Turbulent Mixing in Clusters of Galaxies

We present a spherically-symmetric, steady-state model of galaxy clusters in which radiative cooling from the hot gas is balanced by heat transport through turbulent mixing. We assume that the gas is in hydrostatic equilibrium, and describe the turbulent heat diffusion by means of a mixing length prescription with a dimensionless parameter alpha_mix. Models with alpha_mix ~ 0.01-0.03 yield reasonably good fits to the observed density and temperature profiles of cooling core clusters. Making the strong simplification that alpha_mix is time-independent and that it is roughly the same in all clusters, the model reproduces remarkably well the observed scalings of X-ray luminosity, gas mass fraction and entropy with temperature. The break in the scaling relations at kT \~ 1-2 keV is explained by the break in the cooling function at around this temperature, and the entropy floor observed in galaxy groups is reproduced naturally.Comment: Accepted for publication in ApJ

RXTE and ASCA Constraints on Non-thermal Emission from the A2256 Galaxy Cluster

An 8.3 hour observation of the Abell 2256 galaxy cluster using the Rossi X-ray Timing Explorer proportional counter array produced a high quality spectrum in the 2 - 30 keV range. Joint fitting with the 0.7 - 11 keV spectrum obtained with the Advanced Satellite for Astrophysics and Cosmology gas imaging spectrometer gives an upperlimit of 2.3x10^-7 photons/cm^2/sec/keV for non-thermal emission at 30 keV. This yields a lower limit to the mean magnetic field of 0.36 micro Gauss (uG) and an upperlimit of 1.8x10^-13 ergs/cm^3 for the cosmic-ray electron energy density. The resulting lower limit to the central magnetic field is ~1 - 3 uG While a magnetic field of ~0.1 - 0.2 uG can be created by galaxy wakes, a magnetic field of several uG is usually associated with a cooling flow or, as in the case of the Coma cluster, a subcluster merger. However, for A2256, the evidence for a merger is weak and the main cluster shows no evidence of a cooling flow. Thus, there is presently no satisfactory hypothesis for the origin of an average cluster magnetic field as high as >0.36 uG in the A2256 cluster.Comment: 8 pages, Astrophysical Journal (in press

Off-Center Mergers of Clusters of Galaxies and Nonequipartition of Electrons and Ions in Intracluster Medium

We investigate the dynamical evolution of clusters of galaxies and their observational consequences during off-center mergers, explicitly considering the relaxation process between ions and electrons in intracluster medium by N-body and hydrodynamical simulations. In the contracting phase a bow shock is formed between the two subclusters. The observed temperature between two peaks in this phase depends on the viewing angle even if the geometry of the system seems to be very simple like head-on collisions. Around the most contracting epoch, when we observe merging clusters nearly along the collision axis, they look like spherical relaxed clusters with large temperature gradients. In the expanding phase, spiral bow shocks occur. As in head-on mergers, the electron temperature is significantly lower than the plasma mean one especially in the post-shock regions in the expanding phase. When the systems have relatively large angular momentum, double-peak structures in the X-ray images can survive even after the most contracting epoch. Morphological features in both X-ray images and electron temperature distribution characteristic to off-center mergers are seriously affected by the viewing angle. When the clusters are observed nearly along the collision axis, the distribution of galaxies' line-of-sight (LOS) velocities is a good indicator of mergers. In the contracting phase, an negative kurtosis and a large skewness are expected for nearly equal mass collisions and rather different mass ones, respectively. To obtain statistically significant results, about 1000 galaxies' LOS velocities are required. For nearby clusters ($z<0.05$), large redshift surveys such as 2dF will enable us to study merger dynamics.Comment: 21 pages, 7 figures. Accepted for publication in Ap

Nonthermal Bremsstrahlung and Hard X-ray Emission from Clusters of Galaxies

We have calculated nonthermal bremsstrahlung (NTB) models for the hard X-ray (HXR) tails recently observed by BeppoSAX in clusters of galaxies. In these models, the HXR emission is due to suprathermal electrons with energies of about 10-200 keV. Under the assumption that the suprathermal electrons form part of a continuous spectrum of electrons including highly relativistic particles, we have calculated the inverse Compton (IC) extreme ultraviolet (EUV), HXR, and radio synchrotron emission by the extensions of the same populations. For accelerating electron models with power-law momentum spectra (N[p] propto p^{- mu}) with mu <~ 2.7, which are those expected from strong shock acceleration, the IC HXR emission exceeds that due to NTB. Thus, these models are only of interest if the electron population is cut-off at some upper energy <~1 GeV. Similarly, flat spectrum accelerating electron models produce more radio synchrotron emission than is observed from clusters if the ICM magnetic field is B >~ 1 muG. The cooling electron model produces vastly too much EUV emission as compared to the observations of clusters. We have compared these NTB models to the observed HXR tails in Coma and Abell 2199. The NTB models require a nonthermal electron population which contains about 3% of the number of electrons in the thermal ICM. If the suprathermal electron population is cut-off at some energy above 100 keV, then the models can easily fit the observed HXR fluxes and spectral indices in both clusters. For accelerating electron models without a cutoff, the electron spectrum must be rather steep >~ 2.9.Comment: Accepted for publication in the Astrophysical Journal. 10 pages with 5 embedded Postscript figures in emulateapj.sty. An abbreviated abstract follow

Merger shocks in galaxy clusters A665 and A2163 and their relation to radio halos

We present Chandra gas temperature maps for two hot, intermediate-redshift clusters A665 and A2163. Both show strong temperature variations in their central r=1 Mpc regions, naturally interpreted as product of the subcluster mergers. The A665 map reveals a shock in front of the cool core, while the temperature structure of A2163 is more complicated. On a larger linear scale, our data on A2163 indicate a radial temperature decline in agreement with earlier ASCA results, although the uncertainties are large. Both these clusters exhibit previously known synchrotron radio halos. Comparison of the radio images and the gas temperature maps indicates that radio emission predominantly comes from the hot gas regions, providing a strong argument in favor of the hypothesis that relativistic electrons are accelerated in merger shocks.Comment: Updated radio image for A2163, expanded introduction. ApJ in press. 8 pages, uses emulateapj.sty. Color version is at http://hea-www.harvard.edu/~maxim/papers/665.ps.gz (PS) or http://hea-www.harvard.edu/~maxim/papers/665.pdf (PDF

Magnetic Field Evolution in Merging Clusters of Galaxies

We present initial results from the first 3-dimensional numerical magnetohydrodynamical (MHD) simulations of magnetic field evolution in merging clusters of galaxies. Within the framework of idealized initial conditions similar to our previous work, we look at the gasdynamics and the magnetic field evolution during a major merger event in order to examine the suggestion that shocks and turbulence generated during a cluster/subcluster merger can produce magnetic field amplification and relativistic particle acceleration and, as such, may play a role in the formation and evolution of cluster-wide radio halos. The ICM, as represented by the equations of ideal MHD, is evolved self-consistently within a changing gravitational potential defined largely by the collisionless dark matter component represented by an N-body particle distribution. The MHD equations are solved by the Eulerian, finite-difference code, ZEUS. The particles are evolved by a standard particle-mesh (PM) code. We find significant evolution of the magnetic field structure and strength during two distinct epochs of the merger evolution.Comment: 21 pages, 7 figures, Figure 2 is color postscript. Accepted for publication in Ap

A Powerful Radio Halo in the Hottest Known Cluster of Galaxies 1E0657-56

We report the detection of a diffuse radio halo source in the hottest known cluster of galaxies 1E0657-56 (RXJ0658-5557). The radio halo has a morphology similar to the X-ray emission from the hot intracluster medium. The presence of a luminous radio halo in such a hot cluster is further evidence for a steep correlation between the radio halo power and the X-ray temperature. We favour models for the origin of radio halo sources involving a direct connection between the X-ray emitting thermal particles and the radio emitting relativistic particles.Comment: 21 pages of text, 9 figures, to appear in Ap

Cosmic Ray Electrons in Groups and Clusters of Galaxies: Primary and Secondary Populations from a Numerical Cosmological Simulation

We study the generation and distribution of high energy electrons in cosmic environment and their observational consequences by carrying out the first cosmological simulation that includes directly cosmic ray (CR) particles. Starting from cosmological initial conditions we follow the evolution of primary and secondary electrons (CRE), CR ions (CRI) and a passive magnetic field. CRIs and primary CREs are injected and accelerated at large scale structure shocks. Secondary CREs are continuously generated through inelastic p-p collisions. We include spatial transport, adiabatic expansion/compression, Coulomb collisions, bremsstrahlung, synchrotron (SE)and inverse Compton (IC) emission. We find that, from the perspective of cosmic shock energy and acceleration efficiency, the few detections of hard X-ray radiation excess could be explained in the framework of IC emission of primary CREs in clusters undergoing high accretion/merger phase. Instead, IC emission from both primary and secondary CREs accounts at most for a small fraction of the radiation excesses detected in the extreme-UV (except for the Coma cluster as reported by Bowyer et al.1999). Next, we calculate the SE after normalizing the magnetic field so that for a Coma-like cluster ^1/2~3 \muG. Our results indicate that the SE from secondary CREs reproduces several general properties of radio halos, including the recently found P_1.4GHz vs T relation, the morphology and polarization of the emitting region and, to some extent, the spectral index. Moreover, SE from primary CREs turns out sufficient to power extended regions resembling radio relics observed at the outskirts of clusters. Again we find striking resemblance between morphology, polarization and spectral index of our synthetic maps and those reported in the literature.Comment: emulateapj, 27 pages, 10 figures, 5 tables; ApJ in pres