1,956 research outputs found
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
The Ultimate Halo Mass in a LCDM Universe
In the far future of an accelerating LCDM cosmology, the cosmic web of
large-scale structure consists of a set of increasingly isolated halos in
dynamical equilibrium. We examine the approach of collisionless dark matter to
hydrostatic equilibrium using a large N-body simulation evolved to scale factor
a = 100, well beyond the vacuum--matter equality epoch, a_eq ~ 0.75, and 53/h
Gyr into the future for a concordance model universe (Omega_m ~ 0.3,
Omega_Lambda ~ 0.7). The radial phase-space structure of halos -- characterized
at a < a_eq by a pair of zero-velocity surfaces that bracket a dynamically
active accretion region -- simplifies at a > 10 a_eq when these surfaces merge
to create a single zero-velocity surface, clearly defining the halo outer
boundary, rhalo, and its enclosed mass, mhalo. This boundary approaches a fixed
physical size encompassing a mean interior density ~ 5 times the critical
density, similar to the turnaround value in a classical Einstein-deSitter
model. We relate mhalo to other scales currently used to define halo mass
(m200, mvir, m180b) and find that m200 is approximately half of the total
asymptotic cluster mass, while m180b follows the evolution of the inner zero
velocity surface for a < 2 but becomes much larger than the total bound mass
for a > 3. The radial density profile of all bound halo material is well fit by
a truncated Hernquist profile. An NFW profile provides a somewhat better fit
interior to r200 but is much too shallow in the range r200 < r < rhalo.Comment: 5 pages, 3 figures, submitted to MNRAS letter
High redshift X-ray galaxy clusters. II. The L_X-T relationship revisited
In this paper we re-visit the observational relation between X-ray luminosity
and temperature for high-z galaxy clusters and compare it with the local L_X-T
and with theoretical models. To these ends we use a sample of 17 clusters
extracted from the Chandra archive supplemented with additional clusters from
the literature, either observed by Chandra or XMM-Newton, to form a final
sample of 39 high redshift (0.25 < z < 1.3) objects. Different statistical
approaches are adopted to analyze the L_X-T relation. The slope of the L_X-T
relation of high redshift clusters is steeper than expected from the
self-similar model predictions and steeper, even though still compatible within
the errors, than the local L_X-T slope. The distant cluster L_X-T relation
shows a significant evolution with respect to the local Universe: high-z
clusters are more luminous than the local ones by a factor ~2 at any given
temperature. The evolution with redshift of the L_X-T relation cannot be
described by a single power law nor by the evolution predicted by the
self-similar model. We find a strong evolution, similar or stronger than the
self-similar model, from z = 0 to z <0.3 followed by a much weaker, if any,
evolution at higher redshift. The weaker evolution is compatible with
non-gravitational models of structure formation. According to us a
statistically significant sample of nearby clusters (z < 0.25) should be
observed with the current available X-ray telescopes to completely exclude
observational effects due to different generation detectors and to understand
this novel result.Comment: 14 pages, 10 postscript figures. Accepted for publication in
Astronomy & Astrophysics. Corrected typo
The Organization of the Primate Insular Cortex
Long perceived as a primitive and poorly differentiated brain structure, the primate insular cortex recently emerged as a highly evolved, organized and richly connected cortical hub interfacing bodily states with sensorimotor, environmental, and limbic activities. This insular interface likely substantiates emotional embodiment and has the potential to have a key role in the interoceptive shaping of cognitive processes, including perceptual awareness. In this review, we present a novel working model of the insular cortex, based on an accumulation of neuroanatomical and functional evidence obtained essentially in the macaque monkey. This model proposes that interoceptive afferents that represent the ongoing physiological status of all the organs of the body are first being received in the granular dorsal fundus of the insula or âprimary interoceptive cortex,â then processed through a series of dysgranular poly-modal âinsular stripes,â and finally integrated in anterior agranular areas that serve as an additional sensory platform for visceral functions and as an output stage for efferent autonomic regulation. One of the agranular areas hosts the specialized von Economo and Fork neurons, which could provide a decisive evolutionary advantage for the role of the anterior insula in the autonomic and emotional binding inherent to subjective awareness
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
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
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
Shrinkage Estimation of the Power Spectrum Covariance Matrix
We seek to improve estimates of the power spectrum covariance matrix from a
limited number of simulations by employing a novel statistical technique known
as shrinkage estimation. The shrinkage technique optimally combines an
empirical estimate of the covariance with a model (the target) to minimize the
total mean squared error compared to the true underlying covariance. We test
this technique on N-body simulations and evaluate its performance by estimating
cosmological parameters. Using a simple diagonal target, we show that the
shrinkage estimator significantly outperforms both the empirical covariance and
the target individually when using a small number of simulations. We find that
reducing noise in the covariance estimate is essential for properly estimating
the values of cosmological parameters as well as their confidence intervals. We
extend our method to the jackknife covariance estimator and again find
significant improvement, though simulations give better results. Even for
thousands of simulations we still find evidence that our method improves
estimation of the covariance matrix. Because our method is simple, requires
negligible additional numerical effort, and produces superior results, we
always advocate shrinkage estimation for the covariance of the power spectrum
and other large-scale structure measurements when purely theoretical modeling
of the covariance is insufficient.Comment: 9 pages, 7 figures (1 new), MNRAS, accepted. Changes to match
accepted version, including an additional explanatory section with 1 figur
Cluster Cosmology Redux: A Compact Model of the Halo Mass Function
Massive halos hosting groups and clusters of galaxies imprint coherent,
arcminute-scale features across the spectrophotometric sky, especially
optical-IR clusters of galaxies, distortions in the sub-mm CMB, and extended
sources of X-ray emission. Statistical modeling of such features often rely
upon the evolving space-time density of dark matter halos -- the halo mass
function (HMF) -- as a common theoretical ground for cosmological,
astrophysical and fundamental physics studies. We propose a compact (eight
parameter) representation of the HMF with readily interpretable parameters that
stem from polynomial expansions, first in terms of log-mass, then expanding
those coefficients similarly in redshift. We demonstrate good ()
agreement of this form, referred to as the dual-quadratic (DQ-HMF), with
Mira-Titan N-body emulator estimates for halo masses above over the redshift range , present best-fit
parameters for a Planck 2018 cosmology, and present parameter variation in the
plane. Convolving with a minimal mass-observable
relation (MOR) yields closed-form expressions for counts, mean mass, and mass
variance of cluster samples characterized by some observable property.
Performing information-matrix forecasts of potential parameter constraints from
existing and future surveys under different levels of systematic uncertainties,
we demonstrate the potential for percent-level constraints on model parameters
by an LSST-like optical cluster survey of 300,000 clusters and a richness-mass
variance of . Even better constraints could potentially be achieved by a
survey with one-tenth the sample size but with a reduced selection property
variance of . Potential benefits and extensions to the basic MOR
parameterization are discussed.Comment: 18 pages, 9 figure
Intact Semantic Priming of Critical Lures in Alzheimer's Disease: Implications for False Memory
OBJECTIVES: The present study examines the question of the activation of the critical lure (CL) in Alzheimer\u27s patients with a Deese-Roediger-McDermott (DRM)-like task. More precisely, older adults and Alzheimer\u27s patients performed a lexical decision task in which they were asked to categorize strings of letters as words or nonwords. Contrary to the DRM paradigm in which the activation of the CL is inferred from its production at recall, such a lexical decision task does not require the joint use of intentional recovery strategies and source-monitoring processes that are known to be particularly impaired in Alzheimer\u27s patients. The performance at the lexical decision therefore reflects the activation of the CL without contamination from such strategic processes. METHOD: Twenty-nine older adults and 25 Alzheimer\u27s patients performed a lexical decision task with DRM lists intermixed with neutral words and nonwords. RESULTS: Analysis indicated that older adults as well as Alzheimer\u27s patients showed shorter lexical decision latencies for CLs than for other types of words. DISCUSSION: Contrary to the existing literature, our results suggest that the activation of the CL is preserved in Alzheimer\u27s patients at mild to moderate stages of the disease
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