106 research outputs found
An Objective and Automatic Cluster Finder: An Improvement of the Matched-Filter Method
We describe an objective and automated method for detecting clusters of
galaxies from optical imaging data. This method is a variant of the so-called
`matched-filter' technique pioneered by Postman et al. (1996). With
simultaneous use of positions and apparent magnitudes of galaxies, this method
can, not only find cluster candidates, but also estimate their redshifts and
richnesses as byproducts of detection. We examine errors in the estimation of
cluster's position, redshift, and richness with a number of Monte Carlo
simulations. No systematic discrepancies between the true and estimated values
are seen for either redshift or richness. Spurious detection rate of the method
is about less than 10% of those of conventional ones which use only surface
density of galaxies. A cluster survey in the North Galactic Pole is executed to
verify the performance characteristics of the method with real data. Two known
real clusters are successfully detected. We expect these methods based on
`matched-filter' technique to be essential tools for compiling large and
homogeneous optically-selected cluster catalogs.Comment: 13 pages, 12 PostScript figures, uses LaTeX L-AA, A&AS accepte
Positional Coincidence between the High-latitude Steady Unidentified Gamma-ray Sources and Possibly Merging Clusters of Galaxies
We report an evidence for the first time that merging clusters of galaxies
are a promising candidate for the origin of high galactic-latitude, steady
unidentified EGRET gamma-ray sources. Instead of using past optical catalogs of
eye-selected clusters, we made a matched-filter survey of galaxy clusters over
4\arcdeg \times 4\arcdeg areas around seven steady unidentified EGRET sources
at |b|>45\arcdeg together with a 100 \sq \arcdeg area near the South
Galactic Pole as a control field. In total, 154 Abell-like cluster candidates
and 18 close pairs/groups of these clusters, expected to be possibly merging
clusters, were identified within estimated redshift . Five
among the seven EGRET sources have one or two cluster pairs/groups (CPGs)
within 1\arcdeg from them. We assess the statistical significance of this
result by several methods, and the confidence level of the real excess is
maximally 99.8% and 97.8% in a conservative method. In contrast, we found no
significant correlation with single clusters. In addition to the spatial
correlation, we also found that the richness of CPGs associated with EGRET
sources is considerably larger than those of CPGs in the control field. These
results imply that a part of the steady unidentified EGRET sources at
high-latitude are physically associated with close CPGs, not with single
clusters. We also discuss possible interpretations of these results. We argue
that, if these associations are real, they are difficult to explain by hadronic
processes, but best explained by the inverse-Compton scattering by high energy
electrons accelerated in shocks of cluster formation, as recently proposed.Comment: 9 pages, 2 PostScript figures, uses emulateapj5.sty, added new
analysis and discussion, ApJ accepte
The Aquarius Superclusters - I. Identification of Clusters and Superclusters
We study the distribution of galaxies and galaxy clusters in a 10^deg x 6^deg
field in the Aquarius region. In addition to 63 clusters in the literature, we
have found 39 new candidate clusters using a matched-filter technique and a
counts-in-cells analysis. From redshift measurements of galaxies in the
direction of these cluster candidates, we present new mean redshifts for 31
previously unobserved clusters, while improved mean redshifts are presented for
35 other systems. About 45% of the projected density enhancements are due to
the superposition of clusters and/or groups of galaxies along the line of
sight, but we could confirm for 72% of the cases that the candidates are real
physical associations similar to the ones classified as rich galaxy clusters.
On the other hand, the contamination due to galaxies not belonging to any
concentration or located only in small groups along the line of sight is ~ 10%.
Using a percolation radius of 10 h^{-1} Mpc (spatial density contrast of about
10), we detect two superclusters of galaxies in Aquarius, at z = 0.086 and at z
= 0.112, respectively with 5 and 14 clusters. The latter supercluster may
represent a space overdensity of about 160 times the average cluster density as
measured from the Abell et al. (1989) cluster catalog, and is possibly
connected to a 40 h^{-1} Mpc filament from z ~ 0.11 to 0.14.Comment: LateX text (21 pages) and 12 (ps/eps/gif) figures; figures 5a, 5b and
6 are not included in the main LateX text; to be published in the
Astronomical Journal, March issu
Tobacco Root Endophytic Arthrobacter Harbors Genomic Features Enabling the Catabolism of Host-Specific Plant Specialized Metabolites
Plant roots constitute the primary interface between plants and soilborne microorganisms and harbor microbial communities called the root microbiota. Recent studies have demonstrated a significant contribution of plant specialized metabolites (PSMs) to the assembly of root microbiota. However, the mechanistic and evolutionary details underlying the PSM-mediated microbiota assembly and its contribution to host specificity remain elusive. Here, we show that the bacterial genus Arthrobacter is predominant specifically in the tobacco endosphere and that its enrichment in the tobacco endosphere is partially mediated by a combination of two unrelated classes of tobacco-specific PSMs, santhopine and nicotine. We isolated and sequenced Arthrobacter strains from tobacco roots as well as soils treated with these PSMs and identified genomic features, including but not limited to genes for santhopine and nicotine catabolism, that are associated with the ability to colonize tobacco roots. Phylogenomic and comparative analyses suggest that these genes were gained in multiple independent acquisition events, each of which was possibly triggered by adaptation to particular soil environments. Taken together, our findings illustrate a cooperative role of a combination of PSMs in mediating plant species-specific root bacterial microbiota assembly and suggest that the observed interaction between tobacco and Arthrobacter may be a consequence of an ecological fitting process
Experimental study on mitigation of liquefaction-induced vertical ground displacement by using gravel and geosynthetics
金沢大学理工研究域地球社会基盤学系BackgroundEarthquakes in liquefaction-prone areas are frequently followed by the settlement of surface structures due to subsoil liquefaction. This paper aims to study the influence of geosynthetics along with gravel usage to reduce the vertical soil displacement caused by liquefaction using a shake table equipment. This influence is analyzed by means of measuring soil acceleration, pore water pressures and vertical soil deformation due to the shaking process.ResultsResults of a series of 1-g shaking table tests which have been conducted in different initial relative densities which are 50% (loose sand conditions) and 90% (dense sand conditions) to evaluate the performance of proposed mitigation against settlement problem are presented. It is found that ground settlement reduced around 11.4 mm for loose sand conditions, from 20.9 mm in the case with no countermeasure (Case 1) to 9.5 mm in the Case reinforced with gravel and geosynthetics Type II (Case 4). Correspondingly, for dense sand states, the settlement decreased by about 1.8 mm, from 5.6 mm in the Case 1 to 3.8 mm in Case 4. Moreover, a differential settlement between loose sand and dense sand conditions decreased as well, around 9.6 mm, from 15.3 mm in the Case 1 to 5.7 mm in Case 4.ConclusionsBy conducted a series of shake table tests, it is confirmed that the vertical ground displacement decreased by the use of geosynthetics and gravel up to 54% and 32% for loose sand and dense sand states, respectively. Furthermore, test results also show that there is a decrease in the differential settlement between loose sand and dense sand conditions, around 62%
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.Comment: 20 pages with 5 postscript figures, uses aaspp4.sty, Ap.J. Accepte
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 10^{44}
erg s^{-1}) absorbed narrow-line AGNs (so called type 2 QSOs).Comment: Accepted for publication in ApJ. 57 pages with 13 figures, 9 JPG
plates, 5 additional PS tables. Original EPS plates (gzipped format
~1Mbyte/plate) and TeX tables are available from
ftp://ftp.kusastro.kyoto-u.ac.jp/pub/akiyama/0001289
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