281 research outputs found
A random version of Sperner's theorem
Let denote the power set of , ordered by inclusion, and
let be obtained from by selecting elements
from independently at random with probability . A classical
result of Sperner asserts that every antichain in has size at
most that of the middle layer, . In this note
we prove an analogous result for : If then, with high probability, the size of the largest antichain in
is at most . This
solves a conjecture of Osthus who proved the result in the case when . Our condition on is best-possible. In fact, we prove a
more general result giving an upper bound on the size of the largest antichain
for a wider range of values of .Comment: 7 pages. Updated to include minor revisions and publication dat
XRound : A reversible template language and its application in model-based security analysis
Successful analysis of the models used in Model-Driven Development requires the ability to synthesise the results of analysis and automatically integrate these results with the models themselves. This paper presents a reversible template language called XRound which supports round-trip transformations between models and the logic used to encode system properties. A template processor that supports the language is described, and the use of the template language is illustrated by its application in an analysis workbench, designed to support analysis of security properties of UML and MOF-based models. As a result of using reversible templates, it is possible to seamlessly and automatically integrate the results of a security analysis with a model. (C) 2008 Elsevier B.V. All rights reserved
The Evolution of [OII] Emission from Cluster Galaxies
We investigate the evolution of the star formation rate in cluster galaxies.
We complement data from the CNOC1 cluster survey (0.15<z<0.6) with measurements
from galaxy clusters in the 2dF galaxy redshift survey (0.05<z<0.1) and
measurements from recently published work on higher redshift clusters, up to
almost z=1. We focus our attention on galaxies in the cluster core, ie.
galaxies with r<0.7h^{-1}_{70}Mpc. Averaging over clusters in redshift bins, we
find that the fraction of galaxies with strong [OII] emission is < 20% in
cluster cores, and the fraction evolves little with redshift. In contrast,
field galaxies from the survey show a very strong increase over the same
redshift range. It thus appears that the environment in the cores of rich
clusters is hostile to star formation at all the redshifts studied. We compare
this result with the evolution of the colours of galaxies in cluster cores,
first reported by Butcher & Oemler (1984). Using the same galaxies for our
analysis of the [OII] emission, we confirm that the fraction of blue galaxies,
which are defined as galaxies 0.2 mag bluer in the rest frame B-V than the red
sequence of each cluster, increases strongly with redshift. Since the colours
of galaxies retain a memory of their recent star formation history, while
emission from the [OII] line does not, we suggest that these two results can
best be reconciled if the rate at which the clusters are being assembled is
higher in the past, and the galaxies from which it is being assembled are
typically bluer.Comment: 8 pages, 7 figures, accepted for publication in MNRA
Revisiting the Baryon Fractions of Galaxy Clusters: A Comparison with WMAP 3-year Results
The universal baryonic mass fraction (Omega_b/Omega_m) can be sensitively
constrained using X-ray observations of galaxy clusters. In this paper, we
compare the baryonic mass fraction inferred from measurements of the cosmic
microwave background with the gas mass fractions (f_gas) of a large sample of
clusters taken from the recent literature. In systems cooler than 4 keV, f_gas
declines as the system temperature decreases. However, in higher temperature
systems, f_gas(r500) converges to approx. (0.12 +/- 0.02)(h/0.72)^{-1.5}, where
the uncertainty reflects the systematic variations between clusters at r500.
This is significantly lower than the maximum-likelihood value of the baryon
fraction from the recently released WMAP 3-year results. We investigate
possible reasons for this discrepancy, including the effects of radiative
cooling and non-gravitational heating, and conclude that the most likely
solution is that Omega_m is higher than the best-fit WMAP value (we find
Omega_m = 0.36^{+0.11}_{-0.08}), but consistent at the 2-sigma level.
Degeneracies within the WMAP data require that sigma_8 must also be greater
than the maximum likelihood value for consistency between the data sets.Comment: 8 pages, 3 figures, MNRAS accepte
A Panoramic Halpha Imaging Survey of the z=0.4 cluster Cl0024.0+1652 with Subaru
We employ panoramic, multicolour (BRz') and narrow-band Halpha imaging of the
cluster Cl0024.0+1652 (z=0.39) from Subaru covering a ~30 arcmin field, to
determine cluster membership and star formation rates for a large sample of
galaxies across a wide field in the cluster, ~10 Mpc. We detect over 500
galaxies in narrow-band emission, with broad-band colours consistent with them
lying at z~0.39. Using this sample we determine the Halpha luminosity function
within the cluster and find that its form is approximately independent of local
density, and is consistent with that seen in the intermediate redshift field
population. This suggests that any density-dependent physical mechanisms which
alter the star formation rate must leave the Halpha luminosity function
unchanged; this is possible if the time-scale for star formation to cease
completely is short compared with a Hubble time. Such short time-scale
transformations are also supported by the presence of a population with
late-type morphologies but no detectable Halpha emission. The fraction of blue
galaxies, and the fraction of galaxies detected in Halpha, decreases strongly
with increasing galaxy density in a manner which is qualitatively similar to
that seen at lower redshifts. This trend is significantly steeper than the
trend with galaxy morphology observed from a panoramic Hubble Space Telescope
image of this cluster; this suggests that the physical mechanisms responsible
for transformations in morphology and star formation rates may be partially
independent. Finally, we compare our data with similar data on clusters
spanning a range of redshifts from 0.2<~z<~0.8 and find little evidence for a
trend in the total amount of star formation in clusters with redshift.Comment: 18 pages, 25 figures (of which 2 are provided in jpg format), uses
mn2e.cls. MNRAS in press, a small error in Fig.20 is fixed, Fig.5 and the
relevant text are updated with updated ISO catalo
The GEEC2 spectroscopic survey of Galaxy Groups at
We present the data release of the Gemini-South GMOS spectroscopy in the
fields of 11 galaxy groups at , within the COSMOS field. This forms
the basis of the Galaxy Environment Evolution Collaboration 2 (GEEC2) project
to study galaxy evolution in haloes with across cosmic
time. The final sample includes spectroscopically--confirmed members with
per cent complete for galaxies within the virial
radius, and with stellar mass . Including
galaxies with photometric redshifts we have an effective sample size of galaxies within the virial radii of these groups. We present group
velocity dispersions, dynamical and stellar masses. Combining with the GCLASS
sample of more massive clusters at the same redshift we find the total stellar
mass is strongly correlated with the dynamical mass, with
. This stellar
fraction of per cent is lower than predicted by some halo occupation
distribution models, though the weak dependence on halo mass is in good
agreement. Most groups have an easily identifiable most massive galaxy (MMG)
near the centre of the galaxy distribution, and we present the spectroscopic
properties and surface brightness fits to these galaxies. The total stellar
mass distribution in the groups, excluding the MMG, compares well with an NFW
profile with concentration , for galaxies beyond . This is
more concentrated than the number density distribution, demonstrating that
there is some mass segregation.Comment: Accepted for publication in MNRAS. The appendix is omitted due to
large figures. The full version will be available from the MNRAS website and
from http://quixote.uwaterloo.ca/~mbalogh/papers/GEEC2_data.pdf. Long data
tables are available from MNRAS or by contacting the first autho
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