102 research outputs found
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
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
Luminous Satellites II: Spatial Distribution, Luminosity Function and Cosmic Evolution
We infer the normalization and the radial and angular distributions of the
number density of satellites of massive galaxies
() between redshifts 0.1 and 0.8 as a function
of host stellar mass, redshift, morphology and satellite luminosity. Exploiting
the depth and resolution of the COSMOS HST images, we detect satellites up to
eight magnitudes fainter than the host galaxies and as close as 0.3 (1.4)
arcseconds (kpc). Describing the number density profile of satellite galaxies
to be a projected power law such that P(R)\propto R^{\rpower}, we find
\rpower=-1.1\pm 0.3. We find no dependency of \rpower on host stellar mass,
redshift, morphology or satellite luminosity. Satellites of early-type hosts
have angular distributions that are more flattened than the host light profile
and are aligned with its major axis. No significant average alignment is
detected for satellites of late-type hosts. The number of satellites within a
fixed magnitude contrast from a host galaxy is dependent on its stellar mass,
with more massive galaxies hosting significantly more satellites. Furthermore,
high-mass late-type hosts have significantly fewer satellites than early-type
galaxies of the same stellar mass, likely a result of environmental
differences. No significant evolution in the number of satellites per host is
detected. The cumulative luminosity function of satellites is qualitatively in
good agreement with that predicted using subhalo abundance matching techniques.
However, there are significant residual discrepancies in the absolute
normalization, suggesting that properties other than the host galaxy luminosity
or stellar mass determine the number of satellites.Comment: 23 pages, 12 figures, Accepted for publication in the Astrophysical
Journa
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
CIRS: Cluster Infall Regions in the Sloan Digital Sky Survey I. Infall Patterns and Mass Profiles
We use the Fourth Data Release of the Sloan Digital Sky Survey to test the
ubiquity of infall patterns around galaxy clusters and measure cluster mass
profiles to large radii. We match X-ray cluster catalogs with SDSS, search for
infall patterns, and compute mass profiles for a complete sample of X-ray
selected clusters. Very clean infall patterns are apparent in most of the
clusters, with the fraction decreasing with increasing redshift due to
shallower sampling. All 72 clusters in a well-defined sample limited by
redshift (ensuring good sampling) and X-ray flux (excluding superpositions)
show infall patterns sufficient to apply the caustic technique. This sample is
by far the largest sample of cluster mass profiles extending to large radii to
date. Similar to CAIRNS, cluster infall patterns are better defined in
observations than in simulations. Further work is needed to determine the
source of this difference. We use the infall patterns to compute mass profiles
for 72 clusters and compare them to model profiles. Cluster scaling relations
using caustic masses agree well with those using X-ray or virial mass
estimates, confirming the reliability of the caustic technique. We confirm the
conclusion of CAIRNS that cluster infall regions are well fit by NFW and
Hernquist profiles and poorly fit by singular isothermal spheres. This much
larger sample enables new comparisons of cluster properties with those in
simulations. The shapes (specifically, NFW concentrations) of the mass profiles
agree well with the predictions of simulations. The mass inside the turnaround
radius is on average 2.190.18 times that within the virial radius. This
ratio agrees well with recent predictions from simulations of the final masses
of dark matter haloes.Comment: 34 pages, 24 figures, accepted for publication in AJ, full resolution
version available at http://www.astro.yale.edu/krines
A High Throughput Workflow Environment for Cosmological Simulations
The next generation of wide-area sky surveys offer the power to place
extremely precise constraints on cosmological parameters and to test the source
of cosmic acceleration. These observational programs will employ multiple
techniques based on a variety of statistical signatures of galaxies and
large-scale structure. These techniques have sources of systematic error that
need to be understood at the percent-level in order to fully leverage the power
of next-generation catalogs. Simulations of large-scale structure provide the
means to characterize these uncertainties. We are using XSEDE resources to
produce multiple synthetic sky surveys of galaxies and large-scale structure in
support of science analysis for the Dark Energy Survey. In order to scale up
our production to the level of fifty 10^10-particle simulations, we are working
to embed production control within the Apache Airavata workflow environment. We
explain our methods and report how the workflow has reduced production time by
40% compared to manual management.Comment: 8 pages, 5 figures. V2 corrects an error in figure
Orbital Instabilities in a Triaxial Cusp Potential
This paper constructs an analytic form for a triaxial potential that
describes the dynamics of a wide variety of astrophysical systems, including
the inner portions of dark matter halos, the central regions of galactic
bulges, and young embedded star clusters. Specifically, this potential results
from a density profile of the form , where the radial
coordinate is generalized to triaxial form so that . Using the resulting analytic form of the potential, and the
corresponding force laws, we construct orbit solutions and show that a robust
orbit instability exists in these systems. For orbits initially confined to any
of the three principal planes, the motion in the perpendicular direction can be
unstable. We discuss the range of parameter space for which these orbits are
unstable, find the growth rates and saturation levels of the instability, and
develop a set of analytic model equations that elucidate the essential physics
of the instability mechanism. This orbit instability has a large number of
astrophysical implications and applications, including understanding the
formation of dark matter halos, the structure of galactic bulges, the survival
of tidal streams, and the early evolution of embedded star clusters.Comment: 50 pages, accepted for publication in Ap
Future Evolution of Structure in an Accelerating Universe
Current cosmological data indicate that our universe contains a substantial
component of dark vacuum energy that is driving the cosmos to accelerate. We
examine the immediate and longer term consequences of this dark energy (assumed
here to have a constant density). Using analytic calculations and supporting
numerical simulations, we present criteria for test bodies to remain bound to
existing structures. We show that collapsed halos become spatially isolated and
dynamically relax to a particular density profile with logarithmic slope
steeper than -3 at radii beyond r_200. The asymptotic form of the space-time
metric is then specified. We develop this scenario further by determining the
effects of the accelerating expansion on the background radiation fields and
individual particles. In an appendix, we generalize these results to include
quintessence.Comment: 12 pages, 6 figures. Submitted to ApJ on April 23, 200
A Theoretical Interpretation of the Black Hole Fundamental Plane
We examine the origin and evolution of correlations between properties of
supermassive black holes (BHs) and their host galaxies using simulations of
major galaxy mergers, including the effects of gas dissipation, cooling, star
formation, and BH accretion and feedback. We demonstrate that the simulations
predict the existence of a BH 'fundamental plane' (BHFP), of the form M_BH
sigma^(3.0+-0.3)*R_e^(0.43+-0.19) or M_BH
M_bulge^(0.54+-0.17)*sigma^(2.2+-0.5), similar to relations found
observationally. The simulations indicate that the BHFP can be understood
roughly as a tilted intrinsic correlation between BH mass and spheroid binding
energy, or the condition for feedback coupling to power a pressure-driven
outflow. While changes in halo circular velocity, merger orbital parameters,
progenitor disk redshifts and gas fractions, ISM gas pressurization, and other
parameters can drive changes in e.g. sigma at fixed M_bulge, and therefore
changes in the M_BH-sigma or M_BH-M_bulge relations, the BHFP is robust. Given
the empirical trend of decreasing R_e for a given M_bulge at high redshift, the
BHFP predicts that BHs will be more massive at fixed M_bulge, in good agreement
with recent observations. This evolution in the structural properties of merger
remnants, to smaller R_e and larger sigma (and therefore larger M_BH,
conserving the BHFP) at a given M_bulge, is driven by the fact that bulge
progenitors have characteristically larger gas fractions at high redshifts.
Adopting the observed evolution of disk gas fractions with redshift, our
simulations predict the observed trends in both R_e(M_bulge) and M_BH(M_bulge).Comment: 22 pages, 19 figures, replaced with version accepted to ApJ.
Companion paper to arXiv:0707.400
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