The peculiar motions of early-type galaxies in two distant regions - VII. Peculiar velocities and bulk motions

Abstract

We present peculiar velocities for 84 clusters of galaxies in two large volumes at distances between 6000 and 15000 km/s in the directions of Hercules-Corona Borealis and Perseus-Pisces-Cetus. These velocities are based on Fundamental Plane (FP) distance estimates for early-type galaxies in each cluster. We fit the FP using a maximum likelihood algorithm which accounts for both selection effects and measurement errors, and yields FP parameters with smaller bias and variance than other fitting procedures. We find a best-fit FP with coefficients consistent with the best existing determinations. We measure the bulk motions of the sample volumes using the 50 clusters with the best-determined peculiar velocities. We find the bulk motions in both regions are small, and consistent with zero at about the 5% level. The EFAR results are in agreement with the small bulk motions found by Dale et al. (1999) on similar scales, but are inconsistent with pure dipole motions having the large amplitudes found by Lauer & Postman (1994) and Hudson et al. (1999). The alignment of the EFAR sample with the Lauer & Postman dipole produces a strong rejection of a large-amplitude bulk motion in that direction, but the rejection of the Hudson et al. result is less certain because their dipole lies at a large angle to the main axis of the EFAR sample. We find the bulk motion of our sample is consistent with most cosmological models that approximately reproduce the shape and normalisation of the observed galaxy power spectrum. We conclude that existing measurements of large-scale bulk motions provide no significant evidence against standard models for the formation of structure.