The Galaxy-weighted Small-Scale Velocity Dispersion of the Las Campanas Redshift Survey

The pair-weighted relative velocity dispersion of galaxies provides a measure of the thermal energy of fluctuations of the observed galaxy distribution, but the measure is difficult to interpret and is very sensitive to the existence of rare rich clusters of galaxies. Several alternative statistical procedures have recently been suggested to relieve these problems. We apply a variant of the object-weighted statistical method of Davis, Miller, & White to the Las Campanas Redshift Survey (LCRS), which is the largest and deepest existing redshift survey that is nearly fully sampled. The derived one-dimensional dispersion on scales of ~1 h^-1 Mpc is quite low: σ₁ = 126 ± 10 km s^-1, with a modest decrease at larger scales. The statistic is very stable; the six independent slices of the LCRS all yield consistent results. We apply the same statistical procedure to halos in numerical simulations of an open cosmological model and flat models with and without a cosmological constant. In contrast to the LCRS, all the models show a dispersion that increases for scales >1 h^-1 Mpc; it is uncertain whether this is a numerical artifact or a real physical effect. The standard cluster-normalized cold dark matter model with Ω_m = 1, as well as a tilted variant with n = 0.8, yield dispersions substantially hotter than the LCRS value, while models with low matter density (Ω_m = 0.3) are broadly consistent with the LCRS data. Using a filtered cosmic energy equation, we measure Ω_m ≈ 0.2, with small-scale bias factors b = 1.0-1.5 for high-density models and b = 0.7-1.1 for low-density models.