Inverse Compton Scattering as the Source of Diffuse EUV Emission in the Coma Cluster of Galaxies

We have examined the hypothesis that the majority of the diffuse EUV flux in the Coma cluster is due to inverse Compton scattering of low energy cosmic ray electrons (0.16 < epsilon < 0.31 GeV) against the 3K black-body background. We present data on the two-dimensional spatial distribution of the EUV flux and show that these data provide strong support for a non-thermal origin for the EUV flux. However, we show that this emission cannot be produced by an extrapolation to lower energies of the observed synchrotron radio emitting electrons and an additional component of low energy cosmic ray electrons is required.Comment: 11 pages, 5 figure

Heating and Turbulence Driving by Galaxy Motions in Galaxy Clusters

Using three-dimensional hydrodynamic simulations, we investigate heating and turbulence driving in an intracluster medium (ICM) by orbital motions of galaxies in a galaxy cluster. We consider Ng member galaxies on isothermal and isotropic orbits through an ICM typical of rich clusters. An introduction of the galaxies immediately produces gravitational wakes, providing perturbations that can potentially grow via resonant interaction with the background gas. When Ng^{1/2}Mg_11 < 100, where Mg_11 is each galaxy mass in units of 10^{11} Msun, the perturbations are in the linear regime and the resonant excitation of gravity waves is efficient to generate kinetic energy in the ICM, resulting in the velocity dispersion sigma_v ~ 2.2 Ng^{1/2}Mg_11 km/s. When Ng^{1/2}Mg_11 > 100, on the other hand, nonlinear fluctuations of the background ICM destroy galaxy wakes and thus render resonant excitation weak or absent. In this case, the kinetic energy saturates at the level corresponding to sigma_v ~ 220 km/s. The angle-averaged velocity power spectra of turbulence driven in our models have slopes in the range of -3.7 to -4.3. With the nonlinear saturation of resonant excitation, none of the cooling models considered are able to halt cooling catastrophe, suggesting that the galaxy motions alone are unlikely to solve the cooling flow problem.Comment: 12 pages including 3 figures, To appear in ApJ

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

Contamination of Cluster Radio Sources in the Measurement of the Thermal Sunyaev-Zel'dovich Angular Power Spectrum

We present a quantitative estimate of the confusion of cluster radio halos and galaxies in the measurement of the angular power spectrum of the thermal Sunyaev-Zel'dovich (SZ) effect. To achieve the goal, we use a purely analytic approach to both radio sources and dark matter of clusters by incorporating empirical models and observational facts together with some theoretical considerations. It is shown that the correction of cluster radio halos and galaxies to the measurement of the thermal SZ angular power spectrum is no more than 20% at $l>2000$ for observing frequencies $\nu>30$ GHz. This eliminates the concern that the SZ measurement may be seriously contaminated by the existence of cluster radio sources.Comment: 15 pages, 3 figures, accepted for publication in Ap

Implications of a Nonthermal Origin of the Excess EUV Emission from the Coma Cluster of Galaxies

The inverse Compton (IC) interpretation of the excess EUV emission, that was recently reported from several clusters of galaxies, suggests that the amount of relativistic electrons in the intracluster medium is highly significant, W_e>10^{61} erg. Considering Coma as the prototype galaxy cluster of nonthermal radiation, we discuss implications of the inverse Compton origin of the excess EUV fluxes in the case of low intracluster magnetic fields of order 0.1 muG, as required for the IC interpretation of the observed excess hard X-ray flux, and in the case of high fields of order 1 muG as suggested by Faraday rotation measurements. Although for such high intracluster fields the excess hard X-rays will require an explanation other than by the IC effect, we show that the excess EUV flux can be explained by the IC emission of a `relic' population of electrons driven into the incipient intracluster medium at the epoch of starburst activity by galactic winds, and later on reenergized by adiabatic compression and/or large-scale shocks transmitted through the cluster as the consequence of more recent merger events. For high magnetic fields B > 1 muG the interpretation of the radio fluxes of Coma requires a second population of electrons injected recently. They can be explained as secondaries produced by a population of relativistic protons. We calculate the fluxes of gamma-rays to be expected in both the low and high magnetic field scenarios, and discuss possibilities to distinguish between these two principal options by future gamma-ray observations.Comment: LaTeX, 6 figures; accepted for publication in Ap

Non-Thermal Emission from Relativistic Electrons in Clusters of Galaxies: A Merger Shock Acceleration Model

We have investigated evolution of non-thermal emission from relativistic electrons accelerated at around the shock fronts during merger of clusters of galaxies. We estimate synchrotron radio emission and inverse Compton scattering of cosmic microwave background photons from extreme ultraviolet (EUV) to hard X-ray range. The hard X-ray emission is most luminous in the later stage of merger. Both hard X-ray and radio emissions are luminous only while signatures of merging events are clearly seen in thermal intracluster medium (ICM). On the other hand, EUV radiation is still luminous after the system has relaxed. Propagation of shock waves and bulk-flow motion of ICM play crucial roles to extend radio halos. In the contracting phase, radio halos are located at the hot region of ICM, or between two substructures. In the expanding phase, on the other hand, radio halos are located between two ICM hot regions and shows rather diffuse distribution.Comment: 19 pages, 5 figures, accepted for publication in Ap

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

On The Origin of Radio Halos in Galaxy Clusters

Previously it has been recognized that radio halos in galaxy clusters are preferentially associated with merging systems as indicated by substructure in the X-ray images and temperature maps. Since, however, many clusters without radio halos also possess substructure, the role of mergers in the formation of radio halos has remained unclear. By using power ratios to relate gravitational potential fluctuations to substructure in X-ray images, we provide the first quantitative comparison of the dynamical states of clusters possessing radio halos. A correlation between the 1.4 GHz power (P_{1.4}) of the radio halo (or relic) and the magnitude of the dipole power ratio (P_1/P_0) is discovered such that approximately P_{1.4} ~ P_1/P_0; i.e., the strongest radio halos appear only in those clusters currently experiencing the largest departures from a virialized state. From additional consideration of a small number of highly disturbed clusters without radio halos detected at 1.4 GHz, and recalling that radio halos are more common in clusters with high X-ray luminosity (Giovannini, Tordi, & Feretti), we argue that radio halos form preferentially in massive (L_x >~ 0.5 x 10^{45} erg/s) clusters experiencing violent mergers (P_1/P_0 >~ 0.5 x 10^{-4}) that have seriously disrupted the cluster core. The association of radio halos with massive, large-P_1/P_0, core-disrupted clusters is able to account for both the vital role of mergers in accelerating the relativistic particles responsible for the radio emission as well as the rare occurrence of radio halos in cluster samples.Comment: 4 pages, 1 figure, Accepted for Publication in The Astrophysical Journal Letters, updated reference

Hard X-ray emission from the galaxy cluster A3667

We report the results of a long BeppoSAX observation of Abell 3667, one of the most spectacular galaxy cluster in the southern sky. A clear detection of hard X-ray radiation up to ~ 35 keV is reported, while a hard excess above the thermal gas emission is present at a marginal level that should be considered as an upper limit to the presence of nonthermal radiation. The strong hard excesses reported by BeppoSAX in Coma and A2256 and the only marginal detection of nonthermal emission in A3667 can be explained in the framework of the inverse Compton model. We argue that the nonthermal X-ray detections in the PDS energy range are related to the radio index structure of halos and relics present in the observed clusters of galaxie.Comment: 15 pages, 1 figure, ApJL in pres