Cosmic Mach number as a function of overdensity and galaxy age

We carry out an extensive study of the cosmic Mach number (M) on scales of R=5, 10, and 20 h(-1) Mpc using a Lambda -dominated flat cold dark matter hydrodynamical simulation. We particularly put emphasis on the environmental dependence of M on overdensity, galaxy mass, and galaxy age. We start by discussing the difference in the resulting M according to different definitions of M and different methods of calculation. The simulated Mach numbers are slightly lower than the linear theory predictions even when a nonlinear power spectrum was used in the calculation, reflecting the nonlinear evolution in the simulation. We find that the observed M is higher than the simulated mean <M > 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 Omega (m) is similar to0.2, 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 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 the structure formation history. 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.