The Norma cluster (ACO 3627) – III. The Distance and Peculiar Velocity via the Near-Infrared Ks-band Fundamental Plane.

Abstract

While Norma (ACO 3627) is the richest cluster in the Great Attractor (GA) region, its role in the local dynamics is poorly understood. The Norma cluster has a mean redshift (zCMB) of 0.0165 and has been proposed as the “core” of the GA. We have used the Ks-band Fundamental Plane (FP) to measure Norma cluster’s distance with respect to the Coma cluster. We report FP photometry parameters (effective radii and surface brightnesses), derived from ESO NTT SOFI images, and velocity dispersions, from AAT 2dF spectroscopy, for 31 earlytype galaxies in the cluster. For the Coma cluster we use 2MASS images and SDSS velocity dispersion measurements for 121 early-type galaxies to generate the calibrating FP dataset. For the combined Norma-Coma sample we measure FP coefficients of a = 1.465± 0.059 and b = 0.326± 0.020. We find an rms scatter, in log σ, of ∼ 0.08 dex which corresponds to a distance uncertainty of ∼ 28% per galaxy. The zero point offset between Norma’s and Coma’s FPs is 0.154± 0.014 dex. Assuming that the Coma cluster is at rest with respect to the cosmic microwave background frame and zCMB(Coma) = 0.0240, we derive a distance to the Norma cluster of 5026± 160 km s−1 , and the derived peculiar velocity is −72± 170 km s−1 , i.e., consistent with zero. This is lower than previously reported positive peculiar velocities for clusters/groups/galaxies in the GA region and hence the Norma cluster may indeed represent the GA’s “core”.