749 research outputs found
Biased Estimates of Omega from Comparing Smoothed Predicted Velocity Fields to Unsmoothed Peculiar Velocity Measurements
We show that a regression of unsmoothed peculiar velocity measurements
against peculiar velocities predicted from a smoothed galaxy density field
leads to a biased estimate of the cosmological density parameter Omega, even
when galaxies trace the underlying mass distribution and galaxy positions and
velocities are known perfectly. The bias arises because the errors in the
predicted velocities are correlated with the predicted velocities themselves.
We investigate this bias using cosmological N-body simulations and analytic
arguments. In linear perturbation theory, for cold dark matter power spectra
and Gaussian or top hat smoothing filters, the bias in Omega is always
positive, and its magnitude increases with increasing smoothing scale. This
linear calculation reproduces the N-body results for Gaussian smoothing radii
R_s > 10 Mpc/h, while non-linear effects lower the bias on smaller smoothing
scales, and for R_s < 3 Mpc/h Omega is underestimated rather than
overestimated. The net bias in Omega for a given smoothing filter depends on
the underlying cosmological model. The effect on current estimates of Omega
from velocity-velocity comparisons is probably small relative to other
uncertainties, but taking full advantage of the statistical precision of future
peculiar velocity data sets will require either equal smoothing of the
predicted and measured velocity fields or careful accounting for the biases
discussed here.Comment: 11 pages including 2 eps figures. Submitted to Ap
Mock Catalogs for UHECR Studies
We provide realistic mock-catalogs of cosmic rays above 40 EeV, for a pure
proton composition, assuming their sources are a random subset of ordinary
galaxies in a simulated, volume-limited survey, for various choices of source
density: 10^-3.5 Mpc^-3, 10^-4.0 Mpc^-3 and 10^-4.5 Mpc^-3. The spectrum at the
source is taken to be E^-2.3 and the effects of cosmological redshifting as
well as photo-pion and e^+ e^- energy losses are included.Comment: 7 pages, 4 figure
Sensitivity of orbiting JEM-EUSO to large-scale cosmic-ray anisotropies
The two main advantages of space-based observation of extreme-energy
(~eV) cosmic-rays (EECRs) over ground-based observatories are
the increased field of view, and the all-sky coverage with nearly uniform
systematics of an orbiting observatory. The former guarantees increased
statistics, whereas the latter enables a partitioning of the sky into spherical
harmonics. We have begun an investigation, using the spherical harmonic
technique, of the reach of \J\ into potential anisotropies in the
extreme-energy cosmic-ray sky-map. The technique is explained here, and
simulations are presented. The discovery of anisotropies would help to identify
the long-sought origin of EECRs.Comment: 7 pages, 6 figures. To appear in the proceedings of the Cosmic Ray
Anisotropy Workshop, Madison Wisconsin, September 201
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