Ryo Nagino

Gravitational potential and X-ray luminosities of early-type galaxies observed with XMM-Newton and Chandra

Aims. We study the dark matter content in early-type galaxies and investigate whether X-ray luminosities of early-type galaxies are determined by the surrounding gravitational potential. Methods. We derived gravitational mass profiles of 22 early-type galaxies observed with XMM-Newton and Chandra. Results. Sixteen galaxies show constant or decreasing radial temperature profiles, and their X-ray luminosities are consistent with kinematical energy input from stellar mass loss. The temperature profiles of the other 6 galaxies increase with radius, and their X-ray luminosities are significantly higher. The integrated mass-to-light ratio of each galaxy is constant at that of stars within 0.5–1 re ,a nd increases with radius, where re is the effective radius of a galaxy. The scatter of the central mass-to-light ratio of galaxies was less in K-band light. At 3 re, the integrated mass-to-light ratios of galaxies with flat or decreasing temperature profiles are twice the value at 0.5 re, where the stellar mass dominates, and at 6 re, these increase to three times the value at 0.5 re. Conclusions. This feature should reflect common dark and stellar mass distributions in early-type galaxies: within 3 re, the mass of dark matter is similar to the stellar mass, while within 6 re, the former is larger than the latter by a factor of two. In contrast, X-ray luminous galaxies have higher gravitational mass in the outer regions than X-ray faint galaxies. We describe these X-ray luminous galaxies as the central objects of large potential structures; the presence or absence of this potential is the main source of the large scatter in the X-ray luminosity.

Soft x-ray imager (SXI) onboard ASTRO-H

Soft X-ray Imager (SXI) is a CCD camera onboard the ASTRO-H satellite which is scheduled to be launched in 2015. The SXI camera contains four CCD chips, each with an imaging area of 31mm x 31 mm, arrayed in mosaic, covering the whole FOV area of 38′ x 38′. The CCDs are a P-channel back-illuminated (BI) type with a depletion layer thickness of 200 _m. High QE of 77% at 10 keV expected for this device is an advantage to cover an overlapping energy band with the Hard X-ray Imager (HXI) onboard ASTRO-H. Most of the flight components of the SXI system are completed until the end of 2013 and assembled, and an end-to-end test is performed. Basic performance is verified to meet the requirements. Similar performance is confirmed in the first integration test of the satellite performed in March to June 2014, in which the energy resolution at 5.9 keV of 160 eV is obtained. In parallel to these activities, calibrations using engineering model CCDs are performed, including QE, transmission of a filter, linearity, and response profiles.

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 the virial mass from these data and compare it with the real mass. When a smaller subcluster’s mass is larger than a quarter of that of the larger one, the virial mass can be larger than twice the real mass. The results strongly depend on the observational directions, because of an anisotropic velocity distribution of the member galaxies. We have also made 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, a mass estimation with X-ray data gives us better results than a virial mass estimation. The dependence upon observational directions is weaker than in the case of a 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

ABUNDANCE PATTERNS IN THE INTERSTELLAR MEDIUM OF EARLY-TYPE GALAXIES OBSERVED WITH SUZAKU

We have analyzed 17 early-type galaxies, 13 ellipticals and 4 S0 galaxies, observed with Suzaku, and investigated metal abundances (O, Mg, Si, and Fe) and abundance ratios (O/Fe, Mg/Fe, and Si/Fe) in the interstellar medium (ISM). The emission from each on-source region, which is four times the effective radius, r e, is reproduced with one-temperature (1T) or two-temperature (2T) thermal plasma models as well as a multi-temperature model, using APEC plasma code version 2.0.1. The multi-temperature model gave almost the same abundances and abundance ratios with the 1T or 2T models. The weighted averages of the O, Mg, Si, and Fe abundances of all the sample galaxies derived from the multi-temperature model fits are 0.83 ± 0.04, 0.93 ± 0.03, 0.80 ± 0.02, and 0.80 ± 0.02 solar, respectively, in solar units according to the solar abundance table by Lodders in 2003. These abundances show no significant dependence on the morphology and environment. The systematic differences in the derived metal abundances between versions 2.0.1 and 1.3.1 of the APEC plasma codes were investigated. The derived O and Mg abundances in the ISM agree with the stellar metallicity within an aperture with a radius of one r e derived from optical spectroscopy. From these results, we discuss the past and present Type Ia supernova rates and star formation histories in early-type galaxies.

The Abundance Pattern of O, Ne, Mg, and Fe in the Interstellar Medium of S0 Galaxy NGC 4382 Observed with Suzaku

We derived the O, Ne, Mg, and Fe abundances in the interstellar medium (ISM) of a relatively isolated S0 galaxy, NGC 4382, observed with the Suzaku XIS instruments, and compared the O/Ne/Mg/Fe abundance pattern with that of the ISM in elliptical galaxies. The derived temperature and Fe abundance in the ISM are � 0.3 keV and 0.6–2.9 solar, respectively. The abundance ratios are derived with considerable accuracy than the abundances, themselves; that is, the O=Fe, Ne=Fe, and Mg=Fe ratios are 0.3, 0.7, and 0.6 in solar units, respectively. The O=Fe ratio is smaller than those of the ISM in elliptical galaxies (NGC 720, NGC 1399, NGC 1404, and NGC 4636) observed with Suzaku. Since O, Ne, and Mg are predominantly synthesized by supernovae (SNe) of type II, the observed abundance pattern indicates that the contribution of SN Ia products is higher in the S0 galaxy than in the elliptical galaxy. Since the hot ISM in early-type galaxies is an accumulation of stellar mass and SN Ia products, the low O=Fe ratio in the ISM of NGC 4382 reflects a higher rate of present SNe Ia, or stars containing more SN Ia products than those in elliptical galaxies.

Abundance Patterns in the Interstellar Medium of the S0 Galaxy NGC 1316 (Fornax A) Revealed with Suzaku

The Suzaku X-ray satellite observed the nearby S0 galaxy NGC 1316, a merger remnant aged 3 Gyr. The total good exposure time was 48.7 ksec. The spectra were well represented by a two-temperature thermal model for the interstellar medium (ISM) plus a power-law model. The cool and hot temperatures of the thermal model were 0.48 +/- 0.03 and 0.92 +/- 0.04 keV, respectively. The excellent spectral sensitivity of Suzaku enables for the first time to measure the metal abundances of O, Ne, Mg, Si, and Fe in the ISM. The resultant abundance pattern of O, Ne, Mg, Si, and Fe is close to that of the new solar abundance determined by Lodders (2003). The measured abundance pattern is compared with those of elliptical galaxies and an S0 galaxy, observed with Suzaku. Considering the metal-enrichment from present Type Ia supernovae, the near-solar abundance pattern of the ISM in NGC~1316 indicates an enhanced {\alpha}/Fe ratio of stellar materials in the entire galaxy, like in giant elliptical galaxies.

Search for a non-equilibrium plasma in the merging galaxy cluster Abell 754

Abell 754 is a galaxy cluster in which an ongoing merger is evident on the plane of the sky, from the southeast to the northwest. We study the spatial variation of the X-ray spectra observed with Suzaku along the merging direction, centering on the Fe Ly-alpha / Fe He-alpha line ratio to search for possible deviation from ionization equilibrium. Fitting with a single temperature collisional non-equilibrium plasma model shows that the electron temperature increases from the southeast to the northwest. The ionization parameter is consistent with that in equilibrium (n_et>10^{13} s cm^{-3}) except for a specific region with the highest temperature (kT=13.3^{+1.4}_{-1.1} keV) where n_et=10^{11.6^{+0.6}_{-1.7}} s cm^{-3}. The elapsed time from the plasma heating estimated from the ionization parameter is 0.36-76 Myr at the 90% confidence level. This time scale is quite short but consistent with the traveling time of a shock to pass through that region. We thus interpret that the non-equilibrium ionization plasma in Abell 754 observed is a remnant of the shock heating in the merger process. We, however, note that the X-ray spectrum of the specific region where the non-equilibrium is found can also be fitted with a collisional ionization plasma model with two temperatures, low kT=4.2^{+4.2}_{-1.5} keV and very high kT > 19.3 keV. The very high temperature component is alternatively fitted with a power law model. Either of these spectral models is interpreted as a consequence of the ongoing merger process as in the case of that with the non-equilibrium ionization plasma.

The Soft X-ray Imager (SXI) for the ASTRO-H Mission

The Soft X-ray Imager (SXI) is an X-ray CCD camera onboard the ASTRO-H X-ray observatory. The CCD chip used is a P-channel back-illuminated type, and has a 200-µm thick depletion layer, with which the SXI covers the energy range between 0.4 keV and 12 keV. Its imaging area has a size of 31 mm x 31 mm. We arrange four of the CCD chips in a 2 by 2 grid so that we can cover a large field-of-view of 38’ x 38’. We cool the CCDs to -120 °C with a single-stage Stirling cooler. As was done for the CCD camera of the Suzaku satellite, XIS, artificial charges are injected to selected rows in order to recover charge transfer inefficiency due to radiation damage caused by in-orbit cosmic rays. We completed fabrication of flight models of the SXI and installed them into the satellite. We verified the performance of the SXI in a series of satellite tests. On-ground calibrations were also carried out and detailed studies are ongoing.

Formation Flight Astronomical Survey Telescope (FFAST) mission in hard x-ray

A formation flight astronomical survey telescope (FFAST) is a new project that will cover a large sky area in hard X-ray. In particular, it will focus on the energy range up to 80keV. It consists of two small satellites that will go in a formation flight. One is an X-ray telescope satellite carrying a super mirror, and the other is a detector satellite carrying an SDCCD. Two satellites are put into a low earth orbit in keeping the separation of 12m. This will survey a large sky area at hard X-ray region to study the evolution of the universe.

Discovery of an overlapping cluster in the Abell 1674 field with Suzaku

We present the results of a Suzaku observation of Abell 1674, an optically very rich (richness class 3) cluster cataloged as z = 0.1066. We discover the He-like Fe K-shell line from the central region for the first time, and find that the X-ray spectrum yields a high redshift of 0.215^{+0.007}_{-0.006}. On the other hand, the spectrum of the southwestern region is fitted with a redshift of 0.11 \pm 0.02 by the He-like Fe L-shell lines, consistent with the optically determined value. The gas temperature, metal abundance, and core radius of the X-ray emission in the central region are 3.8 \pm 0.2 keV, 0.20 \pm 0.05 Z_solar and 450 \pm 40 kpc, respectively, while those in the southwestern region are 2.0 \pm 0.2 keV, 0.41^{+0.17}_{-0.13} Z_solar and 220^{+90}_{-70} kpc, respectively. These parameters are typical for clusters. We thus conclude that Abell 1674 consists of two independent clusters, A1674-C at z ~0.22 and A1674-SW at z ~0.11, overlapping along the line of sight. The X-ray luminosities of A1674-C within r = 2 Mpc is 15.9 \pm 0.6 \times 10^{43} erg s^{-1} in the 0.1-2.4 keV energy band, while that for A1674-SW is 1.25 \pm 0.07 \times 10^{43} erg s^{-1}. Both are consistent with those expected from the L-T relation of clusters within a factor of 2. This is another support for our conclusion.