25 research outputs found
Cosmological Numerical Simulations of Radio Relics in Galaxy Clusters: Insights for Future Observations
The acceleration of electrons at shock fronts is thought to be responsible
for radio relics, extended radio features in the vicinity of merging galaxy
clusters. By combining high resolution Adaptive Mesh Refinement Hydro/N-body
cosmological simulations with an accurate shock-finding algorithm and a model
for electron acceleration, we calculate the expected synchrotron emission
resulting from cosmological structure formation. From these simulations, we
produce radio, SZE and X-ray images for a large sample of galaxy clusters along
with radio luminosity functions and scaling relationships. We find that with
upcoming radio arrays, we expect to see an abundance of radio emission
associated with merger shocks in the intracluster medium. By producing
observationally motivated statistics, we provide predictions that can be
compared with observations to further our understanding of electron shock
acceleration and kinematic structure of galaxy clusters.Comment: 4 pages, 4 figures, to appear in proceedings of "Non-thermal
phenomena in colliding galaxy clusters" (Nice, France, Nov 2010
Galaxy Clusters at the Edge: Temperature, Entropy, and Gas Dynamics at the Virial Radius
Recently, Suzaku has produced temperature and entropy profiles, along with
profiles of gas density, gas fraction, and mass, for multiple galaxy clusters
out to ~r_200 (~= virial radius). In this paper, we compare these novel X-ray
observations with results from N-body + hydrodynamic adaptive mesh refinement
cosmological simulations using the Enzo code. There is excellent agreement in
the temperature, density, and entropy profiles between a sample of 27 mostly
substructure-free massive clusters in the simulated volume and the observed
clusters. This supports our previous contention that clusters have "universal"
outer temperature profiles. Furthermore, it appears that the simplest adiabatic
gas physics used in these Enzo simulations is adequate to model the outer
regions of these clusters without other mechanisms (e.g., non-gravitational
heating, cooling, magnetic fields, or cosmic rays). However, the outskirts of
these clusters are not in hydrostatic equilibrium. There is significant bulk
flow and turbulence in the outer intracluster medium created by accretion from
filaments. Thus, the gas is not fully supported by thermal pressure. The
implications for mass estimation from X-ray data are discussed.Comment: 23 pages, 7 figures, submitted to Ap
Disentangling redshift-space distortions and nonlinear bias using the 2D power spectrum
We present the nonlinear 2D galaxy power spectrum, , in redshift
space, measured from the Dark Sky simulations, using galaxy catalogs
constructed with both halo occupation distribution and subhalo abundance
matching methods, chosen to represent an intermediate redshift sample of
luminous red galaxies. We find that the information content in individual
(cosine of the angle to the line of sight) bins is substantially richer then
multipole moments, and show that this can be used to isolate the impact of
nonlinear growth and redshift space distortion (RSD) effects. Using the
simulation data, which we show is not impacted by RSD effects, we can
successfully measure the nonlinear bias to an accuracy of % at Mpc. This use of individual bins to extract the nonlinear bias
successfully removes a large parameter degeneracy when constraining the linear
growth rate of structure. We carry out a joint parameter estimation, using the
low simulation data to constrain the nonlinear bias, and to
constrain the growth rate and show that can be constrained to % to a Mpc from clustering alone using
a simple dispersion model, for a range of galaxy models. Our analysis of
individual bins also reveals interesting physical effects which arise
simply from different methods of populating halos with galaxies. We find a
prominent turnaround scale, at which RSD damping effects are greater then the
nonlinear growth, which differs not only for each bin but also for each
galaxy model. These features may provide unique signatures which could be used
to shed light on the galaxy-dark matter connection.Comment: 15 pages, 11 figures, published in MNRA
Cosmological Simulations of Isotropic Conduction in Galaxy Clusters
Simulations of galaxy clusters have a difficult time reproducing the radial
gas-property gradients and red central galaxies observed to exist in the cores
of galaxy clusters. Thermal conduction has been suggested as a mechanism that
can help bring simulations of cluster cores into better alignment with
observations by stabilizing the feedback processes that regulate gas cooling,
but this idea has not yet been well tested with cosmological numerical
simulations. Here we present cosmological simulations of ten galaxy clusters
performed with five different levels of isotropic Spitzer conduction, which
alters both the cores and outskirts of clusters, but not dramatically. In the
cores, conduction flattens central temperature gradients, making them nearly
isothermal and slightly lowering the central density but failing to prevent a
cooling catastrophe there. Conduction has little effect on temperature
gradients outside of cluster cores because outward conductive heat flow tends
to inflate the outer parts of the intracluster medium (ICM) instead of raising
its temperature. In general, conduction tends reduce temperature inhomogeneity
in the ICM, but our simulations indicate that those homogenizing effects would
be extremely difficult to observe in ~5 keV clusters. Outside the virial
radius, our conduction implementation lowers the gas densities and temperatures
because it reduces the Mach numbers of accretion shocks. We conclude that
despite the numerous small ways in which conduction alters the structure of
galaxy clusters, none of these effects are significant enough to make the
efficiency of conduction easily measurable unless its effects are more
pronounced in clusters hotter than those we have simulated.Comment: 13 pages, 13 figures. Submitted to The Astrophysical Journa
On The Road To More Realistic Galaxy Cluster Simulations: The Effects of Radiative Cooling and Thermal Feedback Prescriptions on the Observational Properties of Simulated Galaxy Clusters
Flux limited X-ray surveys of galaxy clusters show that clusters come in two
roughly equally proportioned varieties: "cool core" clusters (CCs) and
non-"cool core" clusters (NCCs). In previous work, we have demonstrated using
cosmological -body + Eulerian hydrodynamic simulations that NCCs are often
consistent with early major mergers events that destroy embryonic CCs. In this
paper we extend those results and conduct a series of simulationsusing
different methods of gas cooling, and of energy and metal feedback from
supernovae, where we attempt to produce a population of clusters with realistic
central cooling times, entropies, and temperatures. We find that the use of
metallicity-dependent gas cooling is essential to prevent early overcooling,and
that adjusting the amount of energy and metal feedback can have a significant
impact on observable X-ray quantities of the gas. We are able to produce
clusters with more realistic central observable quantities than have previously
been attained. However, there are still significant discrepancies between the
simulated clusters and observations, which indicates that a different approach
to simulating galaxies in clusters is needed. We conclude by looking towards a
promising subgrid method of modeling galaxy feedback in clusters which may help
to ameliorate the discrepancies between simulations and observations.Comment: 20 pages, 16 figures, 2 table
How Much Can We Learn From A Merging Cold Front Cluster? : Insights From X-ray Temperature and Radio Maps of Abell 3667
The galaxy cluster Abell 3667 is an ideal laboratory to study the plasma
processes in the intracluster medium (ICM). High resolution Chandra X-ray
observations show a cold front in Abell 3667. At radio wavelengths, Abell 3667
reveals a double radio-relic feature in the outskirts of the cluster. These
suggest multiple merger events in this cluster. In this paper, we analyze the
substantial archival X-ray observations of Abell 3667 from ChandraX-ray
Observatory and compare these with existing radio observations as well as
state-of-the-art AMR (Adaptive Mesh Refinement) MHD cosmological simulations
using Enzo. We have used two temperature map making techniques, Weighted
Voronoi Tessellation and Adaptive Circular Binning, to produce the high
resolution and largest field-of-view temperature maps of Abell 3667. These high
fidelity temperature maps allow us to study the X-ray shocks in the cluster
using a new 2-dimensional shock-finding algorithm. We have also estimated the
Mach numbers from the shocks inferred from previous ATCA radio observations.
The combined shock statistics from the X-ray and radio data are in agreement
with the shock statistics in a simulated MHD cluster. We have also studied the
profiles of the thermodynamic properties across the cold front using 447 ksec
from the combined Chandra observations on Abell 3667. Our results show that the
stability of the cold front in Abell 3667 can be attributed to the suppression
of the thermal conduction across the cold front by a factor of 100 - 700
compared to the classical Spitzer value.Comment: 23 pages, 16 figures. Accepted for publication in The Astrophysical
Journa
The Concentration Dependence of the Galaxy-Halo Connection: Modeling Assembly Bias with Abundance Matching
Empirical methods for connecting galaxies to their dark matter halos have
become essential for interpreting measurements of the spatial statistics of
galaxies. In this work, we present a novel approach for parameterizing the
degree of concentration dependence in the abundance matching method. This new
parameterization provides a smooth interpolation between two commonly used
matching proxies: the peak halo mass and the peak halo maximal circular
velocity. This parameterization controls the amount of dependence of galaxy
luminosity on halo concentration at a fixed halo mass. Effectively this
interpolation scheme enables abundance matching models to have adjustable
assembly bias in the resulting galaxy catalogs. With the new 400 Mpc/h DarkSky
Simulation, whose larger volume provides lower sample variance, we further show
that low-redshift two-point clustering and satellite fraction measurements from
SDSS can already provide a joint constraint on this concentration dependence
and the scatter within the abundance matching framework.Comment: 15 pages, 11 figures. APJ in press. Matched published versio
Galaxy Cluster Radio Relics in Adaptive Mesh Refinement Cosmological Simulations: Relic Properties and Scaling Relationships
Cosmological shocks are a critical part of large-scale structure formation,
and are responsible for heating the intracluster medium in galaxy clusters. In
addition, they are also capable of accelerating non-thermal electrons and
protons. In this work, we focus on the acceleration of electrons at shock
fronts, which is thought to be responsible for radio relics - extended radio
features in the vicinity of merging galaxy clusters. By combining high
resolution AMR/N-body cosmological simulations with an accurate shock finding
algorithm and a model for electron acceleration, we calculate the expected
synchrotron emission resulting from cosmological structure formation. We
produce synthetic radio maps of a large sample of galaxy clusters and present
luminosity functions and scaling relationships. With upcoming long wavelength
radio telescopes, we expect to see an abundance of radio emission associated
with merger shocks in the intracluster medium. By producing observationally
motivated statistics, we provide predictions that can be compared with
observations to further improve our understanding of magnetic fields and
electron shock acceleration.Comment: 20 pages, 15 figures, further discussion and appendix added, accepted
to Ap
Dark Sky Simulations: Early Data Release
The Dark Sky Simulations are an ongoing series of cosmological N-body
simulations designed to provide a quantitative and accessible model of the
evolution of the large-scale Universe. Such models are essential for many
aspects of the study of dark matter and dark energy, since we lack a
sufficiently accurate analytic model of non-linear gravitational clustering. In
July 2014, we made available to the general community our early data release,
consisting of over 55 Terabytes of simulation data products, including our
largest simulation to date, which used
particles in a volume across. Our simulations were
performed with 2HOT, a purely tree-based adaptive N-body method, running on
200,000 processors of the Titan supercomputer, with data analysis enabled by
yt. We provide an overview of the derived halo catalogs, mass function, power
spectra and light cone data. We show self-consistency in the mass function and
mass power spectrum at the 1% level over a range of more than 1000 in particle
mass. We also present a novel method to distribute and access very large
datasets, based on an abstraction of the World Wide Web (WWW) as a file system,
remote memory-mapped file access semantics, and a space-filling curve index.
This method has been implemented for our data release, and provides a means to
not only query stored results such as halo catalogs, but also to design and
deploy new analysis techniques on large distributed datasets.Comment: 26 pages, 9 figures, project website at
http://darksky.slac.stanford.edu, repository at
http://bitbucket.org/darkskysim
Visual search over billions of aerial and satellite images
We present a system for performing visual search over billions of aerial and
satellite images. The purpose of visual search is to find images that are
visually similar to a query image. We define visual similarity using 512
abstract visual features generated by a convolutional neural network that has
been trained on aerial and satellite imagery. The features are converted to
binary values to reduce data and compute requirements. We employ a hash-based
search using Bigtable, a scalable database service from Google Cloud. Searching
the continental United States at 1-meter pixel resolution, corresponding to
approximately 2 billion images, takes approximately 0.1 seconds. This system
enables real-time visual search over the surface of the earth, and an
interactive demo is available at https://search.descarteslabs.com