0 Submitted to ApJ . Cosmic Mach

We carry out an extensive study of the cosmic Mach number (M) on scales of R = 5, 10 and 20h−1 Mpc using an LCDM hydrodynamical simulation. We discuss the difference in the resultingM according to the different definitions ofM and the different methods of calculation. The simulated Mach numbers are slightly lower than the linear theory predictions, reflecting the non-linear evolution in the simulation. We find that the observed M is higher than the simulated mean 〈M〉 by more than 2-standard deviations at a high confidence level, which suggests either that the Local Group is in a relatively low density region or that the true value of Ωm 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 correlation between galaxy age, overdensity and M for two different samples of galaxies — DWARFs and GIANTs. Older systems cluster in higher density regions with lower M, while younger ones tend to reside in lower density regions with larger M, 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 structure formation. Therefore, for giant systems, one expects blue-selected samples to have higher M than red-selected ones. We briefly comment on the effect of the warm dark matter on the expected Mach number as well. Subject headings: galaxies: formation — cosmology: theory