arXiv subjects

T. Yamada

Publications and source records attributed to T. Yamada.

160 records · Page 9Linked to original sources

arXiv2003

Subaru Deep Survey. IV. Discovery of a Large-Scale Structure at Redshift \simeq 5

We report the discovery of a large-scale structure of Lyman alpha emitters (LAEs) at z=4.86 based on wide-field imaging with the prime-focus camera (Suprime-Cam) on the Subaru telescope. We observed a 25' x 45' area of the Subaru Deep Field in a narrow band (NB, lambdaC=7126 A and FWHM=73 A) together with R and i'. We isolate from these data 43 LAE candidates down to NB=25.5 mag using color criteria. Follow-up spectroscopy of five candidates suggests the contamination by low-z objects to be ~ 20%. We find that the LAE candidates are clustered in an elongated region on the sky of 20 Mpc in width and 50 Mpc in length at z=4.86, which is comparable in size to present-day large-scale structures (we adopt H0=70 km/s/Mpc, Omega0=0.3, lambda0=0.7). This elongated region includes a circular region of 12 Mpc radius of higher surface overdensity (delta=2), which may be the progenitor of a cluster of galaxies. Assuming this circular region to be a sphere with a spatial overdensity of 2, we compare our observation with predictions by Cold Dark Matter models. We find that an Omega0=0.3 flat model with sigma8=0.9 predicts the number of such spheres consistent with the observed number (one sphere in our survey volume) if the bias parameter of LAEs is b \simeq 6. This value suggests that the typical mass of dark haloes hosting LAEs at z \simeq 5 is of the order of 10^12 Msolar. Such a large mass poses an interesting question about the nature of LAEs.

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arXiv2002

Cosmic star formation history associated with QSO activity: an approach using the black hole to bulge mass correlation

The tight correlation between the masses of central black holes and their host spheroids in nearby galaxies and active galactic nuclei suggests that black hole growth is closely related to their spheroid formation. Based on our previous work regarding such a joint evolutionary scheme and the consequential black hole to bulge mass correlation, we use the X-ray luminosity function of AGN and the cosmological evolution rate which are from ROSAT X-ray surveys to estimate the cosmic star formation history associated with the black hole growth. By the Basic assumption that the major black hole growth occurs during the luminous AGN phase, the luminosity function of AGNs as a function of redshift traces not only the accretion history of the black holes but also the cosmic star formation history of the spheroids. Although the space density of the especially luminous QSOs is very low, we show that the total amount of star formation associated with the massive black hole growth is almost the same as that of Lyman Break Galaxies detected by the current optical deep surveys. We thus argue that the optical deep surveys may miss about half of the net star formation in our Universe, This is probably due to in part significant dust extinction as well as the small field of view of previous optical surveys which cannot sample such rare events with relatively short time scale. However, the far infrared emission from the dust heated by star fomation on-going during the black hole growth could sufficiently account for the observed SCUBA number counts, and would be the probable dominating energy source of the SUBA population.

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arXiv2002

Optical Spectropolarimetry of SN 2002ap: High Velocity Asymmetric Explosion

We present spectropolarimetry of the Type Ic supernova SN 2002ap and give a preliminary analysis: the data were taken at two epochs, close to and one month later than the visual maximum (2002 February 8). In addition we present June 9 spectropolarimetry without analysis. The data show the development of linear polarization. Distinct polarization profiles were seen only in the O I λ7773 multiplet/Ca II IR triplet absorption trough at maximum light and in the Ca II IR triplet absorption trough a month later, with the latter showing a peak polarization as high as ~2 %. The intrinsic polarization shows three clear position angles: 80 degs for the February continuum, 120 degs for the February line feature, and 150 degs for the March data. We conclude that there are multiple asymmetric components in the ejecta. We suggest that the supernova has a bulk asymmetry with an axial ratio projected on the sky that is different from 1 by of order 10 %. Furthermore, we suggest very speculatively that a high velocity ejecta component moving faster than ~0.115c (e.g., a jet) contributes to polarization in the February epoch.

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arXiv2002

Witnessing the Hierarchical Assembly of the Brightest Cluster Galaxy in a Cluster at z=1.26

We have obtained a new high-resolution K'-band image of the central region of the rich X-ray cluster RX J0848.9+4452 at z=1.26. We found that the brightest cluster galaxy (BCG) in the cluster is clearly separated into two distinct objects. Whereas the optical to near-infrared colors of the objects are consistent with the predictions of passive evolution models for galaxies formed at high redshift, the luminosities of the two galaxies are both considerably fainter than predicted by passive evolution of BCG's in low and intermediate redshift clusters. We argue that this is evidence of an on-going merger of normal cluster ellipticals to form the dominant galaxy in the core of RX J0848.9+4452. The two galaxies appear to point towards the nearby cluster ClG J0848+4453 and are aligned with the outer X-ray contour of their parent cluster, supporting a model of BCG formation by collimated infall along the surrounding large-scale structure.

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arXiv2002

Discovery of the host galaxy of HDF850.1, the brightest sub-mm source in the Hubble Deep Field

Despite extensive observational efforts, the brightest sub--mm source in the Hubble Deep Field, HDF850.1, has failed to yield a convincing optical/infrared identification almost 4 years after its discovery. This failure is all the more notable given the availability of supporting multi-frequency data of unparalleled depth, and sub-arcsec positional accuracy for the sub-mm/mm source. Consequently, HDF850.1 has become a test case of the possibility that the most violently star-forming objects in the universe are too red and/or distant to be seen in the deepest optical images. Here we report the discovery of the host galaxy of HDF850.1. This object has been revealed by careful analysis of a new, deep K-prime image of the HDF obtained with the Subaru 8.2-m telescope. Its reality is confirmed by a similar analysis of the HST NICMOS F160W image of the same region. This object is extremely faint (K=23.5), clumpy (on sub-arcsec scales) and very red (I-K > 5.2; H-K = 1.4 +- 0.35). The likelihood that it is the correct identification is strongly reinforced by a reanalysis of the combined MERLIN+VLA 1.4-GHz map of the field which yields a new radio detection of HDF850.1 only 0.1 arcsec from the new near-ir counterpart, and with sufficient positional accuracy to exclude all previously considered alternative optical candidates. We have calculated new confidence limits on the estimated redshift of HDF850.1 and find z = 4.1 +- 0.5. We also calculate that the flux density of HDF850.1 has been boosted by a factor of ~3 through lensing by the intervening elliptical 3-586.0, consistent with predictions that a small but significant fraction of blank-field sub-mm sources are lensed by foreground galaxies. We discuss the wider implications of these results for the sub-mm population and cosmic star-formation history.

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arXiv2001

`` $Λ- Σ$ conversion in $^4_Λ$He and $^4_Λ$H based on four-body calculation

Precise four-body calculations for $^4_Λ$He and $^4_Λ$H have been performed in the framework of the variational method with Jacobian-coordinate Gaussian-basis functions. All the rearrangement channels of both $NNN Λ$ and $NNN Σ$ are explicitly taken into account for the first time with the use of realistic $NN$ and $YN$ interactions. The role of $Λ$-$Σ$ conversion and the amount of the virtual $Σ$-component in $^4_Λ$He and $^4_Λ$H are discussed.

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arXiv2001

JHK Spectra of the z=2.39 Radio Galaxy 53W002

We present low-resolution, near-IR JHK spectra of the weak z=2.39 radio galaxy 53W002, obtained with the OH-airglow Suppressor spectrograph (OHS) and Cooled Infrared Spectrograph and Camera for OHS (CISCO) on the Subaru Telescope. They cover rest-frame wavelengths of 3400-7200 A, and the emission lines of [O II]3727, Hb, [O III]4959, 5007, Ha, [N II]6548, 6583 and [S II]6716, 6731 were detected. Using the Ha/Hb line ratio, we find an extinction of E(B-V)=0.14. The emission-line ratios are reproduced by a cloud of electron density n_e=1x10^{3-4}(/cm3) with solar metallicity, ionized by an alpha=-0.7 power-law continuum with ionizing parameter U=1x10^-3. In addition to these emission lines, we make the first spectroscopic confirmation of the Balmer discontinuity in a high-z radio galaxy. Together with rest-frame UV photometry from the literature, we show that at least 1/3 of the present stellar mass was formed in the current starburst. The stellar mass was estimated to be (1-1.4)x10^11 M_sol by one-component model fitting, which is smaller than that of typical z~1 B2/6C radio galaxies. We suggest that 53W002 is currently assembling a large part of its stellar mass through merger events with the surrounding sub-galactic clumps, some of which can be identified with the Lya emitters detected in narrow-band imaging. After a few such events over the next few Gyr, 53W002 will evolve into a massive elliptical galaxy.

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arXiv2001

JHK Spectroscopy of the z=2.39 Radio Galaxy 53W002 and Its Companions

We present low-resolution, near-IR JHK spectra of the weak z=2.39 radio galaxy 53W002 and its companion objects #18 and #19, obtained with OHS/CISCO on the Subaru Telescope. They cover rest-frame wavelengths of 3400-7200A, and many rest-optical emission lines are detected. Contributions to the broad-band flux from these emission lines are found to be very large, up to 40% in the H and K'-bands and 30% in the J-band.

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arXiv2000

Spectroscopic confirmation of a cluster of galaxies at z=1 in the field of the gravitational lens MG2016+112

We present new optical data on the cluster AX J2019+1127 identified by the X-ray satellite ASCA at z\sim 1 (Hattori et al. 1997). The data suggest the presence of a high-redshift cluster of galaxies responsible for the large separation triple quasar MG2016+112. Our deep photometry reveals an excess of z\sim 1 galaxy candidates, as already suspected by Benitez et al. (1999). Our spectroscopic survey of 44 objects in the field shows an excess of 6 red galaxies securely identified at z \sim 1, with a mean redshift of z =1.005 +/- 0.002. We estimate a velocity dispersion of σ= 771 (+430/-160) km s(-1) based on these 6 galaxies and a V-band mass-to-light ratio of 215 (+308/-77) h_50 M/L_sol. Our observations thus confirm the existence of a massive structure acting as the lens, which explains the unusual configuration of the triple quasar. Hence, there is no more need to invoke the existence of a ``dark cluster'' to understand this lens system.

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arXiv2000

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 (0.5 < z < 2) and luminous (Lx(2-10 keV) > 10^{44} erg s^{-1}) absorbed narrow-line AGNs (so called type 2 QSOs).

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arXiv1999

Emission-Line Properties of MG2016+112: A Luminous Type-2 Quasar at High Redshift

We present new high signal-to-noise ratio spectra of the components B and C of the gravitational lensing system MG2016+112. We show that image C displays strong emission lines of Ly$α$, \NV, \CIV, \HeII, and \CIII redshifted to z=3.27, similar to images and B. We examine the emission-line flux ratios in order to put constraints on the lens models as well as to investigate the intrinsic nature of MG2016+112. The observed line ratios of B and C are consistent with those expected in the simple photo-ionization models for narrow-line region of active galactic nuclei (AGN) except for the enhanced \NV\ lines. The line ratios difference of components B and C can be interpreted as a difference in ionization parameters. This result is consistent with lens model prediction that C is a fold image of a slightly outer part of the nucleus. MG2016+112 is known to be very unique among the high-redshift AGN; it is neither an ordinary broad-line quasar nor a powerful radio galaxy as indicated by the width and flux ratio of the emission lines. Together with other observed properties discussed in literature, we argue that MG2016+112 is the highest redshift luminous radio-quiet type-2 quasar.

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arXiv1999

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 square degrees (Large Sky Survey; LSS). To make the best use of ASCA capability, we developed a new source-detection method where 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) band, we detected 107 sources altogether with sensitivity limits of 6 x 10E-14 (0.7-7 keV), 1 x 10E-13 (2-10 keV), and 2 x 10E-14 erg sE-1 cmE-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 effects of the source confusion and accurately derived 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 the GINGA and HEAO1 results with the Euclidean slope of -3/2, while that in the soft band is consistent with the results by ROSAT. At these flux limits, 30 (+/- 3) percent of the CXB in the 0.7-7 keV band and 23 (+/- 3) percent 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 the soft energy band. 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 contribution of sources with hard energy spectra become significant at a flux of 10E-13 erg sE-1 cmE-2 (2-10 keV). The most plausible candidates are type-II AGNs, as indicated by on-going optical identifications.

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arXiv1998

Nature of Hard X-ray Source from Optical Identification of the ASCA Large Sky Survey

We present results of optical identification of the ASCA Large Sky Survey. X-ray sources which have hard X-ray spectra were identified with type-2 AGN at redshifts smaller than 0.5. It is supported that the absorbed X-ray spectrum of type-2 AGN makes the Cosmic X-ray Background harder in the hard X-ray band than type-1 AGN, which is main contributer in the soft X-ray band. Absence of type-2 AGN at redshift larger than 1 in the identified sample, which contrasts to the existence of 6 broad-line QSOs, suggests a deficiency of so-called ``type-2 QSO'' at high redshift.

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arXiv1998

Optical Identification of the Hardest X-ray Source in the ASCA Large Sky Survey

We report the optical identification of the hardest X-ray source (AX J131501+3141) detected in an unbiased wide-area survey in the 0.5--10 keV band, the ASCA Large Sky Survey. The X-ray spectrum of the source is very hard and is well reproduced by a power-law component (Gamma = 1.5^+0.7_-0.6) with N_H = 6^+4_-2 *10^22 cm^-2 (Sakano et al. 1998). We have found a galaxy with R=15.62 mag near the center of the error circle for the X-ray source. The optical spectrum of the galaxy shows only narrow emission lines whose ratios correspond to those of a type 2 Seyfert galaxy at z = 0.072, implying an absorption-corrected X-ray luminosity of 2*10^43 erg sec^-1 (2--10 keV) and M_B = -20.93 mag. A radio point source is also associated with the center of the galaxy. We thus identify the X-ray source with this galaxy as an obscured AGN. The hidden nature of the nucleus of the galaxy in the optical band is consistent with the X-ray spectrum. These results support the idea that the obscured AGNs/QSOs contribute significantly to the cosmic X-ray background in the hard band at the faint flux level.

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arXiv1997

Clustering of red Galaxies near the Radio-loud Quasar 1335.8+2834 at z=1.1

We have obtained new deep optical and near-infrared images of the field of the radio-loud quasar 1335.8+2834 at $z=1.086$ where an excess in the surface number density of galaxies was reported by Hutchings et al. [AJ, 106, 1324] from optical data. We found a significant clustering of objects with very red optical-near infrared colors, $4 \lesssim R-K \lesssim 6$ and $3 \lesssim I-K \lesssim 5$ near the quasar. The colors and magnitudes of the reddest objects are consistent with those of old (12 Gyr old at z=0) passively-evolving elliptical galaxies seen at $z=1.1$, clearly defining a `red envelope' like that found in galaxy clusters at similar or lower redshifts. This evidence strongly suggests that the quasar resides in a moderately-rich cluster of galaxies (richness-class $\geq 0$). There is also a relatively large fraction of objects with moderately red colors ($3.5 < R-K < 4.5$) which have a distribution on the sky similar to that of the reddest objects. They may be interpreted as cluster galaxies with some recent or on-going star formation.

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arXiv1995

Distribution of Blue Galaxies in a Merging Cluster of Galaxies Abell 168

The fraction of star-forming galaxies in rich clusters of galaxies increases rapidly with the redshift. This is interpreted as the result of a rapid evolution of cluster galaxies, though its mechanism is not yet clear. One hypothesis is that if galaxies run into the dense intracluster medium (ICM) regions, starbursts can be induced due to compression of molecular clouds in the galaxies by a raised "external" ICM pressure. In a merging cluster, there would be dense ICM regions and some galaxies may experience a rapid increase of external pressure. Thus, if the mechanism works, starbursts would occur in such galaxies. Such a situation is considered to be realized in Coma cluster, and galaxies showing a recent starburst have a characteristic spatial distribution; they populate an elongated region between two sub-clusters. We examined the above hypothesis by measuring the spatial distribution of blue galaxies, regarded as star-forming galaxies in Abell~168 to check whether a case like Coma is realized in this recently merging cluster. However, we could not find an enhanced blue-galaxy distribution between two sub-clusters. We discuss the causes of our result, including possibilities of surviving the hypothesis.

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