Hydrodynamic Simulations of Merging Galaxy Clusters: Non-Equilibrium Ionization State and Two-Temperature Structure

We investigate a non-equilibrium ionization state and an electron-ion two-temperature structure of the intracluster medium (ICM) in merging galaxy clusters using a series of N-body and hydrodynamic simulations. Mergers with various sets of mass ratios and impact parameters are systematically investigated, and it is found that, in most cases, ICM significantly departs from the ionization equilibrium state at the shock layers with a Mach number of ~1.5-2.0 in the outskirts of the clusters, and the shock layers with a Mach number of ~2-4 in front of the ICM cores. Accordingly, the intensity ratio between Fe xxv and Fe xxvi K alpha line emissions is significantly altered from that in the ionization equilibrium state. If the effect of the two-temperature structure of ICM is incorporated, the electron temperature is ~10-20 % and ~30-50 % lower than the mean temperature of ICM at the shock layers in the outskirts and in front of the ICM cores, respectively, and the deviation from the ionization equilibrium state becomes larger. We also address the dependence of the intensity ratio on the viewing angle with respect to the merging plane.Comment: 11 pages, 10 figures. Submitted to PASJ; Accepted for publication in PAS

Direct Integration of the Collisionless Boltzmann Equation in Six-dimensional Phase Space: Self-gravitating Systems

We present a scheme for numerical simulations of collisionless self-gravitating systems which directly integrates the Vlasov--Poisson equations in six-dimensional phase space. By the results from a suite of large-scale numerical simulations, we demonstrate that the present scheme can simulate collisionless self-gravitating systems properly. The integration scheme is based on the positive flux conservation method recently developed in plasma physics. We test the accuracy of our code by performing several test calculations including the stability of King spheres, the gravitational instability and the Landau damping. We show that the mass and the energy are accurately conserved for all the test cases we study. The results are in good agreement with linear theory predictions and/or analytic solutions. The distribution function keeps the property of positivity and remains non-oscillatory. The largest simulations are run on 64^6 grids. The computation speed scales well with the number of processors, and thus our code performs efficiently on massively parallel supercomputers.Comment: 35 pages, 19 figures. Submitted to the Astrophysical Journa

Spatial and Dynamical Biases in Velocity Statistics of Galaxies

We present velocity statistics of galaxies and their biases inferred from the statistics of the underlying dark matter using a cosmological hydrodynamic simulation of galaxy formation in low-density and spatially flat cold dark matter cosmogony. We find that the pairwise velocity dispersion (PVD) of all galaxies is significantly lower than that of the dark matter particles, and that the PVD of the young galaxies is lower than that of the old types, and even of all galaxies together, especially at small separations. These results are in reasonable agreement with the recent measurements of PVDs in the Las Campanas redshift survey, the PSCz catalogue and the SDSS data. We also find that the low PVD of young galaxies is due to the effects of dynamical friction as well as the different spatial distribution. We also consider the mean infall velocity and the POTENT density reconstruction that are often used to measure the cosmological parameters, and investigate the effects of spatial bias and dynamical friction. In our simulation, the mean infall velocity of young galaxies is significantly lower than that of all the galaxies or of the old galaxies, and the dynamical bias becomes important on scales less than 3Mpc/h. The mass density field reconstructed from the velocity field of young galaxies using the POTENT-style method suffers in accuracy both from the spatial bias and the dynamical friction on the smoothing scale of R_s=8Mpc/h. On the other hand, in the case of R_s=12Mpc/h, which is typically adopted in the actual POTENT analysis, the density reconstruction based on various tracers of galaxies is reasonably accurate.Comment: 29 pages, 11 figures, accepted for publication in the Ap

Non-Equilibrium Ionization State and Two-Temperature Structure in the Bullet Cluster 1E0657-56

We investigate a non-equilibrium ionization state and an electron-ion two-temperature structure of the intracluster medium in the merging galaxy cluster, 1E0657-56 (the Bullet cluster), using a series of N-body and hydrodynamic simulations. We find that the electron temperature at the shock layer associated with the X-ray sub peak (bullet) is quite different depending on the thermal relaxation model between electrons and ions; ~25 keV for the Coulomb thermal relaxation model and ~45 keV for the instantaneous thermal relaxation model in the simulations which reproduce the observed X-ray morphology. Furthermore, both of Fe xxv and Fe xxvi are overabundant compared with the ionization equilibrium state around the shock layer, and thus, the intensity ratio between Fe xxv and Fe xxvi K alpha lines are significantly altered from that in the ionization equilibrium state. We also carry out the simulations with various sets of merger parameters, and discuss a possible range of the non-equilibrium effects in this system. Our results could be tested with future X-ray observations such as Astro-H with better sensitivity in high energy band.Comment: 11 pages, 9 figures. To appear in PAS

Non-Equilibrium Ionization State and Two-Temperature Structure in the Linked Region of Abell 399/401

We investigate a non-equilibrium ionization state and two-temperature structure of the intracluster medium in the linked region of Abell 399/401, using a series of N-body + SPH simulations, and find that there exist significant shock layers at the edge of the linked region, and that the ionization state of iron departs from the ionization equilibrium state at the shock layers and around the center of the linked region. As for the two-temperature structure, an obvious difference of temperature between electrons and ions is found in the edge of the linked regions. K alpha line emissions of Fe xxiv and Fe xxv are not severely affected by the deviation from the ionization equilibrium state around the center of the linked region, suggesting that the detection of relatively high metallicity in this area cannot be ascribed to the non-equilibrium ionization state of the intracluster medium. On the other hand, the K alpha emissions are significantly deviated from the equilibrium values at the shock layers, and the intensity ratio of K alpha lines between Fe xxiv-xxv and Fe xxvi is found to be significantly altered from that in the ionization equilibrium state.Comment: 4 pages, 2 figures, submitted to PASJ Letter; accepted June

Non-equilibrium Ionization State of Warm-Hot Intergalactic Medium

Time evolution of the ionization state of metals in the cosmic baryons is investigated in a cosmological context without the assumption of ionization equilibrium. We find that a significant fraction of ionized oxygen ions (OVII and OVIII) in the warm-hot intergalactic medium (WHIM) is not in the ionization equilibrium state at a redshift of z~0. We also investigate the effect on the detectability and observables of WHIM as a consequence of such deviation from ionization equilibrium. It is found that the detectability of WHIM is not altered very much both through its emission and absorption signatures, but line ratios between OVII and OVIII are significantly different from those in the ionization equilibrium state.Comment: 25 pages, 18 figures, accepted for publication in PAS

Proto-clusters in the Lambda CDM Universe

We compare the highly clustered populations of very high redshift galaxies with proto-clusters identified numerically in a standard $\Lambda$CDM universe ($\Omega_0=0.3, \lambda_0=0.7$) simulation. We evolve 256^3 dark matter particles in a comoving box of side 150h^{-1}Mpc. By the present day there are 63 cluster sized objects of mass in excess of 10^{14}h^{-1}Mo in this box. We trace these clusters back to higher redshift finding that their progenitors at z=4--5 are extended regions of typically 20--40 Mpc (comoving) in size, with dark halos of mass in excess of 10^{12}h^{-1}Mo and are overdense by typically 1.3--13 times the cosmological mean density. Comparison with the observation of Lyman alpha emitting (LAEs) galaxies at z=4.86 and at z=4.1 indicates that the observed excess clustering is consistent with that expected for a proto-cluster region if LAEs typically correspond to massive dark halos of more than 10^{12}h^{-1}Mo. We give a brief discussion on the relation between high redshift concentration of massive dark halos and present day rich clusters of galaxies.Comment: 4 pages, 5 figures, Accepted for publication in ApJ Letter