Deviations from the Fundamental Plane and the Peculiar Velocities of Clusters

Abstract

We have fit the Fundamental Plane of Ellipticals (FP) to over 400 early-type galaxies in 20 nearby clusters (4000 < cz < 11000 km/s), using our own photometry and spectroscopy as well as measurements culled from the literature. We find that the quality-of-fit, r.m.s.[log(sigma)], to the average fundamental plane <FP> varies substantially among these clusters. A statistically significant gap in r.m.s.[log(sigma)] roughly separates the clusters which fit <FP> well from those that do not. In addition, these two groups of clusters show distinctly different behavior in their peculiar velocity (PV) distributions. Assuming galaxies are drawn from a single underlying population, cluster PV should not be correlated with r.m.s.[log(sigma)]. Instead, the clusters with below average scatter display no motion with respect to the cosmic microwave background (CMB) within our measurement errors (~250 km/s), while clusters in the poor-fit group typically show large PVs. Furthermore, we find that all X-ray bright clusters in our sample fit the <FP> well, suggesting that early-type galaxies in the most massive, virialized clusters form a more uniform population than do cluster ellipticals as a whole, and that these clusters participate in a quiet Hubble flow.