89 research outputs found
Cluster-Supercluster Alignments
We study correlations in spatial orientation between galaxy clusters and
their host superclusters using a Hubble Volume N-body realization of a
concordance cosmology and an analytic model for tidally-induced alignments. We
derive an analytic form for distributions of the alignment angle as functions
of halo mass (M), ellipticity (epsilon), distance (r) and velocity (v) and show
that the model, after tuning of three parameters, provides a good fit to the
numerical results. The parameters indicate a high degree of alignment along
anisotropic, collapsed filaments. The degree of alignment increases with M and
epsilon while it decreases with r and is independent of v. We note the
possibility of using the cluster-supercluster alignment effect as a
cosmological probe to constrain the slope of the initial power spectrum.Comment: accepted by ApJ, revised version, new analysis using those
superclusters with more than 5 clusters include
The Asymptotic Form of Cosmic Structure: Small Scale Power and Accretion History
We explore the effects of small scale structure on the formation and
equilibrium of dark matter halos in a universe dominated by vacuum energy. We
present the results of a suite of four N-body simulations, two with a LCDM
initial power spectrum and two with WDM-like spectra that suppress the early
formation of small structures. All simulations are run into to far future when
the universe is 64Gyr/h old, long enough for halos to essentially reach
dynamical equilibrium. We quantify the importance of hierarchical merging on
the halo mass accretion history, the substructure population, and the
equilibrium density profile. We modify the mass accretion history function of
Wechsler et al. (2002) by introducing a parameter, \gamma, that controls the
rate of mass accretion, dln(M) / dln(a) ~ a^(-\gamma), and find that this form
characterizes both hierarchical and monolithic formation. Subhalo decay rates
are exponential in time with a much shorter time scale for WDM halos. At the
end of the simulations, we find truncated Hernquist density profiles for halos
in both the CDM and WDM cosmologies. There is a systematic shift to lower
concentration for WDM halos, but both cosmologies lie on the same locus
relating concentration and formation epoch. Because the form of the density
profile remains unchanged, our results indicate that the equilibrium halo
density profile is set independently of the halo formation process.Comment: 17 pages, submitted to ApJ. Full resolution version avaliable at
http://www-personal.umich.edu/~mbusha/Papers/AccretionHistory.pd
COSMOS: A Hybrid N-Body/Hydrodynamics Code for Cosmological Problems
We describe a new hybrid N-body/hydrodynamical code based on the
particle-mesh (PM) method and the piecewise-parabolic method (PPM) for use in
solving problems related to the evolution of large-scale structure, galaxy
clusters, and individual galaxies. The code, named COSMOS, possesses several
new features which distinguish it from other PM-PPM codes. In particular, to
solve the Poisson equation we have written a new multigrid solver which can
determine the gravitational potential of isolated matter distributions and
which properly takes into account the finite-volume discretization required by
PPM. All components of the code are constructed to work with a nonuniform mesh,
preserving second-order spatial differences. The PPM code uses vacuum boundary
conditions for isolated problems, preventing inflows when appropriate. The PM
code uses a second-order variable-timestep time integration scheme. Radiative
cooling and cosmological expansion terms are included. COSMOS has been
implemented for parallel computers using the Parallel Virtual Machine (PVM)
library, and it features a modular design which simplifies the addition of new
physics and the configuration of the code for different types of problems. We
discuss the equations solved by COSMOS and describe the algorithms used, with
emphasis on these features. We also discuss the results of tests we have
performed to establish that COSMOS works and to determine its range of
validity.Comment: 43 pages, 14 figures, submitted to ApJS and revised according to
referee's comment
The Role of Gas in the Merging of Massive Black Holes in Galactic Nuclei. I. Black Hole Merging in a Spherical Gas Cloud
Using high-resolution SPH numerical simulations, we investigate the effects
of gas on the inspiral and merger of a massive black hole binary. This study is
motivated by both observational and theoretical work that indicate the presence
of large amounts of gas in the central regions of merging galaxies. N-body
simulations have shown that the coalescence of a massive black hole binary
eventually stalls in a stellar background. However, our simulations suggest
that the massive black hole binary will finally merge if it is embedded in a
gaseous background. Here we present results in which the gas is assumed to be
initially spherical with a relatively smooth distribution. In the early
evolution of the binary, the separation dimishes due to the gravitational drag
exerted by the background gas. In the later stages, when the binary dominates
the gravitational potential in its vicinity, the medium responds by forming an
ellipsoidal density enhancement whose axis lags behind the binary axis, and
this offset produces a torque on the binary that causes continuing loss of
angular momentum and is able to reduce the binary separation to distances where
gravitational radiation is efficient. Assuming typical parameters from
observations of Ultra Luminous Infrared Galaxies, we predict that a black hole
binary will merge within yrs; therefore these results imply that in a
merger of gas-rich galaxies, any massive central black holes will coalescence
soon after the galaxies merge. Our work thus supports scenarios of massive
black hole evolution and growth where hierarchical merging plays an important
role. The final coalescence of the black holes leads to gravitational radiation
emission that would be detectable up to high redshift by LISA. We show that
similar physical effects are important for the formation of close binary stars.Comment: 38 pages, 14 figures, submitted to Ap
Shapes and Alignments of Galaxy Cluster Halos
We present distribution functions and spatial correlations of the shapes of
dark matter halos derived from Hubble Volume simulations of a LambdaCDM
universe. We measure both position and velocity shapes within spheres
encompassing mean density 200 times the critical value, and calibrate small-N
systematic errors using Poisson realizations of isothermal spheres and higher
resolution simulations. For halos more massive than 3x10^{14} Msun/h, the shape
distribution function peaks at (minor/major, intermediate/major) axial ratios
of (0.64,0.76) in position, and is rounder in velocity, peaking at (0.72,0.82).
Halo shapes are rounder at lower mass and/or redshift; the mean minor axis
ratio in position follows (M,z) = c_{15,0} [1-\alpha\ln(M/10^{15}Msun/h)]
(1+z)^{-\epsilon}, with c_{15,0}=0.631 \pm 0.001, \alpha=0.023 \pm 0.002 and
\epsilon=0.086 \pm 0.004. Position and velocity principal axes are well aligned
in direction, with median alignment angle , and the axial ratios in
these spaces are correlated in magnitude. We investigate mark correlations of
halo pair orientations using two measures: a simple scalar product shows alignment extending to 30 \hinv \mpc while a filamentary statistic
exhibits non-random alignment extending to scales \sims 200 \hinv \mpc, ten
times the sample two-point correlation length and well into the regime of
negative two-point correlation. Cluster shapes are unaffected by the
large-scale environment; the shape distribution of supercluster members is
consistent with that of the general population.Comment: 12 pages, 8 figures, submitted to Ap
The Beginning and Evolution of the Universe
We review the current standard model for the evolution of the Universe from
an early inflationary epoch to the complex hierarchy of structure seen today.
We summarize and provide key references for the following topics: observations
of the expanding Universe; the hot early Universe and nucleosynthesis; theory
and observations of the cosmic microwave background; Big Bang cosmology;
inflation; dark matter and dark energy; theory of structure formation; the cold
dark matter model; galaxy formation; cosmological simulations; observations of
galaxies, clusters, and quasars; statistical measures of large-scale structure;
and measurement of cosmological parameters. We conclude with discussion of some
open questions in cosmology. This review is designed to provide a graduate
student or other new worker in the field an introduction to the cosmological
literature.Comment: 69 pages. Invited review article for Publications of the Astronomical
Society of the Pacific. Supplementary references, tables, and more concise
PDF file at http://www.physics.drexel.edu/univers
Statistical characteristics of formation and evolution of structure in the universe
An approximate statistical description of the formation and evolution of
structure of the universe based on the Zel'dovich theory of gravitational
instability is proposed. It is found that the evolution of DM structure shows
features of self-similarity and the main structure characteristics can be
expressed through the parameters of initial power spectrum and cosmological
model. For the CDM-like power spectrum and suitable parameters of the
cosmological model the effective matter compression reaches the observed scales
20 -- 25Mpc with the typical mean separation of
wall-like elements 50 -- 70Mpc. This description can be
directly applied to the deep pencil beam galactic surveys and absorption
spectra of quasars. For larger 3D catalogs and simulations it can be applied to
results obtained with the core-sampling analysis.
It is shown that the interaction of large and small scale perturbations
modulates the creation rate of early Zel'dovich pancakes and generates bias on
the SLSS scale. For suitable parameters of the cosmological model and reheating
process this bias can essentially improve the characteristics of simulated
structure of the universe.
The models with give the best description of the
observed structure parameters. The influence of low mass "warm" dark matter
particles, such as a massive neutrino, will extend the acceptable range of
and .Comment: 20pages, 7 figures, MNRAS in pres
Constraints on the Detectability of Cosmic Topology from Observational Uncertainties
Recent observational results suggest that our universe is nearly flat and
well modelled within a CDM framework. The observed values of
and inevitably involve uncertainties. Motivated
by this, we make a systematic study of the necessary and sufficient conditions
for undetectability as well as detectability (in principle) of cosmic topology
(using pattern repetition) in presence of such uncertainties. We do this by
developing two complementary methods to determine detectability for nearly flat
universes. Using the first method we derive analytical conditions for
undetectability for infinite redshift, the accuracy of which is then confirmed
by the second method. Estimates based on WMAP data together with other
measurements of the density parameters are used to illustrate both methods,
which are shown to provide very similar results for high redshifts.Comment: 16 pages, 1 figure, LaTeX2
Dark matter and structure formation a review
This paper provides a review of the variants of dark matter which are thought
to be fundamental components of the universe and their role in origin and
evolution of structures and some new original results concerning improvements
to the spherical collapse model. In particular, I show how the spherical
collapse model is modified when we take into account dynamical friction and
tidal torques
Comprehensive characterization of 536 patient-derived xenograft models prioritizes candidatesfor targeted treatment
Development of candidate cancer treatments is a resource-intensive process, with the research community continuing to investigate options beyond static genomic characterization. Toward this goal, we have established the genomic landscapes of 536 patient-derived xenograft (PDX) models across 25 cancer types, together with mutation, copy number, fusion, transcriptomic profiles, and NCI-MATCH arms. Compared with human tumors, PDXs typically have higher purity and fit to investigate dynamic driver events and molecular properties via multiple time points from same case PDXs. Here, we report on dynamic genomic landscapes and pharmacogenomic associations, including associations between activating oncogenic events and drugs, correlations between whole-genome duplications and subclone events, and the potential PDX models for NCI-MATCH trials. Lastly, we provide a web portal having comprehensive pan-cancer PDX genomic profiles and source code to facilitate identification of more druggable events and further insights into PDXs' recapitulation of human tumors
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