arXiv subjects

Motokazu Takizawa

Publications and source records attributed to Motokazu Takizawa.

At least 37 records · Page 2Linked to original sources

arXiv2012

Suzaku observations of the Hydra A cluster out to the virial radius

We report Suzaku observations of the northern half of the Hydra A cluster out to ~1.4 Mpc, reaching the virial radius. This is the first Suzaku observations of a medium-size (kT ~3 keV) cluster out to the virial radius. Two observations were conducted, north-west and north-east offsets, which continue in a filament direction and a void direction of the large-scale structure of the Universe, respectively. The X-ray emission and distribution of galaxies elongate in the filament direction. The temperature profiles in the two directions are mostly consistent with each other within the error bars and drop to 1.5 keV at 1.5 r_500. As observed by Suzaku in hot clusters, the entropy profile becomes flatter beyond r_500, in disagreement with the r^1.1 relationship that is expected from accretion shock heating models. When scaled with the average intracluster medium (ICM) temperature, the entropy profiles of clusters observed with Suzaku are universal and do not depend on system mass. The hydrostatic mass values in the void and filament directions are in good agreement, and the Navarro, Frenk, and White universal mass profile represents the hydrostatic mass distribution up to ~ 2 r_500. Beyond r_500, the ratio of gas mass to hydrostatic mass exceeds the result of the Wilkinson microwave anisotropy probe, and at r_100, these ratios in the filament and void directions reach 0.4 and 0.3, respectively. We discuss possible deviations from hydrostatic equilibrium at cluster outskirts. We derived radial profiles of the gasmass- to-light ratio and iron-mass-to-light ratio out to the virial radius. Within r_500, the iron-mass-to-light ratio of the Hydra A cluster was compared with those in other clusters observed with Suzaku.

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arXiv2011

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.

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arXiv2011

Suzaku X-ray Follow-up Observation of Weak-lensing-detected Halos in the Field around ZwCl0823.2+0425

We present the results of Suzaku X-ray follow-up observation of weak-lensing-detected halos in the field around galaxy cluster ZwCl0823.2+0425. We clearly detected X-ray emission associated with most of these halos and determined their detailed physical parameters such as X-ray luminosity, temperature, and metal abundance, for the first time. We find that the X-ray luminosity - temperature relation for these halos agrees with former typical results. With mass determined from the weak gravitational lensing data, the mass-temperature relation for them is also investigated and found to be consistent with the prediction from a simple self-similar model and results of the previous studies with both lensing and X-ray data. We would like to emphasize that the self-similar scaling relation of mass and temperature is shown here for the first time using a weak-lensing selected sample, whereas previous studies of the mass scaling relation used X-ray-selected samples of clusters. Therefore, our study demonstrates importance of X-ray follow-up observations of shear-selected clusters, and shows that a joint X-ray and lensing analysis will be crucial for clusters discovered by the forthcoming weak-lensing surveys, such as the one planned with Subaru/Hyper-Suprime-Cam.

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arXiv2010

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.

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arXiv2010

Suzaku Observation of Abell 1689: Anisotropic Temperature and Entropy Distributions Associated with the Large-Scale Structure

(Abridged) We present results of Suzaku observations of the intracluster medium (ICM) in Abell 1689, combined with complementary analysis of the SDSS data and weak and strong lensing analysis of Subaru/Suprime-Cam and HST/ACS observations. Faint X-ray emission from the ICM around the virial radius is detected at 4.0 sigma significance. We find anisotropic gas temperature and entropy distributions in cluster outskirts correlated with large-scale structure of galaxies. The high temperature and entropy region in the northeastern (NE) outskirts is connected to an overdense filamentary structure. The outskirt regions in contact with low density void environments have low gas temperatures and entropies, deviating from hydrostatic equilibrium. These results suggest that thermalization of the ICM occurs faster along the filamentary structures than the void regions. A joint X-ray and lensing analysis shows that the hydrostatic mass is $\sim60-90%$ of spherical lensing one but comparable to a triaxial halo mass within errors in $0.6r_{2500} \simlt r \simlt 0.8r_{500}$, and that it is significantly biased as low as $\simlt60%$ within $0.4r_{2500}$, irrespective of mass models. The thermal gas pressure within $r_{500}$ is, at most, $\sim50$--60% of the total pressure to balance fully the gravity of the spherical lensing mass, and $\sim30$--40% around the virial radius. Although these constitute lower limits when one considers the possible halo triaxiality, these small relative contributions of thermal pressure would require additional sources of pressure, such as bulk and/or turbulent motions.

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arXiv2009

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 $μ$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 $\sim 0.2 μ$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.

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arXiv2008

Hard X-ray Properties of the Merging Cluster Abell 3667 as Observed with Suzaku

Wide-band Suzaku data on the merging cluster Abell 3667 were examined for hard X-ray emission in excess to the known thermal component. Suzaku detected X-ray signals in the wide energy band from 0.5 to 40 keV. The hard X-ray (> 10 keV) flux observed by the HXD around the cluster center cannot be explained by a simple extension of the thermal emission with average temperature of ~7 keV. The emission is most likely an emission from a very hot (kT > 13.2 keV) thermal component around the cluster center, produced via a strong heating process in the merger. In the north-west radio relic, no signature of non-thermal emission was observed. Using the HXD, the overall upper-limit flux within a 34'x34' field-of-view around the relic is derived to be 5.3e-12 erg s-1 cm-2 in the 10-40 keV band, after subtracting the ICM contribution estimated using the XIS or the XMM-Newton spectra. Directly on the relic region, the upper limit is further tightened by the XIS data to be less than 7.3e-13 erg s-1 cm-2, when converted into the 10--40 keV band. The latter value suggest that the average magnetic field within the relic is higher than 1.6 uG. The non-thermal pressure due to magnetic fields and relativistic electrons may be as large as ~20% of the thermal pressure in the region.

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arXiv2008

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.

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arXiv2008

Suzaku Observation of the Ophiuchus Galaxy Cluster: One of the Hottest Cool Core Clusters

We present the analysis of a Suzaku observation of the Ophiuchus galaxy cluster. We confirmed that the cluster has a cool core. While the temperature of the intracluster medium (ICM) decreases toward the center, the metal abundance increases. Except for the core (r<~50 kpc), the cluster is hot (~9-10 keV) and is almost isothermal for r<~1 Mpc; the latter contradicts a previous study. We do not detect the variation of the redshift of the ICM in the cluster; the upper limit of the velocity difference is 3000 km s^-1. The iron line ratios in X-ray spectra indicate that the ICM has reached the ionization equilibrium state. From these results, we conclude that the Ophiuchus cluster is not a major merger cluster but one of the hottest clusters with a cool core. We obtain the upper limit of non-thermal emission from the cluster, which is consistent with both the recent claimed detection with INTEGRAL and the recent upper limits with the Swift/BAT. If the cluster has bright non-thermal emission as suggested by the INTEGRAL measurement, it is probably not due to a recent major cluster merger.

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arXiv2008

High Metallicity of the X-Ray Gas up to the Virial Radius of a Binary Cluster of Galaxies: Evidence of Galactic Superwinds at High-Redshift

We present an analysis of a Suzaku observation of the link region between the galaxy clusters A399 and A401. We obtained the metallicity of the intracluster medium (ICM) up to the cluster virial radii for the first time. We determine the metallicity where the virial radii of the two clusters cross each other (~2 Mpc away from their centers) and found that it is comparable to that in their inner regions (~0.2 Zsun). It is unlikely that the uniformity of metallicity up to the virial radii is due to mixing caused by a cluster collision. Since the ram-pressure is too small to strip the interstellar medium of galaxies around the virial radius of a cluster, the fairly high metallicity that we found there indicates that the metals in the ICM are not transported from member galaxies by ram-pressure stripping. Instead, the uniformity suggests that the proto-cluster region was extensively polluted with metals by extremely powerful outflows (superwinds) from galaxies before the clusters formed. We also searched for the oxygen emission from the warm--hot intergalactic medium in that region and obtained a strict upper limit of the hydrogen density (nH<4.1x10^-5 cm^-3).

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arXiv2006

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 $Λ$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.

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arXiv2005

Hydrodynamic Simulations of A Moving Substructure in A Cluster of Galaxies: Cold Fronts and Turbulence Generation

We perform three dimensional hydrodynamical simulations of a moving substructure in a cluster of galaxies. We investigate dynamical evolution of the intracluster medium (ICM) in and around the substructure moving radially in the larger cluster's gravitational potential, and its observational consequences. After the substructure passes the larger cluster's center, a bow shock and clear contact discontinuity form in front of it. The contact discontinuity looks like a sharp cold front in the X-ray image synthesized from the simulation results. This agrees with a structure found in 1E 0657-56. The flow structure remains laminar before the first turnaround because the ram-pressure stripping is dominant over the development of Kelvin-Helmholtz instabilities on the boundary between the substructure and the ambient ICM. When a subcluster oscillates radially around the larger cluster's center, both Kelvin-Helmholtz and Rayleigh-Taylor instabilities develop well and the flow structure becomes highly turbulent. Around the turnaround, the subcluster's cold gas is pushed out of its potential well. Therefore, the cold gas component appears to be in front of the subcluster. A relatively blunt cold front appears in the simulated X-ray image because the contact discontinuity is perturbed by Rayleigh-Taylor instabilities. This can explain the ICM structure found in A168.

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arXiv2005

Turbulence Generation by Substructure Motion in Clusters of Galaxies

Clusters of galaxies form through major merger and/or absorption of smaller groups. In fact, some characteristic structures such as cold fronts, which are likely relevant to moving substructures, are found by {\it Chandra}. It is expected that moving substructures generate turbulence in the intracluster medium (ICM). Such turbulence probably plays a crucial role in mixture and transport of gas energy and heavy elements, and particle acceleration. The {\it Astro-E2} satellite, which is planned to be launched in 2005, will detect broadened lines due to turbulent motion. In order to explore the above-mentioned issues, it is important to investigate the generation processes and structure of ICM turbulence. We investigate the ICM dynamical evolution in and around a moving substructure with three-dimensional hydrodynamical simulations. Eddy-like structures develop near the boundary between the substructure and the ambient ICM through Kelvin-Helmholtz instabilities. Because of these structures, characteristic patterns appear in the line-of-sight velocity distribution of the ICM.

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arXiv2003

Chandra Observations of the Central Region of Abell 3112

We present the results of a {\it Chandra} observation of the central region of Abell 3112. This cluster has a powerful radio source in the center and was believed to have a strong cooling flow. The X-ray image shows that the intracluster medium (ICM) is distributed smoothly on large scales, but has significant deviations from a simple concentric elliptical isophotal model near the center. Regions of excess emission appear to surround two lobe-like radio-emitting regions. This structure probably indicates that hot X-ray gas and radio lobes are interacting. From an analysis of the X-ray spectra in annuli, we found clear evidence for a temperature decrease and abundance increase toward the center. The X-ray spectrum of the central region is consistent with a single-temperature thermal plasma model. The contribution of X-ray emission from a multiphase cooling flow component with gas cooling to very low temperatures locally is limited to less than 10% of the total emission. However, the whole cluster spectrum indicates that the ICM is cooling significantly as a whole, but in only a limited temperature range ($\geq 2$ keV). Inside the cooling radius, the conduction timescales based on the Spitzer conductivity are shorter than the cooling timescales. We detect an X-ray point source in the cluster center which is coincident with the optical nucleus of the central cD galaxy and the core of the associated radio source. The X-ray spectrum of the central point source can be fit by a 1.3 keV thermal plasma and a power-law component whose photon index is 1.9. The thermal component is probably plasma associated with the cD galaxy. We attribute the power-law component to the central AGN.

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arXiv2002

Nonthermal Emissions from Particles Accelerated by Turbulence in Clusters of Galaxies

We consider nonthermal emission from clusters of galaxies produced by particle acceleration by resonant scattering of Alfvén waves driven by fluid turbulence through the Lighthill mechanism in the intracluster medium. We assume that the turbulence is driven by cluster mergers. We find that the resonant Alfvén waves can accelerate electrons up to γ~10^5 through resonant scattering. We also find that the turbulent resonant acceleration can give enough energy to electrons to produce the observed diffuse radio relic emission from clusters if the clusters have a pool of electrons with γ~10^3. This mechanism can also explain the observed hard X-ray emission from clusters if the magnetic field in a cluster is small enough (~< μG) or the fluid turbulence spectrum is flatter than the Kolmogorov law. The fluid turbulence could be observed with Astro-E2 in the regions where diffuse radio emission is observed. Although non-gravitational heating before cluster formation (preheating) steepens a relation between radio luminosity and X-ray temperature, our predicted relation is still flatter than the observed one.

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arXiv2002

Radio Halo Formation through Magneto-turbulent Particle Acceleration in Clusters of Galaxies

We developed a magneto-turbulent model for the cosmic ray (CR) electrons seen in the radio halo clusters of galaxies. Steady state momentum distribution functions of the CR electrons are calculated for given spectra of the turbulent Alfven waves. The radio spectrum produced by the obtained CR electron distribution is compared to the observed radio spectrum of the Coma radio halo. We find that the observed radio spectrum of the Coma cluster is well reproduced when the spectral index of the turbulent Alfven waves is 2.8. The obtained energy spectrum of the turbulent Alfven waves is steeper than that expected from the turbulence theory, suggesting back reaction of the particle acceleration. The fitting procedure constraints the amplitude of the turbulent Alfven waves as well as the spectral index. Then we estimate the dissipation of the turbulent Alfven waves, which is found to be less than the cooling rate by X-ray radiation. We suggest that the turbulence which is sufficient for particle acceleration is developed in the clusters containing the radio halo. It is most likely that cluster mergers create the turbulence and seed relativistic electrons.

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arXiv2002

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.

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arXiv2000

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.

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