arXiv subjects

Tetsu Kitayama

Publications and source records attributed to Tetsu Kitayama.

At least 55 records · Page 3Linked to original sources

arXiv2012

The ASTRO-H X-ray Observatory

The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the high-energy universe via a suite of four instruments, covering a very wide energy range, from 0.3 keV to 600 keV. These instruments include a high-resolution, high-throughput spectrometer sensitive over 0.3-2 keV with high spectral resolution of Delta E < 7 eV, enabled by a micro-calorimeter array located in the focal plane of thin-foil X-ray optics; hard X-ray imaging spectrometers covering 5-80 keV, located in the focal plane of multilayer-coated, focusing hard X-ray mirrors; a wide-field imaging spectrometer sensitive over 0.4-12 keV, with an X-ray CCD camera in the focal plane of a soft X-ray telescope; and a non-focusing Compton-camera type soft gamma-ray detector, sensitive in the 40-600 keV band. The simultaneous broad bandpass, coupled with high spectral resolution, will enable the pursuit of a wide variety of important science themes.

astro-ph.IM↗

arXiv2011

Far-infrared emission from intergalactic medium in Stephan's Quintet revealed by AKARI

The Stephan's Quintet (SQ, HCG92) was observed with the Far-Infrared Surveyor (FIS) aboard AKARI in four far-infrared (IR) bands at 65, 90, 140, and 160 um. The AKARI four-band images of the SQ show far-IR emission in the intergalactic medium (IGM) of the SQ. In particular, the 160 um band image shows single peak emission in addition to the structure extending in the North-South direction along the shock ridge as seen in the 140 um band, H2 emission and X-ray emission. Whereas most of the far-IR emission in the shocked region comes from the cold dust component, shock-powered [CII]158um emission can significantly contribute to the emission in the 160 um band that shows a single peak at the shocked region. In the shocked region, the observed gas-to-dust mass ratio is in agreement with the Galactic one. The color temperature of the cold dust component (~20 K) is lower than that in surrounding galaxies (~30 K). We discuss a possible origin of the intergalactic dust emission.

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arXiv2010

The ASTRO-H Mission

The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the high-energy universe by performing high-resolution, high-throughput spectroscopy with moderate angular resolution. ASTRO-H covers very wide energy range from 0.3 keV to 600 keV. ASTRO-H allows a combination of wide band X-ray spectroscopy (5-80 keV) provided by multilayer coating, focusing hard X-ray mirrors and hard X-ray imaging detectors, and high energy-resolution soft X-ray spectroscopy (0.3-12 keV) provided by thin-foil X-ray optics and a micro-calorimeter array. The mission will also carry an X-ray CCD camera as a focal plane detector for a soft X-ray telescope (0.4-12 keV) and a non-focusing soft gamma-ray detector (40-600 keV) . The micro-calorimeter system is developed by an international collaboration led by ISAS/JAXA and NASA. The simultaneous broad bandpass, coupled with high spectral resolution of Delta E ~7 eV provided by the micro-calorimeter will enable a wide variety of important science themes to be pursued.

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arXiv2010

Impact of Chandra calibration uncertainties on galaxy cluster temperatures: application to the Hubble Constant

We perform a uniform, systematic analysis of a sample of 38 X-ray galaxy clusters with three different Chandra calibrations. The temperatures change systematically between calibrations. Cluster temperatures change on average by roughly ~6% for the smallest changes and roughly ~13% for the more extreme changes between calibrations. We explore the effects of the changing cluster spectral properties on Sunyaev-Zel'dovich effect (SZE) and X-ray determinations of the Hubble constant. The Hubble parameter changes by +10% and -13% between the current calibration and two previous Chandra calibrations, indicating that changes in the cluster temperature basically explain the entire change in H_0. Although this work focuses on the difference in spectral properties and resultant Hubble parameters between the calibrations, it is intriguing to note that the newer calibrations favor a lower value of the Hubble constant, H_0 ~ 60 km s-1 Mpc-1, typical of results from SZE/X-ray distances. Both galaxy clusters themselves and the details of the instruments must be known precisely to enable reliable precision cosmology with clusters, which will be feasible with combined efforts from ongoing observations and planned missions and observatories covering a wide range of wavelengths.

astro-ph.CO↗

arXiv2010

PAH emission from the dust lane of an elliptical galaxy with the Spitzer IRS

Spitzer and AKARI observations have found that polycyclic aromatic hydrocarbons (PAHs) are present in nearby elliptical galaxies, but their spatial distributions are still unknown. In order to investigate their distributions, we performed deep spectral mapping observations of the PAH-detected elliptical galaxy NGC4589, a merger remnant with a minor-axis optical dust lane. As a result, we obtain clear evidence that the PAH 11.3 um emission comes predominantly from the dust lane of the galaxy. We also detect molecular hydrogen line emissions from the dust lane. The PAH 17 um emission is distributed differently from the PAH 11.3 um emission, and more similarly to the dust continuum emission. From their distinctive distributions, we suggest that the PAHs responsible for the 11.3 um feature are secondary products through the evolution of the ISM brought in by the merger.

astro-ph.CO↗

arXiv2009

Formation of globular clusters induced by external ultraviolet radiation

We present a novel scenario for globular cluster (GC) formation, where the ultraviolet (UV) background radiation effectively works so as to produce compact star clusters. Here, we explore the formation of GCs in UV radiation fields. For this purpose, we calculate baryon and dark matter (DM) dynamics in spherical symmetry, incorporating the self-shielding effects by solving the radiative transfer of UV radiation. In addition, we prescribe the star formation in cooled gas components and pursue the dynamics of formed stars. As a result, we find that the evolution of subgalactic objects in UV background radiation are separated into three types, that is, (1) prompt star formation, where less massive clouds ~10^{5-8} M_sun are promptly self-shielded and undergo star formation, (2) delayed star formation, where photoionized massive clouds >10^8 M_sun collapse despite high thermal pressure and are eventually self-shielded to form stars in a delayed fashion, and (3) supersonic infall, where photoionized less massive clouds ~10^{5-8} M_sun contract with supersonic infall velocity and are self-shielded when a compact core forms. In particular, the type (3) is a novel type found in the present simulations, and eventually produces a very compact star cluster. The resultant mass-to-light ratios, half-mass radii, and velocity dispersions for the three types are compared to the observations of GCs, dwarf spheroidals (dSphs), and ultra-compact dwarfs (UCDs). It turns out that the properties of star clusters resulting from supersonic infall match well with those of observed GCs, whereas the other two types are distinct from GCs. Hence, we conclude that supersonic infall in a UV background is a promising mechanism to form GCs.

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arXiv2008

Extracting Galaxy Cluster Gas Inhomogeneity from X-ray Surface Brightness: A Statistical Approach and Application to Abell 3667

Our previous analysis indicates that small-scale fluctuations in the intracluster medium (ICM) from cosmological hydrodynamic simulations follow the lognormal distribution. In order to test the lognormal nature of the ICM directly against X-ray observations of galaxy clusters, we develop a method of extracting statistical information about the three-dimensional properties of the fluctuations from the two-dimensional X-ray surface brightness. We first create a set of synthetic clusters with lognormal fluctuations. Performing mock observations of these synthetic clusters, we find that the resulting X-ray surface brightness fluctuations also follow the lognormal distribution fairly well. Systematic analysis of the synthetic clusters provides an empirical relation between the density fluctuations and the X-ray surface brightness. We analyze \chandra observations of the galaxy cluster Abell 3667, and find that its X-ray surface brightness fluctuations follow the lognormal distribution. While the lognormal model was originally motivated by cosmological hydrodynamic simulations, this is the first observational confirmation of the lognormal signature in a real cluster. Finally we check the synthetic cluster results against clusters from cosmological hydrodynamic simulations. As a result of the complex structure exhibited by simulated clusters, the empirical relation shows large scatter. Nevertheless we are able to reproduce the true value of the fluctuation amplitude of simulated clusters within a factor of two from their X-ray surface brightness alone. Our current methodology combined with existing observational data is useful in describing and inferring the statistical properties of the three dimensional inhomogeneity in galaxy clusters.

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arXiv2007

Systematic Errors in the Hubble Constant Measurement from the Sunyaev-Zel'dovich effect

The Hubble constant estimated from the combined analysis of the Sunyaev-Zel'dovich effect and X-ray observations of galaxy clusters is systematically lower than those from other methods by 10-15 percent. We examine the origin of the systematic underestimate using an analytic model of the intracluster medium (ICM), and compare the prediction with idealistic triaxial models and with clusters extracted from cosmological hydrodynamical simulations. We identify three important sources for the systematic errors; density and temperature inhomogeneities in the ICM, departures from isothermality, and asphericity. In particular, the combination of the first two leads to the systematic underestimate of the ICM spectroscopic temperature relative to its emission-weighed one. We find that these three systematics well reproduce both the observed bias and the intrinsic dispersions of the Hubble constant estimated from the Sunyaev-Zel'dovich effect.

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arXiv2007

The Habitat Segregation between Lyman Break Galaxies and Lyman alpha Emitters around a QSO at z~5

We carried out a target survey for Lyman break galaxies (LBGs) and Lyman alpha emitters (LAEs) around QSO SDSS J0211-0009 at z=4.87. The deep and wide broadband and narrowband imaging simultaneously revealed the perspective structure of these two high-z populations. The LBGs without Ly-alpha emission form a filamentary structure including the QSO, while the LAEs are distributed around the QSO but avoid it within a distance of ~4.5Mpc. On the other hand, we serendipitously discovered a protocluster with a significant concentration of LBGs and LAEs where no strongly UV ionizing source such as a QSO or radio galaxy is known to exist. In this cluster field, two populations are spatially cross-correlated with each other. The relative spatial distribution of LAEs to LBGs is in stark contrast between the QSO and the cluster fields. We also found a weak trend showing that the number counts based on Ly-alpha and UV continuum fluxes of LAEs in the QSO field are slightly lower than in the cluster field, whereas the number counts of LBGs are almost consistent with each other. The LAEs avoid the nearby region around the QSO where the local UV background radiation could be ~100 times stronger than the average for the epoch. The clustering segregation between LBGs and LAEs seen in the QSO field could be due to either enhanced early galaxy formation in an overdense environment having caused all the LAEs to evolve into LBGs, or local photoionization due to the strong UV radiation from the QSO effectively causing a deficit in low-mass galaxies like LAEs.

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arXiv2007

Early Cosmological HII/HeIII Regions and Their Impact on Second-Generation Star Formation

(Abridged) We present the results of three-dimensional radiation-hydrodynamics simulations of the formation and evolution of early HII/HeIII regions around the first stars. Cooling (by H2 and HD) and recollapse of the gas in the relic HII region is also followed in a full cosmological context, until second-generation stars are formed. A large HII region with a few kiloparsec diameter is formed, within which a smaller HeIII region is embedded. Radiative feedback effect quenches further star-formation within the halo for a hundred million years. Accretion onto remnant blackholes will be inefficient. Recombination radiation within the HII region is weak, but persists for 50 million years. We also follow the thermal and chemical evolution of the photo-ionized gas in the relic HII region. The gas cools by HD line cooling down to a few tens Kelvin. At high redshifts (z>10), the minimum gas temperature is limited by T_CMB. Because of its low temperature, the characteristic mass of a Jeans-unstable gas clump is ~ 40 Msun, and is significantly smaller than a typical clump mass for early primordial gas clouds. We find no evidence of fragmentation by this epoch. Together with the small cloud mass, this result indicates that massive, rather than very massive, primordial stars may form in the relic HII region. Such stars might be responsible for early metal-enrichment of the interstellar medium from which recently discovered hyper metal-poor stars were born.

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arXiv2007

Radial profile and log-normal fluctuations of intra-cluster medium as an origin of systematic bias of spectroscopic temperature

The origin of the recently reported systematic bias in the spectroscopic temperature of galaxy clusters is investigated using cosmological hydrodynamical simulations. We find that the local inhomogeneities of the gas temperature and density, after corrected for the global radial profiles, have nearly a universal distribution that resembles the log-normal function. Based on this log-normal approximation for the fluctuations in the intra-cluster medium, we develop an analytical model that explains the bias in the spectroscopic temperature discovered recently. We conclude that the multi-phase nature of the intra-cluster medium not only from the radial profiles but also from the local inhomogeneities plays an essential role in producing the systematic bias.

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arXiv2006

Systematic bias in the estimate of cluster mass and the fluctuation amplitude from cluster abundance statistics

We revisit the estimate of the mass fluctuation amplitude, sigma_8, from the observational X-ray cluster abundance. In particular, we examine the effect of the systematic difference between the cluster virial mass estimated from the X-ray spectroscopy, M_{vir, spec}, and the true virial mass of the corresponding halo, M_{vir}. Mazzotta et al. (2004) recently pointed out the possibility that alpha_M = M_{vir, spec}/M_{vir} is systematically lower than unity. We perform the statistical analysis combining the latest X-ray cluster sample and the improved theoretical models and find that sigma_8 \sim 0.76 +/- 0.01 + 0.50 (1-alpha_M) for 0.5 \le alpha_M \le 1, where the quoted errors are statistical only. Thus if alpha_M \sim 0.7, the value of sigma_8 from cluster abundance alone is now in better agreement with other cosmological data including the cosmic microwave background, the galaxy power spectrum and the weak lensing data. The current study also illustrates the importance of possible systematic effects in mapping real clusters to underlying dark halos which changes the interpretation of cluster abundance statistics.

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arXiv2005

L_X-T Relation and Related Properties of Galaxy Clusters

An observational approach is presented to constrain the global structure and evolution of the intracluster medium based on the ROSAT and ASCA distant cluster sample. From statistical analysis of the gas density profile and the connection to the LX-T relation under the beta-model, the scaled gas profile is nearly universal for the outer region and the LX(>0.2r500) is tightly related to the temperature through T^3 rather than T^2. On the other hand, a large density scatter exists in the core region and there is clearly a deviation from the self-similar scaling for clusters with a small core size. A direct link between the core size and the radiative cooling timescale suggest that t_cool is a parameter to control the gas structure and the appearance of small cores in regular clusters may be much connected with the thermal evolution. We derive the luminosity-ambient temperature (T') relation, assuming the universal temperature profile to find the dispersion around the relation significantly decreases: L_1keV is almost constant for a wide range of t_cool. We further examined the LX-Tbeta and LX-T'beta relations and showed a trend that merging clusters segregate from the regular clusters on the planes. A good correlation between t_cool and the X-ray morphology on the L_1keV-t_cool/t_age plane leads us to define three phases according to the different level of cooling, and draw a phenomenological picture: after a cluster collapses and t_cool falls below t_age, the core cools radiatively with quasi-hydrostatic balancing in the gravitational potential, and the central density gradually becomes higher to evolve from an outer-core-dominant cluster to inner-core-dominant cluster.

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arXiv2005

Infrared Emission from Intracluster Dust Grains and Constraints on Dust Properties

For 117 clusters of galaxies, we explore the detectability of intracluster dust grains by current and future infrared facilities, taking into account both collisional heating and sputtering of grains by ambient plasma. If the dust grains are injected into the intergalactic space with the amount and size comparable to the Galactic values, the dust-to-gas ratio is typically $10^{-6}$ and the mean dust temperature is $\sim 30$ K near the cluster center. The predicted infrared intensities lie marginally above the detection thresholds for Spitzer Space Telescope, ASTRO-F, Herschel and SPICA missions. For some nearby clusters such as Perseus, A3571, A2319, A3112 and A2204, good detections of intracluster dust signal are expected in the 70 $μ$m band. Given rather tight constraints on the dust temperature from observed electron density and temperature, the dust mass can be inferred directly from the infrared observations. Further constraints on the size distribution will be obtained once multi-band data are available. They will definitely provide a powerful probe of the dust injection processes and dust-gas interactions in the intergalactic space.

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arXiv2005

Supernova Explosions in the Early Universe: Evolution of Radiative Remnants and the Halo Destruction Efficiency

We study the evolution of supernova (SN) remnants of the first stars, taking proper account of the radiative feedback of the progenitor stars on the surroundings. We carry out a series of one-dimensional hydrodynamic simulations with radiative cooling, starting from initial configurations that are drawn from the results of our earlier radiation hydrodynamic simulations of the first HII regions. In low-mass (< 10^6 M_sun) halos, the stellar radiation significantly reduces the ambient gas density prior to the SN explosion. The blastwave quickly propagates over the halo's virial radius, leading to complete evacuation of the gas even with the input energy of 10^50 erg. We find that a large fraction of the remnant's thermal energy is lost in 0.1-10 Myr by line cooling, whereas, for larger explosion energies, the remnant expands even more rapidly with decreasing interior density, and cools predominantly via inverse Compton process. In higher mass halos, the gas density near the explosion site remains high and the SN shock is heavily confined; the thermal energy of the remnant is quickly radiated away by free-free emission, even if the total input energy exceeds the binding energy of halos by two orders of magnitude. We show that the efficiency of halo destruction is determined not only by the explosion energy but also by the gas density profile, and thus controlled by radiative feedback prior to the explosion. Several implications of our results for the formation of first quasars and second-generation stars in the universe are also discussed.

astro-ph↗

arXiv2004

Chandra Observation of a Group of Galaxies HCG 80: Does the Spiral-Only Group Have Hot Intragroup Gas?

We present an analysis of Chandra X-ray observations of a compact group of galaxies, HCG 80 (z=0.03). The system is a spiral-only group composed of four late-type galaxies, and has a high-velocity dispersion of 309 km/s. With high-sensitivity Chandra observations, we searched for diffuse X-ray emission from the intragroup medium (IGM); however, no significant emission was detected. We place a severe upper limit on the luminosity of the diffuse gas as LX < 6e40 erg/s. On the other hand, significant emission from three of the four members were detected. In particular, we discovered huge halo emission from HCG 80a that extends on a scale of ~30 kpc perpendicular to the galactic disk, whose X-ray temperature and luminosity were measured to be ~0.6 keV and ~4e40 erg/s in the 0.5-2 keV band, respectively. It is most likely to be an outflow powered by intense starburst activity. Based on the results, we discuss possible reasons for the absence of diffuse X-ray emission in the HCG 80 group, suggesting that the system is subject to galaxy interactions, and is possibly at an early stage of IGM evolution.

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arXiv2004

X-ray Measurement of Dark Matter "Temperature" in Abell 1795

We present a method from an X-ray observation of a galaxy cluster to measure the radial profile of the dark matter velocity dispersion, sigma_DM, and to compare the dark matter ``temperature'' defined as mu m_p sigma_DM**2 / k_B with the gas temperature. The method is applied to the XMM-Newton observation of Abell 1795. The ratio between the specific energy of the dark matter and that of the intracluster medium (ICM), which can be defined as beta_DM in analogy with beta_spec, is found to be less than unity everywhere ranging \~0.3-0.8. In other words, the ICM temperature is higher than the dark matter ``temperature'', even in the central region where the radiative cooling time is short. A beta_DM value smaller than unity can most naturally be explained by heating of the ICM. The excess energy of ICM is estimated to be ~1-3 keV per particle.

astro-ph↗

arXiv2004

Quasi-hydrostatic intracluster gas under radiative cooling

Quasi-hydrostatic cooling of the intracluster gas is studied. In the quasi-hydrostatic model, work done by gravity on the inflow gas with dP \neq 0, where P is the gas pressure, is taken into account in the thermal balance. The gas flows in from the outer part so as to compensate the pressure loss of the gas undergoing radiative cooling, but the mass flow is so moderate and smooth that the gas is considered to be quasi-hydrostatic. The temperature of the cooling gas decreases toward the cluster center, but, unlike cooling flows with dP = 0, approaches a constant temperature of \sim 1/3 the temperature of the non-cooling ambient gas. This does not mean that gravitational work cancels out radiative cooling, but means that the temperature of the cooling gas appears to approach a constant value toward the cluster center if the gas maintains the quasi-hydrostatic balance. We discuss the mass flow in quasi-hydrostatic cooling, and compare it with the standard isobaric cooling flow model. We also discuss the implication of \dot{M} for the standard cooling flow model.

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