arXiv subjects

Naoki Yasuda

Publications and source records attributed to Naoki Yasuda.

At least 91 records · Page 5Linked to original sources

arXiv2007

Spatial Variations of Galaxy Number Counts in the Sloan Digital Sky Survey. II. Test of Galactic Extinction in High Extinction Regions

Galactic extinction is tested using galaxy number counts at low Galactic latitude obtained from five band photometry of the Sloan Digital Sky Survey. The spatial variation of galaxy number counts for low extinction regions of $E(B-V) < 0.15$ is consistent with the all-sky reddening map of Schlegel, Finkbeiner, & Davis (1998) and the standard extinction law. For higher extinction regions of $E(B-V) > 0.15$, however, the map of Schlegel et al.(1998) overestimates the reddening by a factor up to 1.4, which is likely ascribed to the departure from proportionality of reddening to infrared emissivity of dust. This result is consistent with the analysis of Arce & Goodman (1999) for the Taurus dark cloud complex.

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arXiv2007

Wide-field Survey around Local Group Dwarf Spheroidal Galaxy Leo II: Spatial Distribution of Stellar Content

We carried out a wide-field V, I imaging survey of the Local Group dwarf spheroidal galaxy Leo II using the Subaru Prime Focus Camera on the 8.2-m Subaru Telescope. The survey covered an area of 26.67 x 26.67 arcmin^2, far beyond the tidal radius of Leo II (8.63 arcmin), down to the limiting magnitude of V ~26, which is roughly 1 mag deeper than the turn-off point of the main sequence stars of Leo II. Radial number density profiles of bright and faint red giant branch (RGB) stars were found to change their slopes at around the tidal radius, and extend beyond the tidal radius with shallower slopes. A smoothed surface brightness map of Leo II suggests the existence of a small substructure of globular cluster luminosity beyond the tidal radius. We investigated the properties of the stellar population by means of the color-magnitude diagram. The horizontal branch (HB) morphology index shows a radial gradient in which red HB stars are more concentrated than blue HB stars, which is common to many Local Group dwarf spheroidal galaxies. The color distribution of RGB stars around the mean RGB sequence shows a larger dispersion at the center than in the outskirts, indicating a mixture of stellar populations at the center and a more homogeneous population in the outskirts. Based on the age estimation using subgiant branch (SGB) stars, we found that although the major star formation took place ~8 Gyr ago, a considerable stellar population younger than 8 Gyr is found at the center; such a younger population is insignificant in the outskirts.

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arXiv2007

A Catalogue of Morphologically Classified Galaxies from the Sloan Digital Sky Survey: North Equatorial Region

We present a catalogue of morphologically classified bright galaxies in the north equatorial stripe (230 deg$^2$) derived from the Third Data Release of the Sloan Digital Sky Survey (SDSS). Morphological classification is performed by visual inspection of images in the $g$ band. The catalogue contains 2253 galaxies complete to a magnitude limit of $r=16$ after Galactic extinction correction, selected from 2658 objects that are judged as extended in the photometric catalogue in the same magnitude limit. 1866 galaxies in our catalogue have spectroscopic information. A brief statistical analysis is presented for the frequency of morphological types and mean colours in the catalogue. A visual inspection of the images reveals that the rate of interacting galaxies in the local Universe is approximately 1.5% in the $r\le16$ sample. A verification is made for the photometric catalogue generated by the SDSS, especially as to its bright end completeness.

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arXiv2007

The Kinematics of Intracluster Planetary Nebulae and the On-Going Subcluster Merger in the Coma Cluster Core

The Coma cluster is the richest and most compact of the nearby clusters, yet there is growing evidence that its formation is still on-going. With a new multi-slit imaging spectroscopy technique pioneered at the 8.2 m Subaru telescope and FOCAS, we have detected and measured the line-of-sight velocities of 37 intracluster planetary nebulae associated with the diffuse stellar population of stars in the Coma cluster core, at 100 Mpc distance. We detect clear velocity substructures within a 6 arcmin diameter field. A substructure is present at ~5000 km/s, probably from in-fall of a galaxy group, while the main intracluster stellar component is centered around ~6500 km/s, ~700 km/s offset from the nearby cD galaxy NGC 4874. The kinematics and morphology of the intracluster stars show that the cluster core is in a highly dynamically evolving state. In combination with galaxy redshift and X-ray data this argues strongly that the cluster is currently in the midst of a subcluster merger, where the NGC 4874 subcluster core may still be self-bound, while the NGC 4889 subcluster core has probably dissolved. The NGC 4889 subcluster is likely to have fallen into Coma from the eastern A2199 filament, in a direction nearly in the plane of the sky, meeting the NGC 4874 subcluster arriving from the west. The two inner subcluster cores are presently beyond their first and second close passage, during which the elongated distribution of diffuse light has been created. We predict the kinematic signature expected in this scenario, and argue that the extended western X-ray arc recently discovered traces the arc shock generated by the collision between the two subcluster gas halos. Any preexisting cooling core region would have been heated by the subcluster collision.

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arXiv2007

The Peculiar SN 2005hk: Do Some Type Ia Supernovae Explode as Deflagrations?

We present extensive u'g'r'i'BVRIYJHKs photometry and optical spectroscopy of SN 2005hk. These data reveal that SN 2005hk was nearly identical in its observed properties to SN 2002cx, which has been called ``the most peculiar known type Ia supernova.'' Both supernovae exhibited high ionization SN 1991T-like pre-maximum spectra, yet low peak luminosities like SN 1991bg. The spectra reveal that SN 2005hk, like SN 2002cx, exhibited expansion velocities that were roughly half those of typical type Ia supernovae. The R and I light curves of both supernovae were also peculiar in not displaying the secondary maximum observed for normal type Ia supernovae. Our YJH photometry of SN 2005hk reveals the same peculiarity in the near-infrared. By combining our optical and near-infrared photometry of SN 2005hk with published ultraviolet light curves obtained with the Swift satellite, we are able to construct a bolometric light curve from ~10 days before to ~60 days after B maximum. The shape and unusually low peak luminosity of this light curve, plus the low expansion velocities and absence of a secondary maximum at red and near-infrared wavelengths, are all in reasonable agreement with model calculations of a 3D deflagration which produces ~0.25 M_sun of 56Ni.

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arXiv2007

Kinematic Substructures in the Coma Cluster Core as traced by Intracluster Planetary Nebulae

The Coma cluster is the richest and most compact of the nearby clusters, yet there is growing evidence that its formation is still on-going. A sensitive probe of this evolution is the dynamics of intracluster stars, which are unbound from galaxies while the cluster forms, according to cosmological simulations. With a new multi-slit imaging spectroscopy technique pioneered at the 8.2 m Subaru telescope and FOCAS, we can now detect and measure the line-of-sight velocities of the intracluster planetary nebulae which are associated with the diffuse stellar population of stars, at 100 Mpc distance. We detect significant velocity substructures within a 6 arcmin diameter field, centred on the Coma X-ray cluster emission. One substructure ispresent at ~5000 km/s, probably from infall of a galaxy group, while the main intracluster stellar component moves at ~ 6500 km/s. Hence the ICPNs associated with the diffuse light at the position of the MSIS field are not bound to the nearby cD galaxy NGC 4874, whose radial velocity is ~ 700 km/s greater.

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arXiv2006

Constructing the Subaru Advanced Data and Analysis Service on VO

We present our activity on making the Subaru Data Archive accessible through the Japanese Virtual Observatory (JVO) system. There are a lot of demand to use the archived Subaru data from various fields of astronomers. To be used by those who are not familiar with the way to reduce the Subaru data, the data reduction should be made before providing for them, or at least it should be easily done without precise knowledge about instrument's characteristic and environment where data are taken. For those purposes, data quality assessment system NAQATA is developed, which is presented in this meeting by Nakata et al. (2006), and the science-ready image data are provided for some of the SuprimeCam data at SMOKA data service which is presented by Enoki et al (2006). JVO portal will provide the way to access the reduce data, and also provides the way to reduce from raw data with very few efforts through the user-friendly web browser I/F. To provide such a CPU-intensive service, we have developed a GRID computing system. The architecture of this Subaru Data and Analysis system are discussed.

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arXiv2006

SDSS J1029+2623: A Gravitationally Lensed Quasar with an Image Separation of 22.5 Arcseconds

We report the discovery of a cluster-scale lensed quasar, SDSS J1029+2623, selected from the Sloan Digital Sky Survey. The lens system exhibits two lensed images of a quasar at z_s=2.197. The image separation of 22.5" makes it the largest separation lensed quasar discovered to date. The similarity of the optical spectra and the radio loudnesses of the two components support the lensing hypothesis. Images of the field show a cluster of galaxies at z_l~0.55 that is responsible for the large image separation. The lensed images and the cluster light center are not collinear, which implies that the lensing cluster has a complex structure.

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arXiv2005

Morphological Classification of Galaxies using Photometric Parameters: the Concentration index versus the Coarseness parameter

We devise improved photometric parameters for the morphological classification of galaxies using a bright sample from the First Data Release of the Sloan Digital Sky Survey. In addition to using an elliptical aperture concentration index for classification, we introduce a new texture parameter, coarseness, which quantifies deviations from smooth galaxy isophotes. The elliptical aperture concentration index produces morphological classifications that are in appreciably better agreement with visual classifications than those based on circular apertures. With the addition of the coarseness parameter,the success rate of classifying galaxies into early and late types increases to \~88% with respect to the reference visual classification. A reasonably high success rate ~68% is also attained in classifying galaxies into three types, early-type galaxies (E+S0), early- (Sa+Sb) and late- (Sc+Sdm+Im) type spiral galaxies.

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arXiv2005

The line-of-sight velocity distributions of intracluster planetary nebulae in the Virgo cluster core

Radial velocities of 40 intracluster planetary nebulae (ICPNe) in the Virgo cluster were obtained with the new multi-fiber FLAMES spectrograph on UT2 at VLT. For the first time, the lambda 4959 AA line of the [OIII] doublet is seen in a large fraction (50%) of ICPNe spectra, and a large fraction of the photometric candidates with m(5007) <\= 27.2 is spectroscopically confirmed. ICPNe with the velocity dispersion of the Virgo cluster are found in our CORE field 1 deg from M87. These may have originated from tidal mass loss of smaller galaxies in the M87 subcluster halo. In a field 0.25 deg from M87, we see an extended stellar halo of M87 in approximate dynamical equilibrium, but with few ICPNe. Finally, in a field near M84/M86, the ICPNe velocities are highly correlated with the galaxy velocities, showing that any well-mixed intracluster population is yet to form. Overall, the measured velocity distributions confirm the non-uniform dynamical structure and on-going assembly of the Virgo cluster.(Based on data collected with the FLAMES spectrograph at the UT2 of the VLT at Cerro Paranal, Chile, operated by ESO, during observing run 71.B-0147(A))

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arXiv2005

Detection of Intracluster Planetary Nebulae in the Coma Cluster

[OIII] lambda 5007AA emission lines of 16 intracluster planetary nebulae candidates in the Coma cluster were detected with a Multi-Slit Imaging Spectroscopy (MSIS) technique using FOCAS on the Subaru telescope. The identification of these faint emission sources as PNe is supported by (i) their point-like flux distribution in both space and wavelength, with tight limits on the continuum flux; (ii) the identification of the second [OIII] lambda 4959 line in the only object at high enough velocity that this line too falls into the filter bandpass; (iii) emission line fluxes consistent with PNe at 100 Mpc distance, in the range 2.8 x 10^{-19} - 1.7 x 10^{-18} erg/s/cm^2; and (iv) a narrow velocity distribution approximately centered on the systemic velocity of the Coma cluster. Comparing with the velocities of galaxies in our field, we conclude that the great majority of these candidates would be intracluster PNe, free floating in the Coma cluster core. Their velocity dispersion is ~760 km/s, and their mean velocity is lower than that of the galaxies. The velocity distribution suggests that the intracluster stellar population has different dynamics from the galaxies in the Coma cluster core.

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arXiv2005

A Discovery of Rapid Optical Flares from Low-Luminosity Active Nuclei in Massive Galaxies

We report a serendipitous discovery of six very low-luminosity active galactic nuclei (AGNs) only by optical variability in one-month baseline. The detected flux variability is ~ 1-5% of the total luminosity of host galaxies. Careful subtraction of host galaxy components in nuclear regions indicates that the fractional variability (Delta F / F) of the nuclei is of order unity. At least one of them is showing a compelling flaring activity within just a few days, which appears to be quite different from previously known AGN variability. We obtained spectroscopic data for the one showing the largest flare and confirmed that it is in fact an AGN at z = 0.33 with an estimated black hole mass of ~10^8 M_sun. As a possible interpretation, we suggest that these activities are coming from the region around the black hole event horizon, which is physically similar to the recently discovered near-infrared flares of our Galactic nucleus. It is indicated that our Galaxy is not special, and that surprisingly rapid flaring activity in optical/near-infrared bands may be commonly hidden in nuclei of apparently normal galaxies with low Eddington ratios, in contrast to the variability of well-studied luminous AGNs or quasars.

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arXiv2004

Discovery of a Large Scale Clumpy Structure around the Lynx Supercluster at z~1.27

We report the discovery of a probable large scale structure composed of many galaxy clumps around the known twin clusters at z=1.26 and z=1.27 in the Lynx region. Our analysis is based on deep, panoramic, and multi-colour imaging (26.4'x24.1') in VRi'z'-bands with the Suprime-Cam on the 8.2m Subaru telescope. This unique, deep and wide-field imaging data-set allows us for the first time to map out the galaxy distribution in the highest redshift supercluster known. We apply a photometric redshift technique to extract plausible cluster members at z~1.27 down to i'=26.15 (5sigma) corresponding to M*+2.5 at this redshift. From the 2-D distribution of these photometrically selected galaxies, we newly identify seven candidates of galaxy groups or clusters where the surface density of red galaxies is significantly high (>5sigma), in addition to the two known clusters. These candidates show clear red colour-magnitude sequences consistent with a passive evolution model, which suggests the existence of additional high density regions around the Lynx superclusters.

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arXiv2004

Structured Query Language for Virtual Observatory

Currently two query languages are defined as standards for the Virtual Observatory (VO). Astronomical Data Query Language (ADQL) is used for catalog data query and Simple Image Access Protocol (SIAP) is for image data query. As a result, when we query each data service, we need to know in advance which language is supported and then construct a query language accordingly. The construct of SIAP is simple, but they have a limited capability. For example, there is no way to specify multiple regions in one query, and it is difficult to specify complex query conditions. In this paper, we propose a unified query language for any kind of astronomical database on the basis of SQL99. SQL is a query language optimized for a table data, so to apply the SQL to the image and spectrum data set, the data structure need to be mapped to a table like structure. We present specification of this query language and an example of the architecture for the database system.

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arXiv2003

Spatial Variations of Galaxy Number Counts in the SDSS I.: Extinction, Large-Scale Structure and Photometric Homogeneity

We study the spatial variation of galaxy number counts using five band photometric images from the Sloan Digital Sky Survey. The spatial variation of this sample of 46 million galaxies collected from 2200 sq. degrees can be understood as the combination of Galactic extinction and large-scale clustering. With the use of the reddening map of Schlegel, Finkbeiner & Davis (1998), the standard extinction law is verified for the colour bands from u to z within 5% in the region of small extinction values, E(B-V)<0.15. The residual spatial variations of the number counts suggests that the error of global calibration for SDSS photometry is smaller than 0.02 mag.

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arXiv2003

Searching for a Long Cosmic String Through the Gravitational Lensing Effect

It has been suggested that cosmic strings produced at a phase transition in the early universe can be the origin of the extremely high energy cosmic rays (EHCR) observed by AGASA above 10^20 eV. Superheavy cosmic strings with linear mass density of 10^22 g/cm can be indirectly observed through the gravitational lensing effect the distant galaxies. The lensing effect by a long straight object can be characterized by a line of double galaxies or quasars with angular separation of about 5 arcsec. We have searched for aligned double objects from the archived data taken by the Subaru Prime Focus Camera (Suprime-Cam). The Suprime-Cam has a great advantage in observing the wide field of view (30x30 arcmin^2) with high sensitivity (R<26 400s exposure), so it is suitable for this research. In this paper, we describe the result of simulation study for developing the method of searching the objects lensed by cosmic strings, and present the observational result obtained by this method.

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arXiv2002

Luminosity Function of Morphologically Classified Galaxies in the Sloan Digital Sky Survey

The morphological dependence of the luminosity function is studied using a sample containing approximately 1500 bright galaxies classified into Hubble types by visual inspections for a homogeneous sample obtained from the Sloan Digital Sky Survey (SDSS) northern equatorial stripes. Early-type galaxies are shown to have a characteristic magnitude by 0.45 mag brighter than spiral galaxies in the $r^{\ast}$ band, consistent with the `universal characteristic luminosity' in the $B$ band. The shape of the luminosity function differs rather little among different morphological types: we do not see any symptoms of the sharp decline in the faint end for the luminosity function for early-type galaxies at least 2 mag fainter than the characteristic magnitude, although the faint end behaviour shows a slight decline ($α\lsim -1$) compared with the total sample. We also show that a rather flat faint end slope for early-type galaxies is not due to an increasing mixture of the dwarf galaxies which have softer cores. This means that there are numerous faint early-type galaxies with highly concentrated cores.

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arXiv2002

Subaru Prime Focus Camera -- Suprime-Cam --

We have built an 80-mega pixels (10240 X 8192) mosaic CCD camera, called Suprime-Cam, for the wide-field prime focus of the 8.2 m Subaru telescope. Suprime-Cam covers a field of view 34' X 27', a unique facility among the the 8-10 m class telescopes, with a resolution of 0.''202 per pixel. The focal plane consists of ten high-resistivity 2k X 4k CCDs developed by MIT Lincoln Laboratory, which are cooled by a large stirling-cycle cooler. The CCD readout electronics was designed to be scalable, which allows the multiple read-out of tens of CCDs. It takes 50 seconds to readout entire arrays. We designed a filter-exchange mechanism of the jukebox type that can hold up to ten large filters (205 X 170 X 15 mm^3). The wide-field corrector is basically a three-lens Wynne-type, but has a new type of atmospheric dispersion corrector. The corrector provides a flat focal plane and an un-vignetted field of view of 30' in diameter. The achieved co-planarity of the focal array mosaic is smaller than 30 micron peak-to-peak, which realizes mostly the seeing limited image over the entire field. The median seeing in the Ic-band, measured over one year and a half, is 0.''61. The PSF anisotropy in Suprime-Cam images, estimated by stellar ellipticities, is about 2% under this median seeing condition. At the time of commissioning, Suprime-Cam had the largest survey speed, which is defined as the field of view multiplied by the primary mirror area of the telescope, among those cameras built for sub-arcsecond imaging.

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