Cosmic Mach Number as a Function of Overdensity and Galaxy Age

Abstract

We carry out an extensive study of the cosmic Mach number (\mach) on scales of R=5, 10 and 20h^-1Mpc using an LCDM hydrodynamical simulation. We particularly put emphasis on the environmental dependence of \mach on overdensity, galaxy mass, and galaxy age. We start by discussing the difference in the resulting \mach according to different definitions of \mach and different methods of calculation. The simulated Mach numbers are slightly lower than the linear theory predictions even when a non-linear power spectrum was used in the calculation, reflecting the non-linear evolution in the simulation. We find that the observed \mach is higher than the simulated mean by more than 2-standard deviations, which suggests either that the Local Group is in a relatively low-density region or that the true value of Ω_m is ~ 0.2, significantly lower than the simulated value of 0.37. We show from our simulation that the Mach number is a weakly decreasing function of overdensity. We also investigate the correlations between galaxy age, overdensity and \mach for two different samples of galaxies --- DWARFs and GIANTs. Older systems cluster in higher density regions with lower \mach, while younger ones tend to reside in lower density regions with larger \mach, as expected from the hierarchical structure formation scenario. However, for DWARFs, the correlation is weakened by the fact that some of the oldest DWARFs are left over in low-density regions during the structure formation history. For giant systems, one expects blue-selected samples to have higher \mach than red-selected ones. We briefly comment on the effect of the warm dark matter on the expected Mach number.

43 pages, including 15 figures. Accepted version in ApJ. Included correlation function of different samples of galaxies, and the cumulative number fraction distribution as a fcn. of overdensity. Reorganized figures and added some references

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Cited by in corpus (9)

• Non-Friedmann cosmology for the Local Universe, significance of the universal Hubble constant and short-distance indicators of dark energy
• Local dark energy: HST evidence from the vicinity of the M 81/M 82 galaxy group
• Measuring the Mach number of the Universe via the Sunyaev-Zeldovich effect
• Uneven flows: On cosmic bulk flows, local observers, and gravity
• Local dark energy: HST evidence from the expansion flow around Cen A/M83 galaxy group
• Secondary halo bias through cosmic time II: Reconstructing halo properties using clustering information
• Numerical study of the cosmological velocity field as a function of density
• The Cosmic Mach Number as an Environment Measure for the Underlying Dark Matter Density Field
• Numerical Analyses of Weakly Nonlinear Velocity-Density Coupling