1,770 research outputs found
Red Sequence Cluster Finding in the Millennium Simulation
We investigate halo mass selection properties of red-sequence cluster finders
using galaxy populations of the Millennium Simulation (MS). A clear red
sequence exists for MS galaxies in massive halos at redshifts z < 1, and we use
this knowledge to inform a cluster-finding algorithm applied to 500 Mpc/h
projections of the simulated volume. At low redshift (z=0.4), we find that 90%
of the clusters found have galaxy membership dominated by a single, real-space
halo, and that 10% are blended systems for which no single halo contributes a
majority of a cluster's membership. At z=1, the fraction of blends increases to
22%, as weaker redshift evolution in observed color extends the comoving length
probed by a fixed range of color. Other factors contributing to the increased
blending at high-z include broadening of the red sequence and confusion from a
larger number of intermediate mass halos hosting bright red galaxies of
magnitude similar to those in higher mass halos. Our method produces catalogs
of cluster candidates whose halo mass selection function, p(M|\Ngal,z), is
characterized by a bimodal log-normal model with a dominant component that
reproduces well the real-space distribution, and a redshift-dependent tail that
is broader and displaced by a factor ~2 lower in mass. We discuss implications
for X-ray properties of optically selected clusters and offer ideas for
improving both mock catalogs and cluster-finding in future surveys.Comment: final version to appear in MNRAS. Appendix added on purity and
completeness, small shift in red sequence due to correcting an error in
finding i
GALAXY DYNAMICS IN CLUSTERS
We use high resolution simulations to study the formation and distribution of
galaxies within a cluster which forms hierarchically. We follow both dark
matter and baryonic gas which is subject to thermal pressure, shocks and
radiative cooling. Galaxy formation is identified with the dissipative collapse
of the gas into cold, compact knots. We examine two extreme representations of
galaxies during subsequent cluster evolution --- one purely gaseous and the
other purely stellar. The results are quite sensitive to this choice.
Gas-galaxies merge efficiently with a dominant central object while
star-galaxies merge less frequently. Thus, simulations in which galaxies remain
gaseous appear to suffer an ``overmerging'' problem, but this problem is much
less severe if the gas is allowed to turn into stars. We compare the kinematics
of the galaxy population in these two representations to that of dark halos and
of the underlying dark matter distribution. Galaxies in the stellar
representation are positively biased (\ie over-represented in the cluster) both
by number and by mass fraction. Both representations predict the galaxies to be
more centrally concentrated than the dark matter, whereas the dark halo
population is more extended. A modest velocity bias also exists in both
representations, with the largest effect, , found for the more massive star-galaxies. Phase diagrams show that the
galaxy population has a substantial net inflow in the gas representation, while
in the stellar case it is roughly in hydrostatic equilibrium. Virial mass
estimators can underestimate the true cluster mass by up to a factor of 5. The
discrepancy is largest if only the most massive galaxies are used, reflecting
significant mass segregation.Comment: 30 pages, self-unpacking (via uufiles) postscript file without
figures. Eighteen figures (and slick color version of figure 3) and entire
paper available at ftp://oahu.physics.lsa.umich.edu/groups/astro/fews Total
size of paper with figures is ~9.0 Mb uncompressed. Submitted to Ap.J
Expectations For an Interferometric Sunyaev-Zel'dovich Effect Survey for Galaxy Clusters
Non-targeted surveys for galaxy clusters using the Sunyaev-Zel'dovich effect
(SZE) will yield valuable information on both cosmology and evolution of the
intra-cluster medium (ICM). The redshift distribution of detected clusters will
constrain cosmology, while the properties of the discovered clusters will be
important for studies of the ICM and galaxy formation. Estimating survey yields
requires a detailed model for both cluster properties and the survey strategy.
We address this by making mock observations of galaxy clusters in cosmological
hydrodynamical simulations. The mock observatory consists of an interferometric
array of ten 2.5 m diameter telescopes, operating at a central frequency of 30
GHz with a bandwidth of 8 GHz. We find that clusters with a mass above will be detected at any redshift, with the
exact limit showing a very modest redshift dependence. Using a Press-Schechter
prescription for evolving the number densities of clusters with redshift, we
determine that such a survey should find hundreds of galaxy clusters per year,
many at high redshifts and relatively low mass -- an important regime uniquely
accessible to SZE surveys. Currently favored cosmological models predict
roughly 25 clusters per square degree.Comment: revised to match published versio
Self-similarity of clusters of galaxies and the L_X-T relation
In this paper based on ROSAT/PSPC data we investigate the emission measure
profiles of a sample of hot clusters of galaxies (kT>3.5keV) in order to
explain the differences between observed and theoretically predicted L_X-T
relation. Looking at the form of the emission measure profiles as well as their
normalizations we find clear indication that indeed the profiles have similar
shapes once scaled to the virial radius, however, the normalization of the
profiles shows a strong temperature dependence. We introduce a M_gas-T relation
with the dependence M_gas propto T^1.94. This relationship explains the
observed L_X-T relation and reduces the scatter in the scaled profiles by a
factor of 2 when compared to the classical scaling. We interpret this finding
as strong indication that the M_gas-T relation in clusters deviates from
classical scaling.Comment: 4 pages including 4 figures, accepted for publication in A&A Letter
The X-ray Size-Temperature Relation for Intermediate Redshift Galaxy Clusters
We present the first measurements of the X-ray size-temperature (ST) relation
in intermediate redshift (z~0.30) galaxy clusters. We interpret the local ST
relation (z~0.06) in terms of underlying scaling relations in the cluster dark
matter properties, and then we use standard models for the redshift evolution
of those dark matter properties to show that the ST relation does not evolve
with redshift. We then use ROSAT HRI observations of 11 clusters to examine the
intermediate redshift ST relation; for currently favored cosmological
parameters, the intermediate redshift ST relation is consistent with that of
local clusters. Finally, we use the ST relation and our evolution model to
measure angular diameter distances; with these 11 distances we evaluate
constraints on Omega_M and Omega_L which are consistent with those derived from
studies of Type Ia supernovae. The data rule out a model with Omega_M=1 and
Omega_L=0 with 2.5 sigma confidence. When limited to models where
Omega_M+Omega_L=1, these data are inconsistent with Omega_M=1 with 3 sigma
confidence.Comment: ApJ: submitted April 7, accepted June 28, to appear Dec 1 (vol 544
CURRENT THERAPEUTIC APPROACH OF THE WHITE SPONGE NAEVUS OF THE ORAL CAVITY.
We report a case of White Sponge Naevus of the tongue in a 50 years-old man. White Sponge Naevus of the oral cavity is a rare, benign and dominant autosomic inherited disorder, which presents in the form of a white, hyperplasic and verrucous or spongious lesion of the oral mucosa. Differential diagnosis is clinically difficult with more common white lesions of the oral cavity. Various therapeutic approaches have been proposed. Systemic antibiotics or local applications of retinoic acid provide limited benefits but are poorly effective. To our knowledge, CO2 Laser has never been tried to treat a White Sponge Naevus of the oral cavity. We performed a complete removal of the lesion with CO2 Laser, but complete recurrence occurred. Finally, a surgical resection was realized, which proved to be effective. Two years later, the patient is free of recurrence. This article proposes a review of the literature on what is known on White Sponge Naevus of the oral mucosa. We stress the importance of confrontation between anamnesis, clinical examination and pathologic findings to lead to the proper diagnosis of this rare disease
Can Virialization Shocks be Detected Around Galaxy Clusters Through the Sunyaev-Zel'dovich Effect?
In cosmological structure formation models, massive non-linear objects in the
process of formation, such as galaxy clusters, are surrounded by large-scale
shocks at or around the expected virial radius. Direct observational evidence
for such virial shocks is currently lacking, but we show here that their
presence can be inferred from future, high resolution, high-sensitivity
observations of the Sunyaev-Zel'dovich (SZ) effect in galaxy clusters. We study
the detectability of virial shocks in mock SZ maps, using simple models of
cluster structure (gas density and temperature distributions) and noise
(background and foreground galaxy clusters projected along the line of sight,
as well as the cosmic microwave background anisotropies). We find that at an
angular resolution of 2'' and sensitivity of 10 micro K, expected to be reached
at ~ 100 GHz frequencies in a ~ 20 hr integration with the forthcoming ALMA
instrument, virial shocks associated with massive M ~ 10^15 M_Sun clusters will
stand out from the noise, and can be detected at high significance. More
generally, our results imply that the projected SZ surface brightness profile
in future, high-resolution experiments will provide sensitive constraints on
the density profile of cluster gas.Comment: 15 pages, submitted to Ap
Discovery of a Galaxy Cluster via Weak Lensing
We report the discovery of a cluster of galaxies via its weak gravitational
lensing effect on background galaxies, the first spectroscopically confirmed
cluster to be discovered through its gravitational effects rather than by its
electromagnetic radiation. This fundamentally different selection mechanism
promises to yield mass-selected, rather than baryon or photon-selected, samples
of these important cosmological probes. We have confirmed this cluster with
spectroscopic redshifts of fifteen members at z=0.276, with a velocity
dispersion of 615 km/s. We use the tangential shear as a function of source
photometric redshift to estimate the lens redshift independently and find z_l =
0.30 +- 0.08. The good agreement with the spectroscopy indicates that the
redshift evolution of the mass function may be measurable from the imaging data
alone in shear-selected surveys.Comment: revised version with minor changes, to appear in ApJ
FIREBALL: Detector, data acquisition and reduction
The Faint Intergalactic Redshifted Emission Balloon (FIREBALL) had its first scientific flight in June 2009. The instrument combines microchannel plate detector technology with fiber-fed integral field spectroscopy on an unstable stratospheric balloon gondola platform. This unique combination poses a series of calibration and data reduction challenges that must be addressed and resolved to allow for accurate data analysis. We discuss our approach and some of the methods we are employing to accomplish this task
Four Measures of the Intracluster Medium Temperature and Their Relation to a Cluster's Dynamical State
We employ an ensemble of hydrodynamic cluster simulations to create spatially
and spectrally resolved images of quality comparable to Chandra's expected
performance. Emission from simulation mass elements is represented using the
XSPEC mekal program assuming 0.3 solar metallicity, and the resulting spectra
are fit with a single-temperature model. Despite significant departures from
isothermality in the cluster gas, single-temperature models produce acceptable
fits to 20,000 source photon spectra. The spectral fit temperature T_s is
generally lower than the mass weighted average temperature T_m due to the
influence of soft line emission from cooler gas being accreted as part of the
hierarchical clustering process. In a Chandra-like bandpass of 0.5 to 9.5 keV
we find a nearly uniform fractional bias of (T_m-T_s)/T_s = 20% with occasional
large deviations in smaller clusters. In the more traditional 2.0 to 9.5 keV
bandpass, the fractional deviation is scale-dependent and on average follows
the relation (T_m-T_s)/T_s = 0.2 log(T_m). This bias results in a spectral
mass-temperature relationship with slope about 1.6, intermediate between the
virial relation M ~ T_m^{3/2} and the observed relation M_{ICM} ~ T^2. Imaging
each cluster in the ensemble at 16 epochs in its evolutionary history, we
catalogue merger events with mass ratios exceeding 10% in order to investigate
the relationship between spectral temperature and proximity to a major merger
event. Clusters that are very cool relative to the mean mass-temperature
relationship lie preferentially close to a merger, suggesting a viable
observational method to cull a subset of dynamically young clusters from the
general population.Comment: 34 pages, including 2 tables and 14 figures (one in color). Compiled
using LaTeX 2.09 with graphics package and aaspp4 style. The simulated
spectral data files used in this paper are available for public consumption
at http://redshift.stanford.edu/bfm
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