Showing papers in "Publications of the Astronomical Society of Japan in 2003"

Magnetically Arrested Disk : an Energetically Efficient Accretion Flow

Abstract: OAK-B135 We consider an accretion flow model originally proposed by Bisnovatyi-Kogan and Ruzmaikin (1974), which has been confirmed in recent 3D MHD simulations. In this model, the accreting gas drags in a strong poloidal magnetic field to the center such that the accumulated field disrupts the axisymmetric accretion flow at a relatively large radius. Inside the disruption radius, the gas accretes as discrete blobs or streams with a velocity much less than the free-fall velocity. Almost the entire rest mass energy of the gas is released as heat, radiation and mechanical/magnetic energy. Even for a non-rotating black hole, the efficiency of converting mass to energy is of order 50% or higher. The model is thus a practical analog of an idealized engine proposed by Geroch and Bekenstein.

Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: I. The H I Disk

Abstract: We derived the three-dimensional distribution of H I gas in the Milky Way Galaxy using the latest H I survey data cubes and rotation curves. The distance of the H I gas was determined by the kinematic distance using a rotation curve. We solved the near–far problem in the inner Galaxy by a fitting method which involves introducing a model of vertical H I distribution. In our resultant maps we could trace three prominent arms: the Sagittarius–Carina arm, the Perseus arm, and the Outer arm. These three arms were found to be logarithmic spiral arms. The pitch angles of the Sagittarius–Carina, Perseus, and Outer arms were estimated to be about 11 ◦ ,1 8 ◦ ,a nd 7 ◦ , respectively. The Sun is located in a region rich in H I gas between the Sagittarius–Carina arm and the Perseus arm. The H I disk shows large and asymmetric warping in the outer disk: the H I disk goes up to about 1.5kpc above the Galactic plane in the northern hemisphere, and down to about 1kpc in the southern hemisphere, which means asymmetric warping. The inner H I disk is also found to be tilting. The radius of the HI disk is about 17kpc and the H I mass within this radius is estimated to be 2.5×10 9 M� , which corresponds to 1.5% of the dynamical mass predicted from the rotation curve. We also found that the H I outskirt is largely swelling in the fourth quadrant, and hence the Galaxy is significantly lopsided. The scale-height of the H I layer increases with the radius, and is correlated with the H I volume density at the centroid of the H I layer.

The Discovery of Two Lyman α Emitters beyond Redshift 6 in the Subaru Deep Field

Abstract: We performed a deep optical imaging survey using a narrow-band filter (NB921) centered at λ=9196 u A together

GRAPE-6: Massively-Parallel Special-Purpose Computer for Astrophysical Particle Simulations

Abstract: In this paper, we describe the architecture and performance of the GRAPE-6 system, a massively-parallel special-purpose computer for astrophysical $N$-body simulations. GRAPE-6 is the successor of GRAPE-4, which was completed in 1995 and achieved the theoretical peak speed of 1.08 Tflops. As was the case with GRAPE-4, the primary application of GRAPE-6 is simulation of collisional systems, though it can be used for collisionless systems. The main differences between GRAPE-4 and GRAPE-6 are (a) The processor chip of GRAPE-6 integrates 6 force-calculation pipelines, compared to one pipeline of GRAPE-4 (which needed 3 clock cycles to calculate one interaction), (b) the clock speed is increased from 32 to 90 MHz, and (c) the total number of processor chips is increased from 1728 to 2048. These improvements resulted in the peak speed of 64 Tflops. We also discuss the design of the successor of GRAPE-6.

Non-Linear Resonance in Nearly Geodesic Motion in Low-Mass X-Ray Binaries

Abstract: We have explored the ideas that parametric resonance affects nearly geodesic motion around a black hole or a neutron star, and that it may be relevant to the high-frequency (twin) quasi-periodic oscillations that occur in some low-mass X-ray binaries. We have assumed the particles or uid elements of an accretion disc to be subject to an isotropic perturbation having a hypothetical but rather general form. We nd that the parametric resonance is indeed excited close to the radius where epicyclic frequencies of the radial and meridional oscillations are in a 2 : 3 ratio. The location and frequencies of the highest amplitude excitation vary with the strength of the perturbation. These results agree with actual frequency ratios of twin kHz QPOs that have been reported in some black hole candidates, and they may be consistent also with correlation of the twin peaks in Sco X-1.

Discovery of Interacting Molecular Gas toward the TeV Gamma-Ray Peak of the SNR G 347.3–0.5

Abstract: Supernova remnants (= SNR) are suggested to be sites of cosmic-ray acceleration. In particular, it has been an issue of keen interest whether cosmic ray protons are being accelerated in a SNR which emits TeV γ-rays. A crucial observational test for this is to find dense molecular gas towards the SNR, because such molecular gas can best verify the existence of cosmic-ray protons via pion decay to γ-rays. Here, we show that new high-resolution mm-wave observations of interstellar CO molecule have revealed molecular gas at 1kpc distance interacting with the TeV γ-ray SNR G 347.3−0.5, and that a molecular cloud of ∼ 200 solar masses is clearly associated with the TeV γ-ray peak, providing strong evidence for proton acceleration. We have estimated the total energy of accelerated protons to be ∼ 1048erg, which corresponds to an acceleration efficiency of ∼ 0.001, posing an observational constraint on the proton acceleration.

Hδ-Strong Galaxies in the Sloan Digital Sky Survey: I. The Catalog

Abstract: We present here a new and homogeneous sample of 3340 galaxies selected from the Sloan Digital Sky Survey (SDSS) based solely on the observed strength of their Hδ hydrogen Balmer absorption line. The presence of a strong Hδ line within the spectrum of a galaxy indicates that the galaxy has undergone a significant change in its starformation history within the last Gigayear. Therefore, such galaxies have received considerable attention in recent years, because they provide an opportunity to study galaxy evolution in action. These galaxies are commonly known as “post-starburst”, “E+A”, “k+a”, and Hδ-strong galaxies. Their study has been severely hampered by the lack of a large, statistical sample of such galaxies. In this paper, we rectify this problem by selecting a sample of galaxies which possess an absorption Hδ equivalent width of EW(Hδmax) − ∆EW(Hδmax) > 4 u A from 106682 galaxies in the SDSS. The measured abundance of our Hδ-strong (HDS) galaxies is 2.6 ±0.1% of all galaxies within a volumelimited sample of 0.05 < z < 0. 1a ndMr ∗ < −20.5, which is consistent with previous studies of such galaxies described in the literature. We find that only 25 of our HDS galaxies in this volume-limited sample (3.5 ±0.7%) show no, or little, evidence for [O II ]a nd Hα emission lines, thus indicating that true E+A galaxies (as originally defined by Dressler and Gunn) are extremely rare objects at low redshift, i.e., only 0.09 ±0.02% of all galaxies in this volume-limited sample are true E+A galaxies. In contrast, 89 ±5% of our HDS galaxies in the volume-limited sample have significant detections of the [O II ]a nd Hα emission lines. Of these, only 131 galaxies are robustly classified as Active Galactic Nuclei (AGNs) and, therefore, a majority of these emission-line HDS galaxies are starforming galaxies. We find that 52 ±12% (27/52) of the galaxies in our volume-limited HDS sample that possess no detectable [O II] emission, do however possess detectable Hα emission lines. These may be dusty star-forming galaxies. We provide the community with this new catalog of Hδ-strong galaxies to aid in our understanding of these galaxies, via detailed follow-up observations, as well as providing a low-redshift sample for comparison with higher redshift studies of HDS galaxies.

The Environment of Passive Spiral Galaxies in the SDSS

Abstract: In previous work on galaxy clusters, several authors reported the discovery of an unusual population of galaxies, which have spiral morphologies, but do not show any star-formation activity. These galaxies are called “passive spirals”, and have been interesting since it has been difficult to understand the existence of such galaxies. Using a volume-limited sample (0.05

Evolution of Dust-to-Metal Ratio in Galaxies

Abstract: This paper investigates the evolution of the dust-to-metal ratio in galaxies based on a simple evolution model for the amount of metal and dust with infall. We take into account grain formation in stellar mass-loss gas, grain growth by the accretion of metallic atoms in a cold dense cloud, and grain destruction by SN shocks. Especially, we propose that the accretion efficiency is independent of the star-formation history. This predicts various evolutionary tracks in the metallicity (Z)–dust-to-gas ratio (D) plane depending on the star-formation history. In this framework, the observed linear Z–D relation of nearby spiral galaxies can be interpreted as a sequence of a constant galactic age. We emphasize that an observational study of the Z–D relation of galaxies at z ∼ 1 is very useful to constrain the efficiencies of dust growth and destruction. We also suggest that the Lyman break galaxies at z ∼ 3 have a very low dust-to-metal ratio, typically 0.1. Although the effect of infall on the evolutionary tracks in the Z–D plane is quite small, the dispersion of the infall rate can disturb the Z–D relation with a constant galactic age.

Minkowski Functionals of SDSS Galaxies I : Analysis of Excursion Sets

Abstract: We present a first morphometric investigation of a preliminary sample from the SDSS of 154287 galaxies with apparent magnitude 14.5 < mr < 17.5 and redshift 0.001 < z < 0.4. We measure the Minkowski Functionals, which are a complete set of morphological descriptors. To account for the complicated wedge–like geometry of the present survey data, we construct isodensity contour surfaces from the galaxy positions in redshift space and employ two complementary methods of computing the Minkowski Functionals. We find that the observed Minkowski Functionals for SDSS galaxies are consistent with the prediction of a Λ–dominated spatially–flat Cold Dark Matter model with random–Gaussian initial conditions, within the cosmic variance estimated from the corresponding mock catalogue. We expect that future releases of the SDSS survey will allow us to distinguish morphological differences in the galaxy distribution with regard to different morphological type and luminosity ranges.

ATCA Monitoring Observations of 202 Compact Radio Sources in Support of the VSOP AGN Survey

Abstract: The Australia Telescope Compact Array (ATCA) has been used in support of the VLBI Space Observatory Programme (VSOP) space VLBI mission, to monitor the total and polarised flux densities of 202 objects that make up 83% of the VSOP all-sky Survey of compact extragalactic radio sources south of δ = + 10°. The primary goal of the ATCA observations is to provide information on the total and polarised emission from the compact components in these sources, for correlation with parameters obtained from VSOP imaging observations. These data represent the first high resolution, long timescale flux density monitoring observations of a large number of southern compact radio sources. In the future, comparison of the ATCA and VSOP data will be used to investigate relativistic beaming models and identify similarities or differences between the major classes of extragalactic radio sources. As an illustration of the scientific value of the ATCA data we undertake a comparison of the properties of the gamma-ray loud and gamma-ray quiet AGN in the southern component of the VSOP Survey sample, finding that in a flat-spectrum sub-sample the gamma-ray loud AGN are more variable than the gamma-ray quiet AGN.

Lyman Break Galaxies at z ∼ 5: Luminosity Function

Abstract: We present the results of a search for Lyman break galaxies (LBGs) at z » 5 in a 618 square-arcmin field including the Hubble Deep Field-North (HDF-N) taken by the Subaru Prime Focus Camera. Utilizing the published redshift data of the HDF-N and its flanking fields, the color selection criteria were chosen so that LBGs could be picked out most eciently and least contaminated by foreground objects. The numbers of detected LBG candidates were 96 in 23.0 • Ic (mag) • 24.5 and 310 in 23:0 • Ic • 25:5. There is a hint of a deficiency of bright blue galaxies, although it is not as clear as has been suggested for LBGs at z » 3 to 4. With 305 LBG candidates in a 575 square-arcmin field, the rest-frame UV luminosity function of LBGs at 4:4 < z < 5:3 was derived statistically by considering both the contamination by objects at the intermediate redshift and the incompleteness of the survey. The fraction of the contamination was estimated to be » 50% in the faintest magnitude range. The completeness of the survey was »80% at the bright part of the sample, and »20% in the faintest magnitude range (25:0

Enhanced HCN (1-0) Emission in the Type-1 Seyfert Galaxy NGC 1097

Abstract: The central kpc region of the low-luminosity type-1 Seyfert galaxy NGC 1097 has been observed in the CO (1–0) and HCN (1–0) lines with the Nobeyama Millimeter Array and the NRO 45m telescope. We have found a striking enhancement of the HCN emission toward the active nucleus of NGC 1097; a large fraction of the CO emission comes from a circumnuclear starburst ring (r ∼ 10 �� or 700pc at D =1 4.5Mpc), whereas the HCN emission is dominated by a strong unresolved peak at the nucleus. The HCN-to-CO integrated intensity ratio on the brightness temperature scale, RHCN/CO, is about 0.11 in the aperture of the 45 m telescope beams (15 �� for CO and 19 �� for HCN), but reaches 0.34 in the smaller aperture (10 �� ×4. 4). These CO and HCN properties in NGC 1097 are similar

Morphological Butcher–Oemler Effect in the SDSS “Cut and Enhance” Galaxy Cluster Catalog

Abstract: We investigated the evolution of the fractions of late-type cluster galaxies as a function of redshift using one of the largest, most uniform cluster samples available. The sample consisted of 514 clusters of galaxies in the range of 0.02≤z≤0.3 from the Sloan Digital Sky Survey “Cut and Enhance” galaxy cluster catalog. This catalog was created using a single automated cluster-finding algorithm applied to uniform data from a single telescope, with accurate CCD photometry, thus minimizing the selection biases. We used four independent methods to analyze the evolution of the late-type galaxy fraction. Specifically, we selected late-type galaxies based on each of the rest-frame g− r color, the u− r color, galaxy profile fitting, and the concentration index. The first criterion corresponds to that used in classical Butcher–Oemler analyses. The last two criteria are more sensitive to the morphological type of galaxies. In all four cases, we find an increase in the fraction of late-type galaxies with increasing redshift, significant at the 99.9% level. The results confirm that cluster galaxies do change colors with redshift (the Butcher–Oemler effect) and, in addition, they change their morphology to later-type toward a higher redshift — indicating a morphological equivalence of the Butcher–Oemler effect. We also found a tendency of richer clusters to have lower fractions of late-type galaxies. This trend is consistent with a ram-pressure stripping model, where galaxies in richer clusters are affected by stronger ram pressure due to the higher temperature of clusters.

A Systematic Study of X-Ray Flares from Low-Mass Young Stellar Objects in the $\rho$ Ophiuchi Star-Forming Region with Chandra

Abstract: We report on the results of a systematic study of X-ray flares from low-mass young stellar objects, using two deep exposure Chandra observations of the main region of the ρ Ophiuchi star-forming cloud. From 195 X-ray sources, including class I-III sources and some young brown dwarfs, we detected a total of 71 X-ray flares. Most of the flares have the typical profile of solar and stellar flares, fast rise and slow decay, while some bright flares show unusually long rise timescales. We derived the time-averaged temperature (� kT � ), luminosity (� LX� ), rise and decay timescales (τr and τd) of the flares, finding that (1) class I-II sources tend to have a highkT � , which sometimes exceeds 5keV, (2) the distribution ofLXduring flares is nearly the same for all classes from ∼10 29.5 to ∼10 31.5 ergs −1 , although there is a marginal hint of a higherLXdistribution for class I than class II-III, and (3) positive and negative log-linear correlations are found between τr and τd ,a ndkTand τr. In order to explain these relations, we used the framework of magnetic reconnection model with heat conduction and chromospheric evaporation to formulate the observational parameters (τr, τd ,a ndkT� ) as a function of the pre-flare (coronal) electronic density (nc), the half-length of the reconnected magnetic loop (L), and magnetic field strength (B). The observed correlations are well reproduced if loop lengths are nearly the same for all classes, regardless of the existence of an accretion disk. The estimated loop length is almost comparable to the typical stellar radius of these objects (10 10 -10 11 cm), which indicates that the observed flares are triggered by solar-type loops, rather than larger ones (∼ 10 12 cm) connecting the star with its inner accretion disk. The higherkTobserved for class I sources may be explained by a slightly higher magnetic field strength (≈ 500G) than for class II-III sources (200-300G).

Interferometric Observations of Formaldehyde in the Protoplanetary Disk around LkCa 15

Abstract: Emission from the 212–111 line of H2CO has been detected and marginally resolved toward LkCa 15 by the Nobeyama Millimeter Array. The column density of H2CO is higher than that observed in DM Tau and than predicted by theoretical models of disk chemistry; also, the line-intensity profile is less centrally peaked than that for CO. A similar behavior is observed in other organic gaseous molecules in the LkCa 15 disk.

Detectability of the Warm/Hot Intergalactic Medium through Emission Lines of O VII and O VIII

Abstract: Most of cosmic baryons predicted by the big-bang nucleosynthesis has evaded the direct detection. Recent numerical simulations indicate that approximately 30 to 50 percent of the total baryons in the present universe is supposed to take a form of warm/hot intergalactic medium (WHIM) whose X-ray continuum emission is very weak. To identify those missing baryons, we consider in detail the detectability of WHIM directly through emission lines of OVII (561, 568, 574, 665eV) and OVIII (653eV). For this purpose, we create mock spectra of the emission lines of WHIM using a light-cone output of the cosmological hydrodynamic simulations. Since the predicted fluxes are generally below the current detection limit, the unambiguous detection requires a dedicated X-ray satellite mission that we also discuss in detail. We find that our proposed mission is sensitive to the WHIM with gas temperature $T=10^{6-7}$K and overdensity $\\delta=10-100$ up to a redshift of 0.3 without being significantly contaminated by the cosmic X-ray background and the Galactic emissions. Thus such a mission provides a unique and important tool to identify a large fraction of otherwise elusive baryons in the universe.

First Fringe Detection with VERA's Dual-Beam System and Its Phase-Referencing Capability

Abstract: We present the results of the first dual-beam observations with VERA (VLBI Exploration of Radio Astrometry). The observations of a pair of H2O maser sources, W 49N and OH 43.8−0.1, were carried out on 2002 May 29 and July 23, and fringes of the H2O maser lines at 22GHz were successfully detected. While the residual fringe phases of both sources showed rapid variations over 360 ◦ due to the atmospheric fluctuation, the differential phase between the two sources remained constant for 1 hour with an r.m.s. of 8 ◦ , demonstrating that the atmospheric phase fluctuation was effectively removed by dual-beam phase referencing. An analysis based on the Allan standard deviation reveals that the differential phase is mostly dominated by white phase noise, and the coherence function calculated from the differential phase shows that after phase referencing the fringe visibility can be integrated for an arbitrarily long time. These results demonstrate VERA’s high capability of phase referencing, indicating that it is a promising tool for phase-referencing VLBI astrometry at 10 µas-level accuracy.

Review of discrete x-ray sources in the Small Magellanic Cloud: Summary of the ASCA results and implication on the recent star forming activity

Abstract: We made 22 observations on the Small Magellanic Cloud (SMC) and covered full regions of the main body and the eastern wing by the end of the ASCA mission. We detected 106 discrete sources with a criterion of S/N > 5 and performed systematic analyses on all of the sources. We determined the source positions with an ∼40 error radius (90% confidence) for sources detected in the central 20 radius of the GIS. We detected coherent pulsations from 17 sources. Among them, eight were newly discovered during this study. We classified most of these pulsars as X-ray binary pulsars (XBPs) based on their properties, such as the flux variability and the existence of an optical counterpart. We detected X-ray emission from eight supernova remnants (SNRs). Among them, five SNRs showed emission lines in their spectra, hence we regarded the five as thermal SNRs. We found that XBPs and thermal SNRs in the SMC can be clearly separated by their spectral hardness ratio. Applying this empirical law to faint (thus unclassified) sources, we found 19 XBP candidates and four thermal SNR candidates. We also found several tens of candidates for active galactic nuclei, both from the hardness ratio and the logN–logS relation of extragalactic sources. Based on these ASCA results and further information from ROSAT, SAX, RXTE, CGRO, Chandra, and XMM-Newton, we compiled comprehensive catalogues of discrete X-ray sources in the Small Magellanic Cloud. Using the catalogues, we 1 derived the spatial distributions of XBPs and SNRs. XBPs and SNRs were found to be concentrated in the main body and eastern wing, which resembles the distribution of young stars with ages of ∼ 2× 10 yr. By comparing the source populations in the SMC and our Galaxy, we suggest that the star-forming rate (per unit mass) in the SMC was much higher than the Galaxy ∼ 10 yr ago. We also discuss the recent change of the star-forming rate in the SMC.

Size and spatial distributions of sub-km Main-Belt Asteroids

Abstract: This paper presents the result of the first systematic investigation of very small Main-Belt Asteroids (sub-km MBAs) using the Subaru Prime-Focus Camera (Suprime-Cam) with an 8K ×10K mosaic CCD array attached to the 8.2m Subaru Telescope atop Mauna Kea, Hawaii. We call this survey SMBAS (Sub-km Main-Belt Asteroid Survey). Observations were carried out on 2001 February 22 and 25 (HST) and a ∼ 3.0deg 2 sky area near the opposition and near the ecliptic was searched. We detected 1111 moving objects down to R ∼ 26mag (including very slow TransNeptunian Objects). In this survey, we could not determine the exact orbits of the moving objects, because of their short observational arc of only 2 hours. Instead, we statistically estimated the semi-major axis (a) and inclination (I) of each moving object from its apparent sky-motion vector, and then obtained the size and spatial distributions of sub-km MBAs. The main results of SMBAS are: (1) The sky number density of MBAs is found to be ∼ 290deg −2 down to R ∼ 24.4mag (for MBAs) near the opposition and near the ecliptic. (2) The slope of the cumulative size distribution for sub-km MBAs ranging from 0.5km to 1km in diameter is fairly shallower (∼ 1.2) than that for large MBAs of more than∼ 5km in diameter (∼ 1.8), which was obtained from past asteroid surveys. This means that the number of sub-km MBAs is much more depleted than a result extrapolated from the size distributions for large asteroids. (3) The depletion of sub-km MBAs is clearer in the outer main-belt than in the inner main-belt. (4) It seems that SMBAS asteroids distribute more widely in the I-direction in the outer zone (a =2 .8–3.1AU) of the main-belt than known large asteroids do. We also discuss the possible causes for the characteristics of the distributions of SMBAS-observed small asteroids.

Spectral Energy Distributions of Dusty Galaxies

Abstract: We present a SED model of dusty galaxies, in which the equation of radiative transfer is solved by assuming spherical symmetry. The temperature fluctuation of very small dust particles is calculated consistently with the radiative transfer. The adopted dust model consists of graphite and silicate grains and PAHs, whose relative fractions are determined for each MW, LMC and SMC type extinction curve. This model allows us to derive the intrinsic SEDs of stellar populations embedded in dusty ISM, which are very important indicators for the age of stellar populations. Therefore, the evolutionary phase of starburst galaxies which have frequently very dusty ISM can be investigated with this SED model. We show that the SEDs of Arp220 and M82 can both be explained by the same single stellar population, despite the significant differences in the SEDs and the infrared luminosities. The apparent difference between their SEDs is mainly caused by the difference in the optical depth. In contrast, the SED of prototypical star-forming ERO, HR10, indicates that this galaxy is relatively old comparing to Arp220 and M82. It is found that, in the case of optically thin limit like elliptical galaxies, the optical depth cannot be inferred only from the SED, due to a degeneracy between the optical depth, galactic size, and the spatial distribution of dust; the latter two are important for estimating the average temperature of dust grains in elliptical galaxies. When the observed size of elliptical galaxies is adopted for the model geometry, SEDs can be used to constrain the spatial distribution of dust in elliptical galaxies.

Evolution of Elliptical Galaxies at z 1 Revealed from a Large, Multicolor Sample of Extremely Red Objects ∗

Abstract: We studied the evolution of elliptical galaxies atz 1 using a sample of 247 Extremely Red Objects (EROs) with R − Ks ≥ 3.35 (AB) and Ks ≤ 22.1 (AB) constructed from BV R izJHK s multicolor data of a 114arcmin 2 area in the Subaru/XMM-Newton Deep Field taken with the Subaru Telescope and the UH 2.2m telescope. By fitting template spectra of old galaxies (OGs) and young, dusty starbursts (DSs) to the multicolor data, we classified EROs into these two classes and estimated their redshifts. We found that 58% of the EROs in our sample belong to the OG class and that these OGs have a wide range of colors at any redshift. We derived rest-frame B-band luminosity functions of OGs in our sample at z = 1-1. 5a nd 1.5-2.5, and found that the number density of galaxies obeying passive evolution drops at z 1.5 by a factor of three or more. The spatial correlation length of OGs is estimated to be r0 8 h −1 Mpc, which is comparable to, or larger than, those of present-day early-type galaxies of similar luminosities. We also discussed properties of DSs in our data.

Another Interpretation of the Disk-Line Profile for the Seyfert 1 Galaxy MCG-6-30-15

Abstract: The Seyfert 1 galaxy MCG−6-30-15 is a representative object possessing a “disk-line” (a broad and skewed feature around 5–7 keV in an X-ray spectrum). Using the longest MCG−6-30-15 observation with ASCA, Matsumoto et al. (2003, PASJ, 55, 615) found an interesting time-variability: the RMS variability as a function of a time-bin-width decreases more rapidly in the disk-line energy band (5.0–6.6 keV) than those in the other energy bands, as the time-bin-width increases from ∼ 104 s to ∼ 105 s. In this paper, we argue that this energy-dependent change in the RMS variability as a function of the time-bin-width is difficult to be reproduced by simply considering the variability of a “disk-line”, but that the RMS variability of the continuum component should have an energy-dependent evolution associated with the time-bin-width change. Such an energy-dependent variability of the continuum component can be realized by introducing absorption components with the typical variation time scale longer than that of the continuum. In this interpretation, a significant part of the broad excess feature in 5–7 keV is considered to be the result of a transparent spectral window of the warm absorbers.

The Virgo High-Resolution CO Survey : I. CO Atlas

Abstract: We present the results of the Virgo high-resolution CO survey (ViCS) obtained with the Nobeyama Millimeter-wave Array (NMA). This survey was made in the course of a long-term project at Nobeyama from 1999 December through 2002 April. The objects were selected from Virgo cluster members, considering CO richness from single dish flux, mild inclination, and lack of strong tidal perturbations. The central 1 arc min regions (4.7 kpc) of 15 spiral galaxies were observed with resolutions of 2 to 5 arcsec and 10 to 20 km/s, and sensitivities of 20 mJy/ beam for a 10 km/s channel. The objects lie at the same distance of the Virgo cluster (16.1 Mpc), which is advantageous for comparisons among individual galaxies. We describe the details of observations and data reduction, and present an atlas of integrated CO intensity maps, velocity fields and position-velocity diagrams along the major axes. The molecular gas morphology in the Virgo galaxies shows a wealth of variety, not specifically depending on the Hubble types.

The Virgo High-Resolution CO Survey: II. Rotation Curves and Dynamical Mass Distributions

Abstract: Based on a high-resolution CO survey of Virgo spirals with the Nobeyama Millimeter-wave Array, we determined the dynamical centers using velocity fields, and derived position–velocity diagrams (PVDs) along the major axes of the galaxies across their dynamical centers. We applied a new iteration method to derive rotation curves (RCs), which reproduce the observed PVDs. The obtained high-accuracy RCs generally show a steep rise in the central ∼100 to 200pc regions, followed by flat rotation in the disk. We applied a deconvolution method to calculate the surface-mass density (SMD) using the RCs based on two extreme assumptions that the mass distribution is either spherical or thin-disk shaped. Both assumptions give nearly identical results, agreeing with each other within a fac�� )

Chandra Observation of the Anomalous X-Ray Pulsar 1E 1841-045

Abstract: We present the results from a Chandra ACIS CC mode observation of an anomalous X-ray pulsar (AXP) 1E 1841−045. This was the first observation in which the pulsar spectrum over a wide energy range was spatially discriminated from the surrounding SNR, Kes 73. Like other AXPs, the phase-integrated spectrum was fitted well with a power-law plus blackbody model. The spectral parameters are Γ = 2.0± 0.3, kTBB = 0.44± 0.02keV, and NH = 2.54+0.15 −0.13×10 cm−2. This photon index is the flattest among AXPs, and resembles soft gamma-ray repeaters (SGRs) in a quiescent state. The pulse profile is double-peaked, and we found that the second peak has a significantly hard spectrum. The spectra of all phases are consistent with a power-law plus blackbody model with a constant temperature and photon index. When fitted with a two-blackbody model, we obtained a similarly good fit. These results can be interpreted by saying that there are two emission regions with different energy spectra.

SiO Maser Survey of Cold IRAS Sources

Abstract: We present the results of observations of cold IRAS sources in the Galactic disk area, 10 < l < 100 and jbj < 5 , in the SiO J = 1{0, v = 1 and 2 maser lines. SiO masers were detected in 51 out of 143 observed sources; 45 were new detections in SiO masers. The selected IRAS sources were objects with dust temperatures of between 160 and 280 K. According to a conrmation using 2MASS near-infrared images, a majority of the sample are AGB or post-AGB stars, although dense cores in the starforming regions (or dusty Hii regions) are involved in part of the sample. Among new detections, two were candidates for post-AGB stars: IRAS 18450 0148 (W 43A), and 19312+1950. We found that the intensity ratios of the SiO J = 1{0, v = 2 to the v = 1 line of the objects clearly correlate with those IRAS colors. The detection rates of SiO masers tend to increase toward the Galactic center as well as the cases of previous SiO maser surveys of typical AGB stars. No strong associations of the objects to the spiral arms were found. The radial-velocity dispersion of the present sample is comparable with the dispersion of the SiO maser sample of typical AGB stars. These facts suggest that the present sample of cold IRAS sources with SiO masers has a kinematic property very similar with that of typical AGB stars.

Superhumps in a Rarely Outbursting SU UMa-Type Dwarf Nova, HO Delphini

Abstract: We observed the 1994, 1996, and 2001 outbursts of HO Del. From the detection of secure superhumps, HO Del is confirmed to be an SU UMa-type dwarf nova with a superhump period of 0.06453(6)d. Based on recent observations and past records, the outbursts of HO Del are found to be relatively rare, with the shortest intervals of superoutbursts being ∼740d. Among SU UMa-type dwarf novae with similar outburst intervals, the outburst amplitude (∼5.0mag) is unusually small. HO Del showed a rather rapid decay of the superhump amplitudes, and no regrowth of the amplitudes during the later stage, in contrast to the commonly observed behavior in SU UMa-type dwarf novae with long outburst intervals. We positively identified HO Del with a ROSAT X-ray source, and obtained a relatively large X-ray luminosity of 10 31.1±0.2 ergs −1 . We also performed a literature survey of SU UMa-type dwarf novae and presented a new set of basic statistics. The SU UMa-type dwarf novae with a brightening trend or with a regrowth of superhumps near the termination of a superoutburst are found to be rather tightly confined in a small region on the (superhump period–supercycle length) plane. These features may provide a better observational distinction for the previously claimed subgroup of dwarf novae (Tremendous Outburst Amplitude Dwarf Novae).

Relations between Galaxy Formation and the Environments of Quasars

Abstract: We investigate the environments of quasars using a semi-analytic model which includes both galaxy and quasar formations based on a hierarchical clustering scenario. We assume that a supermassive black hole is formed and fueled by the accretion of cold gas during a major merger of galaxies. Our model can reproduce not only the general form of the galaxy luminosity functions in the local Universe, but also the observed relation between a supermassive black hole mass and a spheroid luminosity, the present black hole mass function and the quasar luminosity functions at different redshifts. Using this model, we predict the mean number of quasars per halo, the bias parameter of quasars and the probability distribution of the number of galaxies around quasars. The results of our model show that the spatial distribution of galaxies is different from that of quasars. Furthermore, we found that at 0.2 z 0.5 most quasars are likely to reside in galaxy groups. On the other hand, at 1 z 2 most quasars seem to reside in environments ranging from small groups of galaxies to clusters of galaxies. Comparing these predictions with observations in the future will enable us to constrain our model of quasar formation.

Accretion Rates and the Accretion Efficiency in AGNs

Abstract: We used the standard geometrical-thin accretion theory to obtain the accretion rates in Seyfert 1 galaxies and quasars. Combining the accretion rates with the bolometric luminosity, we obtained the accretion efficiency. We found that most Seyfert 1 galaxies and radio-quiet quasars have lower accretion efficiencies, while most radio-loud quasars possess higher accretion efficiencies. This finding further implies that most of the radio-loud quasars possess Kerr black holes, while Seyfert 1 galaxies and radio-quiet quasars may not possess Kerr black holes. Considering the difference in the accretion efficiency, we found that there is a strong correlation between the accretion rate in units of the Eddington accretion rate and the width of the Hβ emission line, and that most AGNs are not accreting at super-Eddington rates.