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 ($\gtrsim 10^{19}$~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