Intracluster Plasma Research Articles

Clusters of Galaxies: Setting the Stage

Clusters of galaxies are self-gravitating systems of mass ~10^14-10^15 Msun. They consist of dark matter (~80 %), hot diffuse intracluster plasma (< 20 %) and a small fraction of stars, dust, and cold gas, mostly locked in galaxies. In most clusters, scaling relations between their properties testify that the cluster components are in approximate dynamical equilibrium within the cluster gravitational potential well. However, spatially inhomogeneous thermal and non-thermal emission of the intracluster medium (ICM), observed in some clusters in the X-ray and radio bands, and the kinematic and morphological segregation of galaxies are a signature of non-gravitational processes, ongoing cluster merging and interactions. In the current bottom-up scenario for the formation of cosmic structure, clusters are the most massive nodes of the filamentary large-scale structure of the cosmic web and form by anisotropic and episodic accretion of mass. In this model of the universe dominated by cold dark matter, at the present time most baryons are expected to be in a diffuse component rather than in stars and galaxies; moreover, ~50 % of this diffuse component has temperature ~0.01-1 keV and permeates the filamentary distribution of the dark matter. The temperature of this Warm-Hot Intergalactic Medium (WHIM) increases with the local density and its search in the outer regions of clusters and lower density regions has been the quest of much recent observational effort. Over the last thirty years, an impressive coherent picture of the formation and evolution of cosmic structures has emerged from the intense interplay between observations, theory and numerical experiments. Future efforts will continue to test whether this picture keeps being valid, needs corrections or suffers dramatic failures in its predictive power.

Convection and AGN Feedback in Clusters of Galaxies

A number of studies have shown that the convective stability criterion for the intracluster medium (ICM) is very differentfromtheSchwarzchildcriterion,duetotheeffectsofanisotropicthermalconductionandcosmicrays.Buildingonthesestudies,wedevelopamodeloftheICMinwhichacentralactivegalacticnucleus(AGN)accreteshotintracluster plasma at the Bondi rate and produces cosmic rays that cause the ICM to become convectively unstable. The resulting convection heats the intracluster plasma and regulates its temperature and density profiles. By adjusting a single parameter in the model (the size of the cosmic-ray acceleration region), we are able to achieve a good match to the observed density and temperature profiles in a sample of eight clusters. Our results suggest that convection is an important process in cluster cores. An interesting feature of our solutions isthat the cooling rate is more sharply peaked abouttheclustercenterthanistheconvectiveheatingrate.Asaresult,inseveraloftheclustersinoursample,acompact coolingflowarisesinthecentralregionwithasizercf thatistypicallyafewkpc.ThecoolingflowmatchesontoaBondi flow at smaller radii. The mass accretion rate in the Bondi flow is equal to, and controlled by, the rate at which mass flowsinthroughthecoolingflow.Oursolutionssuggest thattheAGNregulatesthemassaccretionrateintheseclusters bycontrollingrcf: if the AGN power rises above the equilibrium level,rcf decreases, the mass accretion rate drops, and the AGN power drops back down to the equilibrium level. Subject headingg convection — cooling flows — galaxies: active — galaxies: clusters: general — magnetic fields — turbulence

Missing Baryons, from Clusters to Groups of Galaxies

From clusters to groups of galaxies, the powerful bremsstrahlung radiation LX emitted in X-rays by the intracluster plasma is observed to decline sharply with lowering virial temperatures T (i.e., at shallower depths of the gravitational wells) after a steep local LX − T correlation; this implies increasing scarcity of diffuse baryons relative to dark matter, well under the cosmic fraction. We show how the widely debated issue concerning these "missing baryons" is solved in terms of the thermal and/or dynamical effects of the kinetic (at low redshifts z) and radiative (at high z) energy inputs from central active galactic nuclei, of which independent evidence is being observed. From these inputs we compute shape and z-evolution expected for the LX − T correlation which agree with the existing data, and provide a predictive pattern for future observations.

The Sunyaev‐Zel’dovich Effect in a Sample of 31 Clusters: A Comparison between the X‐Ray Predicted andWMAPObserved Cosmic Microwave Background Temperature Decrement

The WMAP Q-, V-, and W-band radial profiles of temperature deviation of the cosmic microwave background (CMB) were constructed for a sample of 31 randomly selected nearby clusters of galaxies in directions of Galactic latitude > 30°. The profiles were compared in detail with the expected CMB Sunyaev-Zel'dovich effect (SZE) caused by these clusters, with the hot gas properties of each cluster inferred observationally by applying gas temperatures as measured by ASCA to isothermal β-models of the ROSAT X-ray surface brightness profiles, with the WMAP point-spread function fully taken into consideration. After co-adding the 31 cluster fields to significantly reduce the systematic and random uncertainties, it appears that WMAP detected the SZE in all three bands. Quantitatively, however, the observed SZE only accounts for about 1/4 of the expected decrement. The discrepancy represents too much unexplained extra flux: in the W band, the detected SZE corresponds on average to 5.6 times less X-ray gas mass within a 10' radius than the mass value given by the ROSAT β-model. We critically examined how the X-ray prediction of the SZE may depend on our uncertainties in the density and temperature of the hot intracluster plasma, and emission by cluster radio sources. Although our comparison between the detected and expected SZE levels is subject to a margin of error, the fact remains that the average observed SZE depth and profile are consistent with those of the primary CMB anisotropy, i.e., the overall WMAP temperature decrement among the 31 rich clusters is too shallow to necessitate an interpretation in terms of an additional effect like the SZE. A unique aspect of this SZE investigation is that because all the data being analyzed are in the public domain, our work is readily open to scrutiny by others.

AGN‐driven Convection in Galaxy‐Cluster Plasmas

This paper describes how active galactic nuclei can heat galaxy-cluster plasmas by driving convection in the intracluster medium. A model is proposed in which a central supermassive black hole accretes intracluster plasma at the Bondi rate Bondi and powers a radio source. The central radio source produces cosmic rays, which mix into the thermal plasma. The cosmic-ray luminosity Lcr is ∝Bondic2. The cosmic-ray pressure gradient drives convection, which causes plasma heating. It is assumed that plasma heating balances radiative cooling. The plasma heating rate is self-regulating because Bondi is a decreasing function of the specific entropy near the cluster center s0; if heating exceeds cooling, then s0 increases, Bondi and Lcr decrease, and the convective heating rate is reduced. This paper focuses on the role of intracluster magnetic fields, which affect convection by causing heat and cosmic rays to diffuse primarily along magnetic field lines. A new stability criterion is derived for convection in a thermal-plasma/cosmic-ray fluid, and equations for the average fluid properties in a convective cluster are obtained with the use of a nonlocal two-fluid mixing-length theory. Numerical solutions of the model equations compare reasonably well with observations without requiring fine-tuning of the model parameters.

Cosmic rays, cooling flows and non-thermal processes in galaxy clusters

Abstract Bremsstrahlung cooling of the intracluster (IC) plasma in galaxy clusters can be quenched by the energy supplied to the plasma by hadronic cosmic rays (CRs). This solution requires an energy deposition more intense and more distributed than in conventional CR models, but this alternative model is also successful in describing the origin of CRs. The model also provides a simple explanation of other puzzling features of galaxy clusters, among which we discuss here the non-thermal tail of their X-ray emission spectrum.

Convection in Galaxy‐Cluster Plasmas Driven by Active Galactic Nuclei and Cosmic‐Ray Buoyancy

Turbulent heating may play an important role in galaxy-cluster plasmas, but if turbulent heating is to balance radiative cooling in a quasi-steady state, some mechanism must regulate the turbulent velocity so that it is neither too large nor too small. This paper explores one possible regulating mechanism associated with an active galactic nucleus at cluster center. A steady-state model for the intracluster medium is presented in which radiative cooling is balanced by a combination of turbulent heating and thermal conduction. The turbulence is generated by convection driven by the buoyancy of cosmic rays produced by a central radio source. The cosmic-ray luminosity is powered by the accretion of intracluster plasma onto a central black hole. The model makes the rather extreme assumption that the cosmic rays and thermal plasma are completely mixed. Although the intracluster medium is convectively unstable near cluster center in the model solutions, the specific entropy of the thermal plasma still increases outward because of the cosmic-ray modification to the stability criterion. The model provides a self-consistent calculation of the turbulent velocity as a function of position but fails to reproduce the steep central density profiles observed in clusters. The principal difficulty is that in order for the fully mixed intracluster medium to become convectively unstable, the cosmic-ray pressure must become comparable to or greater than the thermal pressure within the convective region. The large cosmic ray pressure gradient then provides much of the support against gravity, reducing the thermal pressure gradient near cluster center and decreasing the central plasma density gradient. A more realistic active galactic nucleus (AGN)-feedback model of intracluster turbulence, in which relativistic and thermal plasmas are only partially mixed, may have greater success.

Chemical and Thermal History of the Intracluster Medium

Clusters of galaxies can be seen as giant astrophysical laboratories enclosing matter in a large enough volume, so that the matter composition can be taken as representing the composition of our Universe. X-ray observations allow a very precise investigation of the physical properties of the intracluster plasma allowing us to probe the cluster structure, determine its total mass, and measure the baryon fraction in clusters and in the Universe as a whole. We can determine the abundance of heavy elements from O to Ni which originate from supernova explosions and draw from this important conclusions on the history of star formation in the cluster galaxy population. From the entropy structure of the intracluster medium we obtain constraints on the energy release during early star bursts. with the observational capabilities of the X-ray observatories XMM-Newton and Chandra this field of research is rapidly evolving. In particular, first detailed observations of the intracluster medium of the Virgo cluster around M87 have provided new insights. The present contribution gives an account of the current implications of the intracluster medium observations, but more importantly illustrates the prospects of this research for the coming years.

X-ray spectroscopy of the cluster of galaxies PKS 0745-191 with XMM-Newton

We present the results from XMM-Newton observations of the cluster of galaxies PKS 0745-191. The spatially resolved spectra from the European Photon Imaging Cameras ( both EPN and EMOS) can be fitted with a single temperature thermal plasma model when the interstellar column density is closer to the lower limit of the range allowed by observations. Alternatively, a two-component model can be fitted to the spectra from the inner region for slightly higher values of N-H. The temperature profile shows a drop in the center of the cluster, from about 8 keV at radii larger than similar to350 kpc to about 4 keV at the center, indicating that the gas in the central region is cooling. The total cooling flow rate inferred from the spectral data is only 364(-94)(+97) M-circle dot yr(-1), however, much smaller than the previously obtained value. The metal abundance is found to decrease from 0.5 in solar units at the center to about 0.2 at a radius of similar to300 kpc. Deprojected spectra are also presented, which show a result similar to the projected spectra. The location of the X-ray emission peak is also the site of extended radio emission. Inspection of the higher resolution Chandra image of the cluster shows enhanced X-ray emission about 3 west of the center where also the radio emission is elongated. We explore possible implications on the interaction between the radio lobes and the intracluster plasma. Assuming that the gas is in hydrostatic equilibrium with the cluster potential, we find a total mass of about 1.18 +/- 0.04 x 10(13) M-circle dot within the critical radius of the observed gravitational arc, which is a factor similar to2 lower, however, than that from optical lensing. We also calculated the gas mass fraction profile up to 0.63 r(vir) and obtained a mean gas mass fraction f(gas) = 0.22 +/- 0.02.

Hot plasma in clusters of galaxies, the largest objects in the Universe

Clusters of galaxies are the largest organized structures in the Universe. They are important cosmological probes, since they are large enough to contain a fair sample of the materials in the Universe, but small enough to have achieved dynamical equilibrium. Clusters were first discovered as concentrations of hundreds of bright galaxies in a region about 3 megaparsecs (10 million light years) across. However, the dominant observed form of matter in clusters is hot, diffuse intergalactic gas. This intracluster plasma has typical temperatures of T∼7×107 K, and typical electron densities of ne∼10−3 cm−3. This intracluster plasma mainly emits x rays, and typical cluster x-ray luminosities are LX∼1043–1045 erg/s. The basic properties of and physical processes in the intracluster plasma will be reviewed. Important observational constraints on plasma processes in these systems will be discussed. Recent x-ray observations of clusters of galaxies with the orbiting Chandra X-Ray Observatory will be highlighted.

XMM-Newton observation of M 87

We report the results of a detailed analysis of the temperature structure of the X-ray emitting plasma halo of M 87, the cD galaxy of the Virgo Cluster. Using the MEKAL model, the data provide strong indications that the intracluster medium has a single phase structure locally, except the regions associated with the radio structures. The deprojected spectrum at each radius is well fitted by a single temperature MEKAL model, except for the very central region (<2 arcmin) which seems to be affected by the jet and radio lobe structure. The temperature of the intracluster plasma is 1 keV at the center and gradually increases to 2.5 keV at 80 kpc. We have also fitted spectra using the APEC code. Although the large changes of the strength of Kα lines causes a discrepancy between the Fe-L and Fe-K lines for the APEC results, the overall temperature structure has not changed. There is no sign of excess absorption in the spectral data. The single-phase nature of the intracluster medium is in conflict with the standard cooling flow model which is based on a multi-phase temperature structure. In addition, the signature of gas cooling below 0.8 keV to zero temperature is not observed as expected for a cooling flow. The gravitational mass profile derived from the temperature and density distribution of the intracluster gas shows two distinct contributions that can be assigned to the gravitational potential of the cD galaxy and the cluster. The central temperature of the intracluster medium agrees well with the potential depth and the velocity dispersion of the cD galaxy. The latter result implies that the central region of the intracluster medium is equivalent to a virialized interstellar medium in M 87.

Radiative processes in the intracluster plasma

We present useful analytic fitting formulae for the study of the radiative processes which take place in the hot intracluster plasma (the plasma which exists in the clusters of galaxies). The first is for the frequency-integrated emissivity of the relativistic thermal bremsstrahlung. The Gaunt factor for the relativistic thermal bremsstrahlung as a function of the ionic charge Z j , the electron temperature T e , and the photon frequency ω has been recently calculated by us and its analytic fitting formula has been presented. In this paper we will integrate this Gaunt factor over the photon frequency ω and express the results by accurate analytic fitting formulae. These results will be useful when one wishes to evaluate the total amount of energy emitted by the hot intracluster plasma as well as other hot plasmas that exist in supernova remnants. The present results for the frequency-integrated emissivity of the thermal bremsstrahlung generally have accuracy of the order of 0.1%, thus making the present results the most accurate to date that calculate the thermal bremsstrahlung due to electron-ion scattering. The present accurate results will be especially useful for the analysis of the precision data taken by the Chandra X-Ray Observatory and XMM-Newton . The second analytic fitting formula that we will present in this paper is for the thermal Sunyaev-Zeldovich effect for clusters of galaxies. The thermal Sunyaev-Zeldovich effect for clusters of galaxies has been recently calculated with high precision by the present authors as well as by other groups. We have, in particular, presented an analytic fitting formula for this effect. In this paper we will present an analytic fitting formula which has still higher accuracy. The present fitting formula will be particularly suited for the forthcoming measurements of the kinematical Sunyaev-Zeldovich effect such as the BOLOCAM project that will be carried out in the crossover frequency region where the thermal Sunyaev-Zeldovich signal changes from negative to positive sign.

Dust in Clusters of Galaxies and the Sunyaev Zel'dovich Effect

We review here the unique opportunity provided by the sub-millimeter/millimeter domain to characterize both the warm intra-cluster plasma via the Sunyaev Zel'dovich Effect (SZE) on the cosmic microwave background, and the dust within this plasma via its thermal emission. However, multi-wavelength measurements are needed in order to separate these effects from the foreground galactic dust and the background infrared galaxies.

-model and cooling flows in X-ray clusters of galaxies

The spatial emission from the core of cooling-flow clusters of galaxies is inadequately described by a β-model. Spectrally, the central region of these clusters is well approximated with a two-temperature model, where the inner temperature represents the multiphase status of the core and the outer temperature is a measure of the ambient gas temperature. Following this observational evidence, I extend the use of the β-model to a two-phase gas emission, where the two components coexist within a boundary radius rcool and the ambient gas alone fills the volume shell at a radius above rcool. This simple model still provides an analytic expression for the total surface brightness profile (Note in the first term the different sign with respect to the standard β-model.) Based upon a physically meaningful model for the X-ray emission, this formula can be used (i) to improve significantly the modelling of the surface brightness profile of cooling flow clusters of galaxies when compared to the standard β-model results, (ii) to constrain properly the physical characteristics of the intracluster plasma in the outskirts, like, e.g., the ambient gas temperature.

Chandra constraints on the thermal conduction in the intracluster plasma of A2142

In this Letter, we use the recent Chandra observation of Abell 2142 reported by Markevitch et al. to put constraints on thermal conduction in the intracluster plasma. We show that the observed sharp temperature gradient requires that classical conductivity has to be reduced at least by a factor of between 250 and 2500. The result provides a direct constraint on an important physical process relevant to the gas in the cores of clusters of galaxies. Key words: galaxies: clusters: general ± galaxies: clusters: individual: A2142 ± X-rays: galaxies. 1 I N T R O D U C T I O N The heat stored in the intracluster plasma is conducted down any temperature gradient present in the gas in a way that can be described through the following equations (Spitzer 1956; Sarazin 1988): q ˆ k d kTe† dr ; 1† where q is the heat flux, Te is the electron temperature, and k is the thermal conductivity that can be expressed in terms of the density, ne, the electron mass, me, and the electron mean free path, le, as (Cowie & McKee 1977) k ˆ 1:31nele kTe me 1=2 2† In a fully ionized gas of (mostly) hydrogen, the electron mean free path is a function of the gas temperature, the density and the impact parameter of the Coulomb collisions, L: le ˆ 30:2 Tn lnL 37:9‡ ln T=n1=2† 21

Relativistic Thermal Bremsstrahlung Gaunt Factor for the Intracluster Plasma. II. Analytic Fitting Formulae

We present accurate analytic fitting formulae which summarize the results of the recent calculation by Nozawa, Itoh and Kohyama on the relativistic thermal bremsstrahlung Gaunt factor for the intracluster plasma. The fitting is carried out for Z = 1 to 28, 6.0 < log T < 8.5, -4.0 < log (omega/kT) < 1.0, where Z is the charge number of the ion, T is the temperature, and omega is the angular frequency of the emitted photon. The present analytic fitting formulae will be useful for the analysis of the X-ray emission which comes from the intracluster plasma as well as the other X-ray sources.

Diffuse Baryons in Groups and Clusters of Galaxies

To predict the X-ray observables associated to the diffuse baryons in clusters of galaxies, we develop a new physical approach to model such a hot intra-cluster plasma. Such approach is based on punctuated equilibria. and comprises the following blocks: Monte Carlo ``merging histories'' of dark matter potential wells; the central hydrostatic disposition for the ICP, reset to a new equilibrium after each merging episode; conditions of shock, or of closely adiabatic compression at the boundary with the external gas, preheated by stellar energy feedbacks. We predict the L-T relation, consistent with the data as for shape and scatter. This we combine with the mass distribution provided by the hierarchical clustering for different COBE-normalized CDM models, to predict the z-resolved luminosity functions, the source counts, the redshift distributions and contribution of the unresolved groups and clusters to the soft X-ray background. When compared with the recent ROSAT surveys, our results confirm that the critical cosmology with Standard CDM is ruled out by its overproduction of local clusters. On account of underproduction, instead, we rule out open cosmologies except for a narrow range around \Omega_o=0.5; even there, we find the consistency with the full data base to be hardly marginal. For \Omega_o=0.3 in flat geometry, we obtain acceptable fits. For the tilted CDM perturbation spectrum with high baryonic content in the critical universe, we obtain marginal consistency. Finally, we discuss the effective limitations of X-ray clusters and groups as cosmological signposts, and their brighter prospects toward the astrophysics of the ICP and the cosmogony of large, high-contrast structures.

The Sunyaev‐Zeldovich Infrared Experiment: A Millimeter‐Wave Receiver for Cluster Cosmology

Measurements of the Sunyaev-Zeldovich (S-Z) effect toward distant clusters of galaxies can be used to determine the Hubble constant and the radial component of cluster peculiar velocities. Determination of the cluster peculiar velocity requires the separation of the two components of the S-Z effect, which are due to the thermal and bulk velocities of the intracluster plasma. The two components can be separated practically only at millimeter wavelengths. Measurements of the S-Z effect at millimeter wavelengths are subject to minimal astrophysical confusion and, therefore, provide an important test of results obtained at longer wavelengths. We describe the instrument used to make the first significant detections of the S-Z effect at millimeter wavelengths. This instrument employs new filter, detector, and readout technologies to produce sensitive measurements of differential sky brightness stable on long timescales. These advances allow drift-scan observations that achieve high sensitivity while minimizing common sources of systematic error.

The X-ray Intracluster Medium

Clusters of galaxies are the largest aggregates of matter that have decoupled from the universal expansion and have approximately evolved to a proper dynamical equilibrium configuration. While in the optical they are just observed as dense concentrations of galaxies, they are seen in X-rays as continuously connected entities through the emission of the X-ray luminous intracluster plasma (e.g. Sarazin 1986). Relativistic particles and magnetic fields are observed in clusters in radio halos, in regions around radio galaxies and through Faraday rotation of the radiation from background radio sources (see contribution by L. Feretti and e.g. Kronberg, 1994). Thus Clusters of galaxies are the largest, well characterized astrophysical laboratories in which plasma physical processes of diffuse media can be studied. In this brief summary the bulk properties of the intracluster medium (ICM) are discussed and two studies of the interaction of galactic radio lobes with the intracluster plasma are presented (related topics are discussed by F. Owen and L. Feretti in this volume).

Clusters of Galaxies

Clusters of galaxies, as the largest, well-defined astronomical objects, are interesting giant astrophysical laboratories and important probes for the large scale structure of the Universe. In X-rays they can be observed and studied as whole entities. In particular, recent X-ray observations with the ROSAT Observatory have advanced our understanding of the dynamical structure and the mass distribution in clusters. The Sunyaev-Zel’dovich effect makes use of the intracluster plasma to determine cosmological distances independent of the traditional cosmological distance ladder.