Predicting the peculiar velocities of nearby PSCz galaxies using the Least Action Principle

Abstract

We use the Least Action Principle to predict the peculiar velocities of PSCz galaxies inside cz=2000 km s−1. Linear theory is used to account for tidal effects to cz=15 000 km s−1, and we iterate galaxy positions to account for redshift distortions. As the Least Action Principle is valid beyond linear theory, we can predict reliable peculiar velocities even for very nearby galaxies (i.e., cz≤500 km s−1). These predicted peculiar velocities are then compared with the observed velocities of 12 galaxies with Cepheid distances. The combination of the PSCz galaxy survey (with its large sky coverage and uniform selection) with the accurate Cepheid distances makes this comparison relatively free from systematic effects. We find that galaxies are good tracers of the mass, even at small (≤10 h−1 Mpc) scales; under the assumption of no biasing, 0.25≤β≤0.75 (at 90 per cent confidence). We use the reliable predicted peculiar velocities to estimate the Hubble constant H0 from the local volume without ‘stepping up’ the distance ladder, finding a confidence range of 65–75 km s−1 Mpc−1 (at 90 per cent confidence).