Takeshi Go Tsuru

Missing Link Found? The "Runaway" Path to Supermassive Black Holes

Observations of stellar kinematics, gasdynamics, and masers around galactic nuclei have now firmly established that many galaxies host central supermassive black holes (SMBHs) with masses in the range of ~106-109 M☉. However, how these SMBHs formed is not well understood. One reason for this situation is the lack of observations of intermediate-mass BHs (IMBHs), which could bridge the gap between stellar mass BHs and SMBHs. Recently, this missing link (i.e., an IMBH) has been found in observations made by ASCA and Chandra of the central region of the starburst galaxy M82. Subsequent observations by Subaru have revealed that this IMBH apparently coincides with a young compact star cluster. Based on these findings, we suggest a new formation scenario for SMBHs. In this scenario, IMBHs first form in young compact star clusters through runaway merging of massive stars. While these IMBHs are forming, the host star clusters sink toward the galactic nucleus through dynamical friction and upon evaporation deposit their IMBHs near the galactic center. The IMBHs then form binaries and eventually merge via gravitational radiation, forming an SMBH.

X-Ray Properties of Early-Type Galaxies as Observed with ASCA

We have systematically investigated the Advanced Satellite for Cosmology and Astrophysics (ASCA) spectra of 12 early-type galaxies. This paper presents the global spectral properties of these systems based on a larger sample than in any previous ASCA study. The X-ray spectra were uniformly fitted by a two-component model consisting of hard X-rays from thermal emission with a temperature of about 10 keV or from a power law with index 1.8, plus soft X-rays from a thin thermal plasma with temperature ranging from 0.3 to 1 keV. The X-ray luminosities of the hard component are found to be proportional to the blue-band luminosities, while those of the soft component show large scatter with no clear correlation. The metal abundances determined from the soft component are systematically lower than solar, with a mean value of about 0.3 solar. We examine the relationships between the temperature and volume emission measure and between the gas temperature and the stellar velocity dispersion. The volume emission measures for early-type galaxies plotted as a function of the gas temperature are well below the extrapolated line found in clusters of galaxies, indicating that early-type galaxies are relatively gas poor compared with galaxy clusters. The ratio of the stellar kinetic energy per unit mass to the thermal energy of the hot gas per unit mass (βspec) is less than unity. We found no systematic relationship between X-ray properties and environment, suggesting that the interaction between interstellar matter and the intracluster medium is not strong.

Optical Identification of the ASCA Large Sky Survey

We present results of optical identification of the X-ray sources detected in the ASCA Large Sky Survey. Optical spectroscopic observations were done for 34 X-ray sources which were detected with the SIS in the 2-7 keV band above 3.5 sigma. The sources are identified with 30 AGNs, 2 clusters of galaxies, and 1 galactic star. Only 1 source is still unidentified. The flux limit of the sample corresponds to 1 x 10^{-13} erg s^{-1} cm^{-2} in the 2-10 keV band. Based on the sample, the paper discusses optical and X-ray spectral properties of the AGNs, contribution of the sources to the Cosmic X-ray Background, and redshift and luminosity distributions of the AGNs. An interesting result is that the redshift distribution of the AGNs suggests a deficiency of high-redshift (0.510^{44} erg s^{-1}) absorbed narrow-line AGNs (so called type 2 QSOs).We present results of optical identification of the X-ray sources detected in the ASCA Large Sky Survey. Optical spectroscopic observations were done for 34 X-ray sources that were detected with the SIS in the 2-7 keV band above 3.5 ?. The flux limit corresponds to ~1 ? 10-13 ergs cm-2 s-1 in the 2-10 keV band. The sources are identified with 30 active galactic nuclei (AGNs), two clusters of galaxies, and one Galactic star. Only one source is still unidentified. All of the X-ray sources that have a hard X-ray spectrum with an apparent photon index of smaller than 1 in the 0.7-10 keV band are identified with narrow-line or weak-broad-line AGNs at redshifts smaller than 0.5. This fact supports the idea that absorbed X-ray spectra of narrow-line and weak-broad-line AGNs make the cosmic X-ray background (CXB) spectrum harder in the hard X-ray band than that of a broad-line AGN, which is the main contributor in the soft X-ray band. Assuming their intrinsic spectra are same as a broad-line AGN (a power-law model with a photon index of 1.7), their X-ray spectra are fitted with hydrogen column densities of log NH(cm-2) = 22-23 at the objects redshift. On the other hand, X-ray spectra of the other AGNs are consistent with that of a nearby type 1 Seyfert galaxy. In the sample, four high-redshift luminous broad-line AGNs show a hard X-ray spectrum with an apparent photon index of 1.3 ? 0.3. The hardness may be explained by the reflection component of a type 1 Seyfert galaxy. The hard X-ray spectra may also be explained by absorption with log NH(cm-2) = 22-23 at the objects redshift, if we assume an intrinsic photon index of 1.7. The origin of the hardness is not clear yet. Based on the log N- log S relations of each population, contributions to the CXB in the 2-10 keV band are estimated to be 9% for less-absorbed AGNs (log NH(cm-2) < 22) including the four high-redshift broad-line AGNs with a hard X-ray spectrum, 4% for absorbed AGNs (22 < log NH(cm-2) < 23, without the four hard broad-line AGNs), and 1% for clusters of galaxies in the flux range from 3 ? 10-11 ergs cm-2 s-1 to 2 ? 10-13 ergs cm-2 s-1. If the four hard broad-line AGNs are included in the absorbed AGNs, the contribution of the absorbed AGNs to the CXB is estimated to be 6%. In optical spectra, there is no high-redshift luminous cousin of a narrow-line AGN in our sample. The redshift distribution of the absorbed AGNs is limited below z = 0.5 excluding the four hard broad-line AGNs, in contrast to the existence of 15 less-absorbed AGNs above z = 0.5. The redshift distribution of the absorbed AGNs suggests a deficiency of AGNs with column densities of log NH(cm-2) = 22-23 in the redshift range 0.5-2, or in the X-ray luminosity range larger than 1044 ergs s-1, or both. If the large column densities of the four hard broad-line AGNs are real, they could complement the deficiency of X-ray absorbed luminous high-redshift AGNs.

log N-log S Relations and Spectral Properties of Sources from the ASCA Large Sky Survey: Their Implications for the Origin of the Cosmic X-Ray Background (CXB)

We carried out the first wide-area unbiased survey with the ASCA satellite in the 0.7-10 keV band around a north Galactic-pole region covering a continuous area of 7 deg2 (Large Sky Survey; LSS). To make the best use of ASCAs capabilities, we developed a new source-detection method in which the complicated detector responses are fully taken into account. Applying this method to the entire LSS data independently in the total (0.7-7 keV), hard (2-10 keV), and soft (0.7-2 keV) bands, we detected 107 sources altogether, with sensitivity limits of 6 × 10-14 (0.7-7 keV), 1 × 10-13 (2-10 keV), and 2 × 10-14 ergs s-1 cm-2 (0.7-2 keV), respectively. A complete list of the detected sources is presented. Based on detailed studies by Monte Carlo simulations, we evaluated the effects of the source confusion and accurately derived the log N-log S relation in each survey band. The log N-log S relation in the hard band is located on the extrapolation from Ginga and HEAO1 results with a Euclidean slope of -3/2, while that in the soft band is consistent with the results from ROSAT. At these flux limits, 30% ± 3% of the CXB in the 0.7-7 keV band and 23% ± 3% in the 2-10 keV band have been resolved into discrete sources. The average spectrum of faint sources detected in the total band shows a photon index of 1.63 ± 0.07 in the 0.7-10 keV range, consistent with the comparison of source counts between the hard and soft energy bands. Those detected in the hard band show a photon index of 1.49 ± 0.10 in the 2-10 keV range. These spectral properties suggest that the contribution of sources with hard energy spectra becomes significant at a flux of 10-13 ergs s-1 cm-2 (2-10 keV). The most plausible candidates are type II active galactic nuclei, as indicated by on-going optical identifications.

Evolution in an oncogenic bacterial species with extreme genome plasticity: Helicobacter pylori East Asian genomes

BackgroundThe genome of Helicobacter pylori, an oncogenic bacterium in the human stomach, rapidly evolves and shows wide geographical divergence. The high incidence of stomach cancer in East Asia might be related to bacterial genotype. We used newly developed comparative methods to follow the evolution of East Asian H. pylori genomes using 20 complete genome sequences from Japanese, Korean, Amerind, European, and West African strains.ResultsA phylogenetic tree of concatenated well-defined core genes supported divergence of the East Asian lineage (hspEAsia; Japanese and Korean) from the European lineage ancestor, and then from the Amerind lineage ancestor. Phylogenetic profiling revealed a large difference in the repertoire of outer membrane proteins (including oipA, hopMN, babABC, sabAB and vacA-2) through gene loss, gain, and mutation. All known functions associated with molybdenum, a rare element essential to nearly all organisms that catalyzes two-electron-transfer oxidation-reduction reactions, appeared to be inactivated. Two pathways linking acetyl~CoA and acetate appeared intact in some Japanese strains. Phylogenetic analysis revealed greater divergence between the East Asian (hspEAsia) and the European (hpEurope) genomes in proteins in host interaction, specifically virulence factors (tipα), outer membrane proteins, and lipopolysaccharide synthesis (human Lewis antigen mimicry) enzymes. Divergence was also seen in proteins in electron transfer and translation fidelity (miaA, tilS), a DNA recombinase/exonuclease that recognizes genome identity (addA), and DNA/RNA hybrid nucleases (rnhAB). Positively selected amino acid changes between hspEAsia and hpEurope were mapped to products of cagA, vacA, homC (outer membrane protein), sotB (sugar transport), and a translation fidelity factor (miaA). Large divergence was seen in genes related to antibiotics: frxA (metronidazole resistance), def (peptide deformylase, drug target), and ftsA (actin-like, drug target).ConclusionsThese results demonstrate dramatic genome evolution within a species, especially in likely host interaction genes. The East Asian strains appear to differ greatly from the European strains in electron transfer and redox reactions. These findings also suggest a model of adaptive evolution through proteome diversification and selection through modulation of translational fidelity. The results define H. pylori East Asian lineages and provide essential information for understanding their pathogenesis and designing drugs and therapies that target them.

X-ray evidence for Seyfert activity buried in the infrared galaxy NGC 4945

We have observed the infrared galaxy NGC 4945 using the Japanese X-ray astronomy satellite, Ginga. The X-ray spectrum is found to be composed of three components: hard X-ray emission which is heavily absorbed by cold material with column density of about 10 24.7 cm −2 ; a soft X-ray component seen in the 2-10 keV band; and an iron emission line. The hard component exhibits a power-law spectrum with photon index of about 1.7. The mean X-ray luminosity in the 2-20 keV band, after correction for absorption is 3×10 42 ergs s −1 with significant intensity variation on a time scale of several hours. These results are thought to be evidence for an AGN (active galactic nucleus) in the galaxy

Interacting Cosmic Rays with Molecular Clouds: A Bremsstrahlung Origin of Diffuse High-energy Emission from the Inner 2??1? of the Galactic Center

The high-energy activity in the inner few degrees of the Galactic center is traced by diffuse radio, X-ray, and ?-ray emission. The physical relationship between different components of diffuse gas emitting at multiple wavelengths is a focus of this work. We first present radio continuum observations using the Green Bank Telescope and model the nonthermal spectrum in terms of a broken power-law distribution of ~GeV electrons emitting synchrotron radiation. We show that the emission detected by Fermi is primarily due to nonthermal bremsstrahlung produced by the population of synchrotron emitting electrons in the GeV energy range interacting with neutral gas. The extrapolation of the electron population measured from radio data to low and high energies can also explain the origin of Fe I 6.4?keV line and diffuse TeV emission, as observed with Suzaku, XMM-Newton, Chandra, and the H.E.S.S. observatories. The inferred physical quantities from modeling multiwavelength emission in the context of bremsstrahlung emission from the inner ~300 ? 120 pc of the Galactic center are constrained to have the cosmic-ray ionization rate ~1-10 ? 10?15?s?1, molecular gas heating rate elevating the gas temperature to 75-200?K, fractional ionization of molecular gas 10?6-10?5, large-scale magnetic field 10-20 ?G, the density of diffuse and dense molecular gas ~100 and ~103?cm?3 over 300?pc and 50?pc path lengths, and the variability of Fe I K? 6.4?keV line emission on yearly timescales. Important implications of our study are that GeV electrons emitting in radio can explain the GeV ?-rays detected by Fermi and that the cosmic-ray irradiation model, like the model of the X-ray irradiation triggered by past activity of Sgr?A*, can also explain the origin of the variable 6.4?keV emission from Galactic center molecular clouds.

New CTI Correction Method for Spaced-Row Charge Injection of the Suzaku X-Ray Imaging Spectrometer

The charge transfer inefficiency (CTI) of the X-ray CCDs on board the Suzaku satellite (X-ray Imaging Spectrometers; XIS) has increased since the launch due to radiation damage, and the energy resolution has been degraded. To improve the CTI, we have applied a spaced-row charge injection (SCI) technique to the XIS in orbit; by injecting charges into CCD rows periodically, the CTI is actively decreased. The CTI in the SCI mode depends on the distance between a signal charge and a preceding injected row, and the pulse height shows periodic positional variations. Using in-flight data of onboard calibration sources and of the strong iron line from the Perseus cluster of galaxies, we studied the variation in detail. We developed a new method to correct the variation. By applying the new method, the energy resolution (FWHM) at 5.9 keV at March 2008 is ~155 eV for the front-illuminated CCDs and ~175 eV for the back-illuminated CCD.

Formation of a Massive Black Hole at the Center of the Superbubble in M82

We performed 12CO (1-0), 13CO (1-0), and HCN (1-0) interferometric observations of the central region (about 450 pc in radius) of M82 with the Nobeyama Millimeter Array and have successfully imaged a molecular superbubble and spurs. The center of the superbubble is clearly shifted from the nucleus by 140 pc. This position is close to that of the massive black hole (BH) of 460 M☉ and the 2.2 μm secondary peak (a luminous supergiant-dominated cluster), which strongly suggests that these objects may be related to the formation of the superbubble. Consideration of star formation in the cluster based on the infrared data indicates that (1) the energy release from supernovae can account for the kinetic energy of the superbubble, (2) the total mass of stellar-mass BHs available for building up the massive BH may be much higher than 460 M☉, and (3) it is possible to form the middle-mass BH of 102-103 M☉ within the timescale of the superbubble. We suggest that the massive BH was produced and is growing in the intense starburst region.

Performance of the Charge-Injection Capability of Suzaku XIS

A charge-injection technique is applied to the X-ray CCD camera, XIS (X-ray Imaging Spectrometer) aboard Suzaku. The charge transfer inefficiency (CTI) in each CCD column (vertical transfer channel) is measured by the injection of charge packets into a transfer channel and subsequent readout. This paper reports on the performances of the charge-injection capability based on the ground experiments using a radiation-damaged device, and in-orbit measurements of the XIS. The ground experiments show that charges are stably injected with a dispersion of 91 eV in FWHM in a specific column for the charges equivalent to an X-ray energy of 5.1 keV. This dispersion width is significantly smaller than that of the X-ray events of 113 eV (FWHM) at approximately the same energy. The amount of charge loss during transfer in a specific column, which is measured with the charge-injection capability, is consistent with that measured with the calibration source. These results indicate that the charge-injection technique can accurately measure column-dependent charge losses, rather than the calibration sources. The column-to-column CTI correction to the calibration source spectra significantly reduces the line widths compared to those with a columnaveraged CTI correction (from 193 eV to 173 eV in FWHM on an average at the time of one year after the launch). In addition, this method significantly reduces the low-energy tail in the line profile of the calibration source spectrum.