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Intracluster Medium (intracluster + medium)
Selected AbstractsFeedback under the microscope , I. Thermodynamic structure and AGN-driven shocks in M87MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010E. T. Million ABSTRACT We present the first in a series of papers discussing the thermodynamic properties of M87 and the central regions of the Virgo Cluster in unprecedented detail. Using a deep Chandra exposure (574 ks), we present high-resolution thermodynamic maps created from the spectra of ,16 000 independent regions, each with ,1000 net counts. The excellent spatial resolution of the thermodynamic maps reveals the dramatic and complex temperature, pressure, entropy and metallicity structure of the system. The ,X-ray arms', driven outwards from M87 by the central active galactic nuclei (AGN), are prominent in the brightness, temperature and entropy maps. Excluding the ,X-ray arms', the diffuse cluster gas at a given radius is strikingly isothermal. This suggests either that the ambient cluster gas, beyond the arms, remains relatively undisturbed by AGN uplift or that conduction in the intracluster medium (ICM) is efficient along azimuthal directions, as expected under action of the heat-flux-driven buoyancy instability (HBI). We confirm the presence of a thick (,40 arcsec or ,3 kpc) ring of high-pressure gas at a radius of ,180 arcsec (,14 kpc) from the central AGN. We verify that this feature is associated with a classical shock front, with an average Mach number M= 1.25. Another, younger shock-like feature is observed at a radius of ,40 arcsec (,3 kpc) surrounding the central AGN, with an estimated Mach number M, 1.2. As shown previously, if repeated shocks occur every ,10 Myr, as suggested by these observations, then AGN-driven weak shocks could produce enough energy to offset radiative cooling of the ICM. A high significance enhancement of Fe abundance is observed at radii 350,400 arcsec (27,31 kpc). This ridge is likely formed in the wake of the rising bubbles filled with radio-emitting plasma that drag cool, metal-rich gas out of the central galaxy. We estimate that at least ,1.0 × 106 solar masses of Fe has been lifted and deposited at a radius of 350,400 arcsec; approximately the same mass of Fe is measured in the X-ray bright arms, suggesting that a single generation of buoyant radio bubbles may be responsible for the observed Fe excess at 350,400 arcsec. [source] The impact of mergers on relaxed X-ray clusters , III.MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2008Effects on compact cool cores ABSTRACT We use the simulations presented in Poole et al. to examine the effects of mergers on the properties of cool cores in X-ray clusters. Motivated by recent Chandra and XMM,Newton observations, we propose a scheme for classifying the morphology of clusters based on their surface brightness and entropy profiles. Three dominant morphologies emerge: two hosting compact cores and central temperatures which are cool [CCC (compact cool core) systems] or warm [CWC (compact warm core) systems] and one hosting extended cores which are warm [EWC (extended warm core) systems]. In the cases we have studied, CCC states are disrupted only after direct collisions with merging cluster cores. This can happen in head-on collisions or during second pericentric passage in off-axis mergers. By the time they are relaxed, our remnant cores have generally been heated to warm core (CWC or EWC) states but subsequently recover CCC states by the end of the simulation. The only case resulting in a long-lived EWC state is a slightly off-axis 3:1 merger for which the majority of shock heating occurs during the accretion of a low-entropy stream formed from the disruption of the secondary's cool core. Since tdyn,tcool for all our relaxing merger remnant cores, compression prevents their core temperatures from falling until after they relax to the compact states allowed by their remnant central entropies. This naturally explains the population of relaxed CWC systems observed in recent Chandra and XMM,Newton observations with no need to invoke active galactic nuclei feedback. The morphological segregation in the Lx,Tx scaling relation noted by McCarthy et al. is qualitatively reflected in the results of our mergers as well. However, none of the cases we have studied produces systems with sufficiently high central entropies to account for the most underluminous EWC systems observed. Lastly, mergers do not efficiently mix the intracluster medium in our simulations. As a result, merging systems which initially host central metallicity gradients do not yield merger remnants with flat metallicity profiles. Taken together, these results suggest that once formed, compact core systems are remarkably stable against disruption from mergers. It remains to be demonstrated exactly how the sizable observed population of extended core systems was formed. [source] The Bullet Cluster 1E0657-558 evidence shows modified gravity in the absence of dark matterMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2007J. R. Brownstein ABSTRACT A detailed analysis of the 2006 November 15 data release X-ray surface density ,-map and the strong and weak gravitational lensing convergence ,-map for the Bullet Cluster 1E0657-558 is performed and the results are compared with the predictions of a modified gravity (MOG) and dark matter. Our surface density ,-model is computed using a King ,-model density, and a mass profile of the main cluster and an isothermal temperature profile are determined by the MOG. We find that the main cluster thermal profile is nearly isothermal. The MOG prediction of the isothermal temperature of the main cluster is T= 15.5 ± 3.9 keV, in good agreement with the experimental value T= 14.8+2.0,1.7 keV. Excellent fits to the 2D convergence ,-map data are obtained without non-baryonic dark matter, accounting for the 8, spatial offset between the ,-map and the ,-map reported in Clowe et al. The MOG prediction for the ,-map results in two baryonic components distributed across the Bullet Cluster 1E0657-558 with averaged mass fraction of 83 per cent intracluster medium (ICM) gas and 17 per cent galaxies. Conversely, the Newtonian dark matter ,-model has on average 76 per cent dark matter (neglecting the indeterminant contribution due to the galaxies) and 24 per cent ICM gas for a baryon to dark matter mass fraction of 0.32, a statistically significant result when compared to the predicted ,-cold dark matter cosmological baryon mass fraction of 0.176+0.019,0.012. [source] Impact of tangled magnetic fields on fossil radio bubblesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007M. Ruszkowski ABSTRACT There is growing consensus that feedback from active galactic nuclei (AGN) is the main mechanism responsible for stopping cooling flows in clusters of galaxies. AGN are known to inflate buoyant bubbles that supply mechanical power to the intracluster gas [intracluster medium (ICM)]. High Reynolds number hydrodynamical simulations show that such bubbles get entirely disrupted within 100 Myr, as they rise in cluster atmospheres, which is contrary to observations. This artificial mixing has consequences for models trying to quantify the amount of heating and star formation in cool core clusters of galaxies. It has been suggested that magnetic fields can stabilize bubbles against disruption. We perform magnetohydrodynamical simulations of fossil bubbles in the presence of tangled magnetic fields using the high-order pencil code. We focus on the physically motivated case where thermal pressure dominates over magnetic pressure and consider randomly oriented fields with and without maximum helicity and a case where large-scale external fields drape the bubble. We find that helicity has some stabilizing effect. However, unless the coherence length of magnetic fields exceeds the bubble size, the bubbles are quickly shredded. As observations of Hydra A suggest that length-scale of magnetic fields may be smaller than typical bubble size, this may suggest that other mechanisms, such as viscosity, may be responsible for stabilizing the bubbles. However, since Faraday rotation observations of radio lobes do not constrain large-scale ICM fields well if they are aligned with the bubble surface, the draping case may be a viable alternative solution to the problem. A generic feature found in our simulations is the formation of magnetic wakes where fields are ordered and amplified. We suggest that this effect could prevent evaporation by thermal conduction of cold H, filaments observed in the Perseus cluster. [source] H i imaging of galaxies in X-ray bright groupsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2007Chandreyee Sengupta ABSTRACT Environment plays an important role in the evolution of the gas contents of galaxies. Gas deficiency of cluster spirals and the role of the hot intracluster medium in stripping gas from these galaxies is a well-studied subject. Loose groups with diffuse X-ray emission from the intragroup medium (IGM) offer an intermediate environment between clusters and groups without a hot IGM. These X-ray bright groups have smaller velocity dispersion and lower temperature than clusters, but higher IGM density than loose groups without diffuse X-ray emission. A single-dish comparative study of loose groups with and without diffuse X-ray emission from the IGM, showed that the galaxies in X-ray bright groups have lost more gas on average than the galaxies in non X-ray bright groups. In this paper we present GMRT H i observations of 13 galaxies from four X-ray bright groups: NGC 5044, 720, 1550 and IC1459. The aim of this work is to study the morphology of H i in these galaxies and to see if the hot IGM has in any way affected their H i content or distribution. In addition to disturbed H i morphology, we find that most galaxies have shrunken H i discs compared to the field spirals. This indicates that IGM-assisted stripping processes like ram pressure may have stripped gas from the outer edges of the galaxies. [source] Chemical enrichment of the intracluster medium by FR II radio sourcesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007D. Heath ABSTRACT We present 2D axisymmetric hydrodynamic simulations investigating the long-term effect of Fanaroff,Riley type II radio galaxies on the metal distribution of the surrounding intracluster medium (ICM). A light jet is injected into a cooling flow atmosphere for 10,30 Myr. We then follow the subsequent evolution for 3 Gyr on a spherical grid spanning 3 Mpc in radius. A series of passive tracer particles were placed in an annulus about the cluster core to simulate metal carrying clouds in order to calculate the metallicity (Z) as a function of time and radial distance from the cluster centre. The jet has a significant effect on the ICM over the entire 3-Gyr period. By the end of the simulations, the jets produced metallicities of ,10 per cent of the initial metallicity of the cluster core throughout much of the cluster. The jets transport the metals not only in mixing regions, but also through upwelling ICM behind the jet, enriching the cluster over both long and short distances. [source] The evolution of the cluster X-ray scaling relations in the Wide Angle ROSAT Pointed Survey sample at 0.6 < z < 1.0MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006B. J. Maughan ABSTRACT The X-ray properties of a sample of 11 high-redshift (0.6 < z < 1.0) clusters observed with Chandra and/or XMM,Newton are used to investigate the evolution of the cluster scaling relations. The observed evolution in the normalization of the L,T, M,T, Mg,T and M,L relations is consistent with simple self-similar predictions, in which the properties of clusters reflect the properties of the Universe at their redshift of observation. Under the assumption that the model of self-similar evolution is correct and that the local systems formed via a single spherical collapse, the high-redshift L,T relation is consistent with the high- z clusters having virialized at a significantly higher redshift than the local systems. The data are also consistent with the more realistic scenario of clusters forming via the continuous accretion of material. The slope of the L,T relation at high redshift (B= 3.32 ± 0.37) is consistent with the local relation, and significantly steeper than the self-similar prediction of B= 2. This suggests that the same non-gravitational processes are responsible for steepening the local and high- z relations, possibly occurring universally at z, 1 or in the early stages of the cluster formation, prior to their observation. The properties of the intracluster medium at high redshift are found to be similar to those in the local Universe. The mean surface-brightness profile slope for the sample is ,= 0.66 ± 0.05, the mean gas mass fractions within R2500(z) and R200(z) are 0.069 ± 0.012 and 0.11 ± 0.02, respectively, and the mean metallicity of the sample is 0.28 ± 0.11 Z,. [source] Turbulent gas motions in galaxy cluster simulations: the role of smoothed particle hydrodynamics viscosityMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005K. Dolag ABSTRACT Smoothed particle hydrodynamics (SPH) employs an artificial viscosity to properly capture hydrodynamic shock waves. In its original formulation, the resulting numerical viscosity is large enough to suppress structure in the velocity field on scales well above the nominal resolution limit, and to damp the generation of turbulence by fluid instabilities. This could artificially suppress random gas motions in the intracluster medium (ICM), which are driven by infalling structures during the hierarchical structure formation process. We show that this is indeed the case by analysing results obtained with an SPH formulation where an individual, time-variable viscosity is used for each particle, following a suggestion by Morris & Monaghan. Using test calculations involving strong shocks, we demonstrate that this scheme captures shocks as well as the original formulation of SPH, but, in regions away from shocks, the numerical viscosity is much smaller. In a set of nine high-resolution simulations of cosmological galaxy cluster formation, we find that this low-viscosity formulation of SPH produces substantially higher levels of turbulent gas motions in the ICM, reaching a kinetic energy content in random gas motions (measured within a 1-Mpc cube) of up to 5,30 per cent of the thermal energy content, depending on cluster mass. This also has significant effects on radial gas profiles and bulk cluster properties. We find a central flattening of the entropy profile and a reduction of the central gas density in the low-viscosity scheme. As a consequence, the bolometric X-ray luminosity is decreased by about a factor of 2. However, the cluster temperature profile remains essentially unchanged. Interestingly, this tends to reduce the differences seen in SPH and adaptive mesh refinement simulations of cluster formation. Finally, invoking a model for particle acceleration by magnetohydrodynamics waves driven by turbulence, we find that efficient electron acceleration and thus diffuse radio emission can be powered in the clusters simulated with the low-viscosity scheme provided that more than 5,10 per cent of the turbulent energy density is associated with fast magneto-sonic modes. [source] The effects of thermal conduction on the intracluster medium of the Virgo clusterMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2005Edward C. D. Pope ABSTRACT Thermal conduction has been suggested as a possible mechanism by which sufficient energy is supplied to the central regions of galaxy clusters to balance the effect of radiative cooling. Recent high-resolution observations of the nearby Virgo cluster make it an ideal subject for an attempt to reproduce the properties of the cluster by numerical simulations, because most of the defining parameters are comparatively well known. Here, we present the results of a simulated high-resolution, 3D Virgo cluster for different values of thermal conductivity ( times the full Spitzer value). Starting from an initially isothermal cluster atmosphere, we allow the cluster to evolve freely over time-scales of roughly 1.3,4.7 × 109 yr. Our results show that thermal conductivity at the Spitzer value can increase the central intracluster medium (ICM) radiative cooling time by a factor of roughly 3.6. In addition, for larger values of thermal conductivity, the simulated temperature and density profiles match the observations significantly better than for the lower values. However, no physically meaningful value of thermal conductivity was able to postpone the cooling catastrophe (characterized by a rapid increase in the central density) for longer than a fraction of the Hubble time nor explain the absence of a strong cooling flow in the Virgo cluster today. We also calculate the effective adiabatic index of the cluster gas for both simulation and observational data and compare the values with theoretical expectations. Using this method, it appears that the Virgo cluster is being heated in the cluster centre by a mechanism other than thermal conductivity. Based on this and our simulations, it is also likely that the thermal conductivity is suppressed by a factor of at least 10 and probably more. Thus, we suggest that thermal conductivity, if present at all, has the effect of slowing down the evolution of the ICM, by radiative cooling, but only by a factor of a few. [source] Rising jet-inflated bubbles in clusters of galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008Assaf Sternberg ABSTRACT We conduct two-dimensional axisymmetric (referred to as 2.5D) hydrodynamical numerical simulations of bubble evolution in clusters of galaxies. We inflate bubbles using slow, massive jets with a wide opening angle, and follow their evolution as they rise through the intracluster medium. We find that these jet-inflated bubbles are quite stable, and can reach large distances in the cluster while maintaining their basic structure. The stability of the jet-inflated bubble comes mainly from the dense shell that forms around it during its inflation stage, and from the outward momentum of the bubble and shell. On the contrary, bubbles that are inserted by hand on to the grid and not inflated by a jet, i.e. an artificial bubble, lack these stabilizing factors; therefore, they are rapidly destroyed. The stability of the jet-inflated bubble removes the demand for stabilizing magnetic fields in the bubble. [source] On the evolution of the Fe abundance and of the Type Ia supernova rate in clusters of galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2007F. Calura ABSTRACT The study of the Fe abundance in the intracluster medium (ICM) provides strong constraints on the integrated star formation history and supernova rate of the cluster galaxies, as well as on the ICM enrichment mechanisms. In this Letter, using chemical evolution models for galaxies of different morphological types, we study the evolution of the Fe content of clusters of galaxies. We assume that the ICM Fe enrichment occurs by means of galactic winds arising from elliptical galaxies and from gas stripped from the progenitors of S0 galaxies via external mechanisms, due to the interaction of the interstellar medium with the ICM. The Fe-rich gas ejected by ellipticals accounts for the XFe,ICM values observed at z > 0.5, whereas the gas stripped from the progenitors of the S0 galaxies accounts for the increase of XFe,ICM observed at z < 0.5. We test two different scenarios for Type Ia supernova (SN) progenitors and we model the Type Ia SN rate observed in clusters, finding a good agreement between our predictions and the available observations. [source] Ram pressure stripping of disk galaxies in galaxy clustersASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009E. Roediger Abstract While galaxies move through the intracluster medium of their host cluster, they experience a ram pressure which removes at least a significant part of their interstellar medium. This ram pressure stripping appears to be especially important for spiral galaxies: this scenario is a good candidate to explain the differences observed between cluster spirals in the nearby universe and their field counterparts. Thus, ram pressure stripping of disk galaxies in clusters has been studied intensively during the last decade. I review advances made in this area, concentrating on theoretical work, but continuously comparing to observations (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Metal enrichment of the intracluster medium: SN-driven galactic windsASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009V. Baumgartner Abstract We investigate the role of supernova (SN)-driven galactic winds in the chemical enrichment of the intracluster medium (ICM). Such outflows on galactic scales have their origin in huge star forming regions and expel metal enriched material out ofthe galaxies into their surroundings as observed, for example, in the nearby starburst galaxy NGC 253. As massive stars in OB-associations explode sequentially, shock waves are driven into the interstellar medium (ISM) of a galaxy and merge, forming a superbubble (SB). These SBs expand in a direction perpendicular to the disk plane following the density gradient of the ISM. We use the 2D analytical approximation by Kompaneets (1960) to model the expansion of SBs in an exponentially stratified ISM. This is modified in order to describe the sequence of SN-explosions as a time-dependent process taking into account the main-sequence life-time of the SN-progenitors and using an initial mass function to get the number of massive stars per mass interval. The evolution of the bubble in space and time is calculated analytically, from which the onset of Rayleigh-Taylor instabilities in the shell can be determined. In its further evolution, the shell will break up and high-metallicity gas will be ejected into the halo ofthe galaxy and even into the ICM. We derive the number of stars needed for blow-out depending on the scale height and density ofthe ambient medium, as well as the fraction of alpha- and iron peak elements contained in the hot gas. Finally, the amount of metals injected by Milky Way-type galaxies to the ICM is calculated confirming the importance ofthis enrichment process (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Observations of magnetic fields in regular and irregular clustersASTRONOMISCHE NACHRICHTEN, Issue 5-6 2006F. Govoni Abstract The existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. The most detailed evidence for the presence of cluster magnetic fields comes from radio observations. Magnetic fields can be investigated through the synchrotron emission of cluster-wide diffuse sources and from studies of the rotation measure of polarized radio galaxies. I will review efforts to measure magnetic field strengths and power spectra and the main issues that have led to our knowledge on magnetic fields in regular and irregular clusters of galaxies. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The origin and evolution of cluster magnetismASTRONOMISCHE NACHRICHTEN, Issue 5-6 2006A. Shukurov Abstract Random motions can occur in the intergalactic gas of galaxy clusters at all stages of their evolution. Depending on the poorly known value of the Reynolds number, these motions can or cannot become turbulent, but in any case they can generate random magnetic fields via dynamo action. We argue that magnetic fields inferred observationally for the intracluster medium require dynamo action, and then estimate parameters of random flows and magnetic fields at various stages of the cluster evolution. Polarization in cluster radio halos predicted by the model would be detectable with the SKA. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||