The Cosmic Mach Number: Comparison from Observations, Numerical Simulations and Nonlinear Predictions

Abstract

We calculate the cosmic Mach number M - the ratio of the bulk flow of the velocity field on scale R to the velocity dispersion within regions of scale R. M is effectively a measure of the ratio of large-scale to small-scale power and can be a useful tool to constrain the cosmological parameter space. Using a compilation of existing peculiar velocity surveys, we calculate M and compare it to that estimated from mock catalogues extracted from the LasDamas (a LCDM cosmology) numerical simulations. We find agreement with expectations for the LasDamas cosmology at ~ 1.5 sigma CL. We also show that our Mach estimates for the mocks are not biased by selection function effects. To achieve this, we extract dense and nearly-isotropic distributions using Gaussian selection functions with the same width as the characteristic depth of the real surveys, and show that the Mach numbers estimated from the mocks are very similar to the values based on Gaussian profiles of the corresponding widths. We discuss the importance of the survey window functions in estimating their effective depths. We investigate the nonlinear matter power spectrum interpolator PkANN as an alternative to numerical simulations, in the study of Mach number.

12 pages, 9 figures, 3 tables

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• SFI++ I: A New I-band Tully-Fisher Template, the Cluster Peculiar Velocity Dispersion and H0
• Impact of massive neutrinos on nonlinear matter power spectrum
• Nonlinear power spectrum in the presence of massive neutrinos: perturbation theory approach, galaxy bias and parameter forecasts
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• Uneven flows: On cosmic bulk flows, local observers, and gravity
• Secondary halo bias through cosmic time II: Reconstructing halo properties using clustering information
• The Cosmic Mach Number as an Environment Measure for the Underlying Dark Matter Density Field