This is the final version of the article. It first appeared from Oxford University Press via https://doi.org/10.1093/mnras/stw170
The effect of AGN feedback on the X-ray morphologies of clusters: Simulations vs. observations
Clusters of galaxies probe the large-scale distribution of matter and are a useful tool to test the cosmological models by constraining cosmic structure growth and the expansion of the Universe. It is the scaling relations between mass observables and the true mass of a cluster through which we obtain the cosmological constraints by comparing to theoretical cluster mass functions. These scaling relations are, however, heavily influenced by cluster morphology. The presence of the slight tension in recent cosmological constraints on and based on the CMB and clusters has boosted the interests in looking for possible sources for the discrepancy. Therefore we study here the effect of active galactic nucleus (AGN) feedback as one of the major mechanisms modifying the cluster morphology influencing scaling relations. It is known that AGN feedback injects energies up to 10 erg into the intracluster medium, controls the heating and cooling of a cluster, and re-distributes cold gas from the centre to outer radii. We have also learned that cluster simulations with AGN feedback can reproduce observed cluster properties, for example, the X-ray luminosity, temperature, and cooling rate at the centre better than without the AGN feedback. In this paper using cosmological hydrodynamical simulations we investigate how the AGN feedback changes the X-ray morphology of the simulated systems, and compare this to the observed Representative Cluster Structure Survey (REXCESS) clusters. We apply two substructure measures, centre shifts () and power ratios (e.g. /), to characterise the cluster morphology, and find that our simulated clusters are more substructured than the observed clusters based on the values of and /. We also show that the degree of this discrepancy is affected by the inclusion of AGN feedback. While the clusters simulated with the AGN feedback are in much better agreement with the REXCESS relation, they are also more substructured, which increases the tension with observations. When classified as non-relaxed or relaxed according to their and / values, we find that there are no relaxed clusters in the simulations with the AGN feedback. This suggests that not only global cluster properties, like and T, and radial profiles should be used to compare and to calibrate simulations with observations, but also substructure measures like centre shifts and power ratios. Finally, we discuss what changes in the simulations might ease the tension with observational constraints on these quantities.H.B. and G.C. acknowledge support from the DFG Transregio Program TR33 and the Munich Excellence Cluster “Structure and Evolution of the Universe”. G.C. acknowledges support by the DLR under grant No. 50 OR 1405. E.P. acknowledges support by the Kavli Foundation and the FP7 ERC Advanced Grant Emergence-320596
Importance of resolving the spectral support of beam-plasma instabilities in simulations
Many astrophysical plasmas are prone to beam-plasma instabilities. For
relativistic and dilute beams, the {\it spectral} support of the beam-plasma
instabilities is narrow, i.e., the linearly unstable modes that grow with rates
comparable to the maximum growth rate occupy a narrow range of wave numbers.
This places stringent requirements on the box-sizes when simulating the
evolution of the instabilities. We identify the implied lower limits on the box
size imposed by the longitudinal beam plasma instability, i.e., typically the
most stringent condition required to correctly capture the linear evolution of
the instabilities in multidimensional simulations. We find that sizes many
orders of magnitude larger than the resonant wavelength are typically required.
Using one-dimensional particle-in-cell simulations, we show that the failure to
sufficiently resolve the spectral support of the longitudinal instability
yields slower growth and lower levels of saturation, potentially leading to
erroneous physical conclusion.Comment: 7 pages, 9 figures, accepted by Ap
Growth of beam-plasma instabilities in the presence of background inhomogeneity
We explore how inhomogeneity in the background plasma number density alters
the growth of electrostatic unstable wavemodes of beam plasma systems. This is
particularly interesting for blazar-driven beam-plasma instabilities, which may
be suppressed by inhomogeneities in the intergalactic medium as was recently
claimed in the literature. Using high resolution Particle-In-Cell simulations
with the SHARP code, we show that the growth of the instability is local, i.e.,
regions with almost homogeneous background density will support the growth of
the Langmuir waves, even when they are separated by strongly inhomogeneous
regions, resulting in an overall slower growth of the instability. We also show
that if the background density is continuously varying, the growth rate of the
instability is lower; though in all cases, the system remains within the linear
regime longer and the instability is not extinguished. In all cases, the beam
loses approximately the same fraction of its initial kinetic energy in
comparison to the uniform case at non-linear saturation. Thus, inhomogeneities
in the intergalactic medium are unlikely to suppress the growth of
blazar-driven beam-plasma instabilities.Comment: 10 pages, 6 figures, Accepted by ApJ, comments welcom
Large 21 cm signals from AGN-dominated reionization
We present predictions for the spatial distribution of 21-cm brightness temperature fluctuations from high-dynamic-range simulations for active galactic nucleus (AGN)-dominated reionization histories that have been tested against available Ly and cosmic microwave background (CMB) data. We model AGNs by extrapolating the observed − σ relation to high redshifts and assign them ionizing emissivities consistent with recent UV luminosity function measurements. We assess the observability of the predicted spatial 21-cm fluctuations in the late stages of reionization in the limit in which the hydrogen 21-cm spin temperature is significantly larger than the CMB temperature. Our AGN-dominated reionization histories increase the variance of the 21-cm emission by a factor of up to 10 compared to similar reionization histories dominated by faint galaxies, to values close to 100 mK at scales accessible to experiments ( 1 cMpc ). This is lower than the sensitivity reached by ongoing experiments only by a factor of about 2 or less. When reionization is dominated by AGNs, the 21-cm power spectrum is enhanced on all scales due to the enhanced bias of the clustering of the more massive haloes and the peak in the large scale 21-cm power is strongly enhanced and moved to larger scales due to bigger characteristic bubble sizes. AGN-dominated reionization should be easily detectable by Low Frequency Array (and later Hydrogen Epoch of Reionization Array and Phase 1 of the Square Kilometre Array) at their design sensitivity, assuming successful foreground subtraction and instrument calibration. Conversely, these could become the first non-trivial reionization scenarios to be ruled out by 21-cm experiments, thereby constraining the contribution of AGNs to reionization.Support by ERC Advanced grant 320596 ‘The Emergence of Structure During the Epoch of Reionization’ is gratefully acknowledged. EP gratefully acknowledges support by the Kavli Foundation. We acknowledge PRACE for awarding us access to the Curie supercomputer, based in France at the Tres Grand Centre de Calcul ´ (TGCC). This work used the DiRAC Data Centric system at Durham University, operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment was funded by BIS National E-infrastructure capital grant ST/K00042X/1, STFC capital grants ST/H008519/1 and ST/K00087X/1, STFC DiRAC Operations grant ST/K003267/1 and Durham University. DiRAC is part of the National E-Infrastructure. This research was supported by the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG cluster of excellence ‘Origin and Structure of the Universe’
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