2,100 research outputs found
Dark Energy and the Hubble Age
I point out that an effective upper limit of approximately 20 Gyr (for a
Hubble constant of 72 km/s/Mpc) or alternatively on the -independent
quantity , exists on the age of the Universe, essentially
independent of the unknown equation of state of the dominant dark energy
component in the Universe. Unless astrophysical constraints on the age of the
Universe can convincingly reduce the upper limit to below this value no useful
lower limit on the equation of state parameter for this component can be
obtained. Direct dating by stars does not provide a useful constraint, but
model-dependent cosmological limits from supernovae and the CMB observations
may. For a constant value of , a bound Comment: 4 pages, submitted to Ap. J. Lett (analytic asymptotic upper bound
now added
The Effects of Clumping and Substructure on ICM Mass Measurements
We examine an ensemble of 48 simulated clusters to determine the effects of
small-scale density fluctuations and large-scale substructure on X-ray
measurements of the intracluster medium (ICM) mass. We measure RMS density
fluctuations in the ICM which can be characterized by a mean mass-weighted
clumping factor C = /^2 between 1.3 and 1.4 within a density
contrast of 500 times the critical density. These fluctuations arise from the
cluster history of accretion shocks and major mergers, and their presence
enhances the cluster's luminosity relative to the smooth case. We expect,
therefore, that ICM mass measurements utilizing models which assume uniform
density at a given radius carry a bias of order sqrt(C) = 1.16. We verify this
result by performing ICM mass measurements on X-ray images of the simulations
and finding the expected level of bias.
The varied cluster morphologies in our ensemble also allow us to investigate
the effects of departures from spherical symmetry on our measurements. We find
that the presence of large-scale substructure does not further bias the
resulting gas mass unless it is pronounced enough to produce a second peak in
the image of at least 1% the maximum surface brightness. We analyze the subset
of images with no secondary peaks and find a bias of 9% and a Gaussian random
error of 4% in the derived mass.Comment: To appear in ApJ
Velocity bias in a LCDM model
We use N-body simulations to study the velocity bias of dark matter halos,
the difference in the velocity fields of dark matter and halos, in a flat low-
density LCDM model. The high force, 2kpc/h, and mass, 10^9Msun/h, resolution
allows dark matter halos to survive in very dense environments of groups and
clusters making it possible to use halos as galaxy tracers. We find that the
velocity bias pvb measured as a ratio of pairwise velocities of the halos to
that of the dark matter evolves with time and depends on scale. At high
redshifts (z ~5) halos move generally faster than the dark matter almost on all
scales: pvb(r)~1.2, r>0.5Mpc/h. At later moments the bias decreases and gets
below unity on scales less than r=5Mpc/h: pvb(r)~(0.6-0.8) at z=0. We find that
the evolution of the pairwise velocity bias follows and probably is defined by
the spatial antibias of the dark matter halos at small scales. One-point
velocity bias b_v, defined as the ratio of the rms velocities of halos and dark
matter, provides a more direct measure of the difference in velocities because
it is less sensitive to the spatial bias. We analyze b_v in clusters of
galaxies and find that halos are ``hotter'' than the dark matter: b_v=(1.2-1.3)
for r=(0.2-0.8)r_vir, where r_vir is the virial radius. At larger radii, b_v
decreases and approaches unity at r=(1-2)r_vir. We argue that dynamical
friction may be responsible for this small positive velocity bias b_v>1 found
in the central parts of clusters. We do not find significant difference in the
velocity anisotropy of halos and the dark matter. The dark matter the velocity
anisotropy can be approximated as beta(x)=0.15 +2x/(x^2+4), where x is measured
in units of the virial radius.Comment: 13 pages, Latex, AASTeXv5 and natbi
Mass Estimates of X-Ray Clusters
We use cosmological gas dynamic simulations to investigate the accuracy of
galaxy cluster mass estimates based on X-ray observations. The experiments
follow the formation of clusters in different cosmological models and include
the effects of gravity, pressure gradients, and hydrodynamical shocks. A subset
of our ensemble also allows for feedback of mass and energy from galactic winds
into the intracluster medium. We find that mass estimates based on the
hydrostatic, isothermal beta-model are remarkably accurate when evaluated at
radii where the cluster mean density is between 500-2500 times the critical
density. Applied to 174 artificial ROSAT images constructed from the
simulations, the distribution of the estimated-to-true mass ratio is nearly
unbiased and has a standard deviation of 14-29%. The scatter can be
considerably reduced (to 8-15%) by using an alternative mass estimator that
exploits the tightness of the mass-temperature relation found in the
simulations. The improvement over beta-model estimates is due to the
elimination of the variance contributed by the gas outer slope parameter. We
discuss these findings and their implications for recent measurements of
cluster baryon fractions.Comment: TeX, 24p; 11 Postscript figs. Submitted to the Astrophysical Journa
Gravitational Lensing as a Probe of Quintessence
A large number of cosmological studies now suggest that roughly two-thirds of
the critical energy density of the Universe exists in a component with negative
pressure. If the equation of state of such an energy component varies with
time, it should in principle be possible to identify such a variation using
cosmological probes over a wide range in redshift. Proper detection of any time
variation, however, requires cosmological probes beyond the currently studied
range in redshift of 0.1 to 1. We extend our analysis to gravitational
lensing statistics at high redshift and suggest that a reliable sample of
lensed sources, out to a redshift of 5, can be used to constrain the
variation of the equation of state, provided that both the redshift
distribution of lensed sources and the selection function involved with the
lensed source discovery process are known. An exciting opportunity to catalog
an adequate sample of lensed sources (quasars) to probe quintessence is now
available with the ongoing Sloan Digital Sky Survey. Writing , we study the expected accuracy to which the equation of state
today and its rate of change can simultaneously be
constrained. Such a determination can rule out some missing-energy candidates,
such as classes of quintessence models or a cosmological constant.Comment: Accepted for publication in ApJ Letters (4 pages, including 4
figures
Hydrodynamic simulations of correlation and scatter in galaxy cluster maps
The two dimensional structure of hot gas in galaxy clusters contains
information about the hydrodynamical state of the cluster, which can be used to
understand the origin of scatter in the thermodynamical properties of the gas,
and to improve the use of clusters to probe cosmology. Using a set of
hydrodynamical simulations, we provide a comparison between various maps
currently employed in the X-ray analysis of merging clusters and those cluster
maps anticipated from forthcoming observations of the thermal
Sunyaev-Zel'dovich effect. We show the following: 1) an X-ray pseudo-pressure,
defined as square root of the soft band X-ray image times the temperature map
is a good proxy for the SZ map; 2) we find that clumpiness is the main reason
for deviation between X-ray pseudo-pressure and SZ maps; 3) the level of
clumpiness can be well characterized by X-ray pseudo-entropy maps. 4) We
describe the frequency of deviation in various maps of clusters as a function
of the amplitude of the deviation. This enables both a comparison to
observations and a comparison to effects of introduction of complex physical
processes into simulation.Comment: 7 pages, A&A in pres
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