1,794 research outputs found
Entropy and Long range correlations in literary English
Recently long range correlations were detected in nucleotide sequences and in
human writings by several authors. We undertake here a systematic investigation
of two books, Moby Dick by H. Melville and Grimm's tales, with respect to the
existence of long range correlations. The analysis is based on the calculation
of entropy like quantities as the mutual information for pairs of letters and
the entropy, the mean uncertainty, per letter. We further estimate the number
of different subwords of a given length . Filtering out the contributions
due to the effects of the finite length of the texts, we find correlations
ranging to a few hundred letters. Scaling laws for the mutual information
(decay with a power law), for the entropy per letter (decay with the inverse
square root of ) and for the word numbers (stretched exponential growth with
and with a power law of the text length) were found.Comment: 8 page
The colour-magnitude relations of ClJ1226.9+3332, a massive cluster of galaxies at z=0.89
(Abridged) The colour-magnitude relations of one of the most massive, high
redshift clusters of galaxies known have been studied. Photometry has been
measured in the V, R, I, z, F606W, F814W, J and K bands to a depth of K*+2.5
and spectroscopy confirms 27 K band selected cluster members. The V-K colours
are equivalent to a rest-frame colour of ~2700A-J, and provide a very sensitive
measure of star-formation activity. HST ACS imaging has been used to
morphologically classify the galaxies.
The cluster has a low early-type fraction compared to nearby clusters, with
only 33% of the cluster members having types E or S0. The early-type member
galaxies form a clear red-sequence in all colours. The scatter and slope of the
relations show no evolution compared to the equivalent Coma cluster relations,
suggesting the stellar populations are already very old. The normalisation of
the relations has been compared to models based on synthetic stellar
populations, and are most consistent with stellar populations forming at z>3.
Some late-type galaxies were found to lie on the red-sequence, suggesting that
they have very similar stellar populations to the early-types.
These results present a picture of a cluster in which the early-type galaxies
are all old, but in which there must be future morphological transformation of
galaxies to match the early-type fraction of nearby clusters. In order to
preserve the tight colour-magnitude relation of early-types seen in nearby
clusters, the late-type galaxies must transform their colours, through the
cessation of star-formation, before the morphological transformation occurs.
Such evolution is observed in the late-types lying on the colour-magnitude
relation.Comment: Accepted for publication in MNRAS. 14 pages, 5 figure
The WARPS survey: III. The discovery of an X-ray luminous galaxy cluster at z=0.833 and the impact of X-ray substructure on cluster abundance measurements
The WARPS team reviews the properties and history of discovery of
ClJ0152.7-1357, an X-ray luminous, rich cluster of galaxies at z=0.833. At L_X
= 8 x 10^44 h^(-2) erg/s (0.5-2.0 keV) ClJ0152.7-1357 is the most X-ray
luminous cluster known at redshifts z>0.55. The high X-ray luminosity of the
system suggests that massive clusters may begin to form at redshifts
considerably greater than unity. This scenario is supported by the high degree
of optical and X-ray substructure in ClJ0152.7-1357, which is similarly complex
as that of other X-ray selected distant clusters and consistent with the
picture of cluster formation by mass infall along large-scale filaments. X-ray
emission from ClJ0152.7-1357 was detected already in 1980 with the EINSTEIN
IPC. However, because the complex morphology of the emission caused its
significance to be underestimated, the corresponding source was not included in
the EMSS cluster sample and hence not previously identified. Simulations of the
EMSS source detection and selection procedure suggest a general bias of the
EMSS against X-ray luminous clusters with pronounced substructure. If highly
unrelaxed, merging clusters are common at high redshift, they could create a
bias in some samples as the morphological complexity of mergers may cause them
to fall below the flux limit of surveys that assume a unimodal spatial source
geometry. Conversely, the enhanced X-ray luminosity of mergers might cause them
to, temporarily, rise above the flux limit. Either effect could lead to
erroneous conclusions about the evolution of the comoving cluster space
density. A high fraction of morphologically complex clusters at high redshift
would also call into question the validity of cosmological studies that assume
that the systems under investigation are virialized.Comment: 17 pages, 7 figures; revised to focus on possible detection biases
caused by substructure in clusters; accepted for publication in ApJ; uses
emulateapj.sty; eps files of figures 1 and 2 can be obtained from
ftp://hubble.ifa.hawaii.edu/pub/ebeling/warp
The WARPS Survey. VIII. Evolution of the Galaxy Cluster X-ray Luminosity Function
We present measurements of the galaxy cluster X-ray Luminosity Function (XLF)
from the Wide Angle ROSAT Pointed Survey (WARPS) and quantify its evolution.
WARPS is a serendipitous survey of the central region of ROSAT pointed
observations and was carried out in two phases (WARPS-I and WARPS-II). The
results here are based on a final sample of 124 clusters, complete above a flux
limit of 6.5 10E-15 erg/s/cm2, with members out to redshift z ~ 1.05, and a sky
coverage of 70.9 deg2. We find significant evidence for negative evolution of
the XLF, which complements the majority of X-ray cluster surveys. To quantify
the suggested evolution, we perform a maximum likelihood analysis and conclude
that the evolution is driven by a decreasing number density of high luminosity
clusters with redshift, while the bulk of the cluster population remains nearly
unchanged out to redshift z ~ 1.1, as expected in a low density Universe. The
results are found to be insensitive to a variety of sources of systematic
uncertainty that affect the measurement of the XLF and determination of the
survey selection function. We perform a Bayesian analysis of the XLF to fully
account for uncertainties in the local XLF on the measured evolution, and find
that the detected evolution remains significant at the 95% level. We observe a
significant excess of clusters in the WARPS at 0.1 < z < 0.3 and LX ~ 2 10E42
erg/s compared with the reference low-redshift XLF, or our Bayesian fit to the
WARPS data. We find that the excess cannot be explained by sample variance, or
Eddington bias, and is unlikely to be due to problems with the survey selection
function.Comment: 13 pages, 12 figures, accepted for publication in MNRA
Images, structural properties and metal abundances of galaxy clusters observed with Chandra ACIS-I at 0.1<z<1.3
We have assembled a sample of 115 galaxy clusters at 0.1<z<1.3 with archived
Chandra ACIS-I observations. We present X-ray images of the clusters and make
available region files containing contours of the smoothed X-ray emission. The
structural properties of the clusters were investigated and we found a
significant absence of relaxed clusters (as determined by centroid shift
measurements) at z>0.5. The slope of the surface brightness profiles at large
radii were steeper on average by 15% than the slope obtained by fitting a
simple beta-model to the emission. This slope was also found to be correlated
with cluster temperature, with some indication that the correlation is weaker
for the clusters at z>0.5. We measured the mean metal abundance of the cluster
gas as a function of redshift and found significant evolution, with the
abundances dropping by 50% between z=0.1 and z~1. This evolution was still
present (although less significant) when the cluster cores were excluded from
the abundance measurements, indicating that the evolution is not solely due to
the disappearance of relaxed, cool core clusters (which are known to have
enhanced core metal abundances) from the population at z>0.5.Comment: 23 pages, 12 figures. Accepted for publication in ApJS. Updated to
match published version. Redshifts of two clusters (RXJ1701 and CL0848)
corrected and two observations of MACSJ0744.8 have been combined into one.
Conclusions unchanged. A version with images of all of the clusters is
available at http://hea-www.harvard.edu/~bmaughan/clusters.htm
The ROSAT All-Sky Survey: a Catalog of Clusters of Galaxies in a Region of 1 Ster around the South Galactic Pole
A field of 1.013 ster in the ROSAT all-sky survey (RASS), centered on the
south galactic pole (SGP), has been searched in a systematic, objective manner
for clusters of galaxies. The procedure relied on a correlation of the X-ray
positions and properties of ROSAT sources with the distribution of galaxies in
the COSMOS digitised data base, which was obtained by scanning the plates of
the UK Schmidt IIIa-J optical southern sky survey. The study used the second
ROSAT survey data base (RASS-2) and included several optical observing
campaigns to measure redshifts. The search, a precursor to the larger REFLEX
survey of the whole southern sky, reached the detection limits of the RASS and
the COSMOS data, and yielded a catalog of 186 clusters in which the lowest flux
is 1.5e-12 erg/cm2/s in the 0.1-2.4 keV band. Of these 157 have measured
redshifts. Using a limit of 3.0e-12 erg/cm2/s a complete subset of 112 clusters
was obtained, of which 110 have measured redshifts. The spatial distribution of
the X-ray clusters out to z = 0.15 shows an extension of the Local Supercluster
to the Pisces-Cetus supercluster (z<~0.07), and a more distant orthogonal
structure at 0.07<z<0.15.Comment: To be published in ApJ Supplements in February 2002: 53 pages: 18
figure
A Flux-limited Sample of Bright Clusters of Galaxies from the Southern Part of the ROSAT All-Sky Survey: the Catalog and the LogN-LogS
We describe the selection of an X-ray flux-limited sample of bright clusters
of galaxies in the southern hemisphere, based on the first analysis of the
ROSAT All-Sky Survey data (RASS1). The sample is constructed starting from an
identification of candidate clusters in RASS1, and their X-ray fluxes are
remeasured using the Steepness Ratio Technique. This method is better suited
than the RASS1 standard algorithm for measuring flux from extended sources. The
final sample is count-rate-limited in the ROSAT hard band (0.5-2.0 keV), so
that due to the distribution of NH, its effective flux limit varies between
about 3-4 x 10**-12 ergs cm**-2 s**-1 over the selected area. This covers the
Decl<2.5 deg part of the south Galactic cap region (b<-20 deg) - with the
exclusion of patches of low RASS1 exposure time and of the Magellanic Clouds
area - for a total of 8235 deg**2. 130 candidate sources fulfill our selection
criteria for bonafide clusters of galaxies in this area. Of these, 101 are
Abell/ACO clusters, while 29 do not have a counterpart in these catalogs. Of
these clusters, 126 (97%) clusters have a redshift and for these we compute an
X-ray luminosity. 20% of the cluster redshifts come from new observations, as
part of the ESO Key Program REFLEX Cluster Survey that is under completion.
Considering the intrinsic biases and incompletenesses introduced by the flux
selection and source identification processes, we estimate the overall
completeness to be better than 90%. The observed number count distribution,
LogN-LogS, is well fitted by a power law with slope alpha = 1.34 +/- 0.15 and
normalization A = 11.87 +/- 1.04 sr**-1 (10**-11 ergs cm**-2 s**-1)**alpha, in
good agreement with other measurements.Comment: 27 pages, 8 figures and 3 tables included, LaTex, emulateapj.sty and
epsf.sty, accepted for publication in ApJ: scheduled for the March 20, 1999,
Vol.514. The cluster catalog is available at
http://www.merate.mi.astro.it/~degrand
Measurements of Sunyaev-Zel'dovich Effect Scaling Relations for Clusters of Galaxies
We present new measurements of the Sunyaev-Zel'dovich (SZ) effect from
clusters of galaxies using the Sunyaev-Zel'dovich Infrared Experiment (SuZIE
II). We combine these new measurements with previous cluster observations with
the SuZIE instrument to form a sample of 15 clusters of galaxies. For this
sample we calculate the central Comptonization, y, and the integrated SZ flux
decrement, S, for each of our clusters. We find that the integrated SZ flux is
a more robust observable derived from our measurements than the central
Comptonization due to inadequacies in the spatial modelling of the
intra-cluster gas with a standard Beta model. This is highlighted by comparing
our central Comptonization results with values calculated from measurements
using the BIMA and OVRO interferometers. On average, the SuZIE calculated
central Comptonizations are approximately 60% higher in the cooling flow
clusters than the interferometric values, compared to only approximately 12%
higher in the non-cooling flow clusters. We believe this discrepancy to be in
large part due to the spatial modelling of the intra-cluster gas. From our
cluster sample we construct y-T and S-T scaling relations. The y-T scaling
relation is inconsistent with what we would expect for self-similar clusters;
however this result is questionable because of the large systematic uncertainty
in the central Comptonization. The S-T scaling relation has a slope and
redshift evolution consistent with what we expect for self-similar clusters
with a characteristic density that scales with the mean density of the
universe. We rule out zero redshift evolution of the S-T relation at 90%
confidence.Comment: Accepted to Astrophysical Journal. 52 pages, 14 tables, 7 figures
;replaced to match ApJ accepted versio
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