67 research outputs found
Self Calibration of Tomographic Weak Lensing for the Physics of Baryons to Constrain Dark Energy
Numerical studies indicate that uncertainties in the treatment of baryonic
physics can affect predictions for shear power spectra at a level that is
significant for forthcoming surveys such as DES, SNAP, and LSST.
Correspondingly, we show that baryonic effects can significantly bias dark
energy parameter measurements. Eliminating such biases by neglecting
information in multipoles beyond several hundred leads to weaker parameter
constraints by a factor of approximately 2 to 3 compared with using information
out to multipoles of several thousand. Fortunately, the same numerical studies
that explore the influence of baryons indicate that they primarily affect power
spectra by altering halo structure through the relation between halo mass and
mean effective halo concentration. We explore the ability of future weak
lensing surveys to constrain both the internal structures of halos and the
properties of the dark energy simultaneously as a first step toward self
calibrating for the physics of baryons. This greatly reduces parameter biases
and no parameter constraint is degraded by more than 40% in the case of LSST or
30% in the cases of SNAP or DES. Modest prior knowledge of the halo
concentration relation greatly improves even these forecasts. Additionally, we
find that these surveys can constrain effective halo concentrations near
m~10^14 Msun/h and z~0.2 to better than 10% with shear power spectra alone.
These results suggest that inferring dark energy parameters with measurements
of shear power spectra can be made robust to baryonic effects and may
simultaneously be competitive with other methods to inform models of galaxy
formation. (Abridged)Comment: 18 pages, 11 figures. Minor changes reflecting referee's comments.
Results and conclusions unchanged. Accepted for publication in Physical
Review
Non-Equilibrium Electrons and the Sunyaev-Zel'dovich Effect of Galaxy Clusters
We present high-resolution cosmological hydrodynamic simulations of three
galaxy clusters employing a two-temperature model for the intracluster medium.
We show that electron temperatures in cluster outskirts are significantly lower
than the mean gas temperature, because Coulomb collisions are insufficient to
keep electrons and ions in thermal equilibrium. This deviation is larger in
more massive and less relaxed systems, ranging from 5% in relaxed clusters to
30% for clusters undergoing major mergers. The presence of non-equilibrium
electrons leads to significant suppression of the SZE signal at large
cluster-centric radius. The suppression of the electron pressure also leads to
an underestimate of the hydrostatic mass. Merger-driven, internal shocks may
also generate significant populations of non-equilibrium electrons in the
cluster core, leading to a 5% bias on the integrated SZ mass proxy during
cluster mergers.Comment: 5 pages, 4 figures, Accepted for publication in ApJ
Effects of Baryons and Dissipation on the Matter Power Spectrum
We study the importance of baryonic physics on predictions of the matter
power spectrum as it is relevant for forthcoming weak lensing surveys. We
quantify the impact of baryonic physics using a set of three cosmological
numerical simulations. Each simulation has the same initial density field, but
models a different set of physical processes. The first simulation evolves the
density field using gravity alone, the second includes non-radiative
gasdynamics, and the third includes radiative heating and cooling of baryons,
star formation, and supernova feedback. We find that baryonic processes alter
predictions for the matter power spectrum significantly relative to models that
include only gravitational interactions. Our results imply that future weak
lensing experiments such as LSST and SNAP will be very sensitive to the
poorly-understood physics governing the nonlinear evolution of the baryonic
component of the universe. The net effect is significantly larger in the case
of the model with cooling and star formation, in which case our results imply
that contemporary surveys such as the CFHT Wide survey may also be sensitive to
baryonic processes. In particular, this effect could be important for forecasts
of the constraining power of future surveys if information from scales larger
than l ~ 1000 is included in the analysis. We find that deviations are caused
primarily by the rearrangement of matter within individual dark matter halos
relative to the gravity-only case, rather than a large-scale rearrangement of
matter. Consequently, we propose a simple model, based on the phenomenological
halo model of dark matter clustering, for baryonic effects that can be used to
aid in the interpretation of forthcoming weak lensing data.Comment: 14 pages, 8 figures. Submitted to Ap
Computational Eulerian Hydrodynamics and Galilean Invariance
Eulerian hydrodynamical simulations are a powerful and popular tool for
modeling fluids in astrophysical systems. In this work, we critically examine
recent claims that these methods violate Galilean invariance of the Euler
equations. We demonstrate that Eulerian hydrodynamics methods do converge to a
Galilean-invariant solution, provided a well-defined convergent solution
exists. Specifically, we show that numerical diffusion, resulting from
diffusion-like terms in the discretized hydrodynamical equations solved by
Eulerian methods, accounts for the effects previously identified as evidence
for the Galilean non-invariance of these methods. These velocity-dependent
diffusive terms lead to different results for different bulk velocities when
the spatial resolution of the simulation is kept fixed, but their effect
becomes negligible as the resolution of the simulation is increased to obtain a
converged solution. In particular, we find that Kelvin-Helmholtz instabilities
develop properly in realistic Eulerian calculations regardless of the bulk
velocity provided the problem is simulated with sufficient resolution (a factor
of 2-4 increase compared to the case without bulk flows for realistic
velocities). Our results reiterate that high-resolution Eulerian methods can
perform well and obtain a convergent solution, even in the presence of highly
supersonic bulk flows.Comment: Version accepted by MNRAS Oct 2, 2009. Figures degraded. For
high-resolution color figures and movies of the numerical simulations, please
visit
http://www.astro.caltech.edu/~brant/Site/Computational_Eulerian_Hydrodynamics_and_Galilean_Invariance.htm
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