On the X-Ray and Mass Distribution in the Merging Galaxy Cluster 1E 0657-56: Ram Pressure-Stripping in Substructures with an NFW Density Profile

We investigate the X-ray and mass distribution in the merging galaxy cluster 1E 0657-56. We study head-on collisions of two virialized clusters with an NFW density profile in the $\Lambda$CDM universe using an N-body + hydrodynamical code. A clear off-set of an X-ray peak from a mass peak, which is like what is reported in Clow et al. (2004), is first reproduced in the N-body + hydrodynamical simulations. We estimate the ram pressure-stripping conditions of the substructure in mergers of two NFW dark halos using a simple analytical model. We find that the ram pressure dominates the gravity of the substructure when the smaller cluster's mass is less than approximately one tenth of the larger cluster's mass. The characteristic X-ray and mass structures found in 1E 0657-56 suggest that neither the ram pressure nor the gravitational bound force overwhelms the other and that the mass ratio between the progenitors is near the critical value mentioned above.Comment: 5 pages, 3 figures, Accepted for publication in PAS

Evolution of Hard X-Ray Radiation from Clusters of Galaxies: Bremsstrahlung or Inverse Compton Scattering?

We have calculated evolution of a non-thermal electron population from super-thermal but weakly relativistic to highly relativistic energy range in clusters of galaxies. We investigate evolution of hard X-ray radiation due to both bremsstrahlung and inverse Compton scattering of the cosmic microwave background photons. The bremsstrahlung component is more significant than the inverse Compton scattering one when the momentum spectra of electron sources are steeper than $\sim P_e^{-3.0}$ and vice versa in the case of Coma, where $P_e$ is an electron momentum. The resultant hard X-ray spectra are flatter when the bremsstrahlung component is dominant. When the spectral indices of the source term are in the intermediate range ($-2.5 \sim -3.5$), too much extreme ultraviolet emission is produced. Inverse Compton dominant models can reproduce Coma cluster results with reasonable injection rates, which are possible in cluster mergers and/or ambient gas accretion.Comment: 11 pages with 6 figures, Accepted for publication in PAS

Suzaku Observation of the Radio Halo Cluster Abell 2319: Gas Dynamics and Hard X-ray Properties

We present the results of Suzaku observation of the radio halo cluster Abell 2319. The metal abundance in the central cool region is found to be higher than the surrounding region, which was not resolved in the former studies. We confirm that the line-of-sight velocities of the intracluster medium in the observed region are consistent with those of the member galaxies of entire A2319 and A2319A subgroup for the first time, though any velocity difference within the region is not detected. On the other hand, we do not find any signs of gas motion relevant to A2319B subgroup. Hard X-ray emission from the cluster is clearly detected, but its spectrum is likely thermal. Assuming a simple single temperature model for the thermal component, we find that the upper limit of the non-thermal inverse Compton component becomes $2.6 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$ in the 10-40 keV band, which means that the lower limit of the magnetic field is 0.19 $\mu$G with the radio spectral index 0.92. Although the results slightly depend on the detailed spectral modeling, it is robust that the upper limit of the power-law component flux and lower limit of the magnetic field strength become $\sim 3 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$ and $\sim 0.2 \mu$G, respectively. Considering the lack of a significant amount of very hot ($\sim 20$ keV) gas and the strong bulk flow motion, it is more likely that the relativistic non-thermal electrons responsible for the radio halo are accelerated through the intracluster turbulence rather than the shocks.Comment: 12 pages, 15 figures, accepted for publication in PAS

Mass Estimation of Merging Galaxy Clusters

We investigate the impact of mergers on the mass estimation of galaxy clusters using $N$-body + hydrodynamical simulation data. We estimate virial mass from these data and compare it with real mass. When the smaller subcluster's mass is larger than a quarter of that of the larger one, virial mass can be larger than twice of the real mass. The results strongly depend on the observational directions, because of anisotropic velocity distribution of the member galaxies. We also make the X-ray surface brightness and spectroscopic-like temperature maps from the simulation data. The mass profile is estimated from these data on the assumption of hydrostatic equilibrium. In general, mass estimation with X-ray data gives us better results than virial mass estimation. The dependence upon observational directions is weaker than in case of virial mass estimation. When the system is observed along the collision axis, the projected mass tends to be underestimated. This fact should be noted especially when the virial and/or X-ray mass are compared with gravitational lensing results.Comment: 21 pages, 13 figures, accepted for publication in PAS

Time evolution of galactic warps in prolate haloes

A recent observation with the Hipparcos satellite and some numerical simulations imply that the interaction between an oblate halo and a disc is inappropriate for the persistence of galactic warps. Then, we have compared the time evolution of galactic warps in a prolate halo with that in an oblate halo. The haloes were approximated as fixed potentials, while the discs were represented by N-body particles. We have found that the warping in the oblate halo continues to wind up, and finally disappears. On the other hand, for the prolate halo model, the precession rate of the outer disc increases when the precession of the outer disc recedes from that of the inner disc, and vice versa. Consequently, the warping in the prolate halo persisted to the end of the simulation by retaining the alignment of the line of nodes of the warped disc. Therefore, our results suggest that prolate haloes could sustain galactic warps. The physical mechanism of the persistence of warp is discussed on the basis of the torque between a halo and a disc and that between the inner and outer regions of the disc.Comment: 8 pages, 8 figures. Accepted for publication in MNRA

N-body + Magnetohydrodynamical Simulations of Merging Clusters of Galaxies: Characteristic Magnetic Field Structures Generated by Bulk Flow Motion

We present results from N-body + magnetohydrodynamical simulations of merging clusters of galaxies. We find that cluster mergers cause various characteristic magnetic field structures because of the strong bulk flows in the intracluster medium. The moving substructures result in cool regions surrounded by the magnetic field. These will be recognized as magnetized cold fronts in the observational point of view. A relatively ordered magnetic field structure is generated just behind the moving substructure. Eddy-like field configurations are also formed by Kelvin-Helmholtz instabilities. These features are similarly seen even in off-center mergers though the detailed structures change slightly. The above-mentioned characteristic magnetic field structures are partly recognized in Faraday rotation measure maps. The higher absolute values of the rotation measure are expected when observed along the collision axis, because of the elongated density distribution and relatively ordered field structure along the axis. The rotation measure maps on the cosmic microwave background radiation, which covers clusters entirely, could be useful probes of not only the magnetic field structures but also the internal dynamics of the intracluster medium.Comment: Accepted for publication in Ap

X-Ray View of the Shock Front in the Merging Cluster Abell 3376 with Suzaku

We report on a Suzaku measurement of the shock feature associated with the western radio relic in the merging cluster A3376. The temperature profile is characterized by an almost flat radial shape with kT ~ 4 keV within 0.5 r200 and a rise by about 1 keV inside the radio relic. Across the relic region (0.6-0.8 r200), the temperature shows a remarkable drop from about 4.7 keV to 1.3 keV. This is a clear evidence that the radio relic really corresponds to a shock front possibly caused by a past major merger. The observed sharp changes of the temperature and electron density indicate the Mach number M~3. The radial entropy profile is flatter than the prediction (r^1.1) of numerical simulations within 0.5 r200}, and becomes steeper around the relic region. These observed features and time-scale estimation consistently imply that the ICM around the radio relic has experienced a merger shock and is in the middle of the process of dynamical and thermal relaxation.Comment: Accepted for publication in PASJ (12 pages, 6 figures