The Norma Cluster (ACO 3627): I. A Dynamical Analysis of the Most Massive Cluster in the Great Attractor

TL;DR

This paper presents a detailed dynamical analysis of the Norma cluster, revealing its velocity structure, subpopulations, and evidence of ongoing mergers, contributing to understanding the cluster's mass and its role in the Great Attractor.

Contribution

It provides the first comprehensive dynamical analysis of the Norma cluster, including velocity measurements, substructure identification, and mass estimation, highlighting its relaxed E/S0 population and active merger processes.

Findings

Mean velocity of 4871 km/s with 925 km/s dispersion

E/S0 population is relaxed and aligned with large-scale structures

Spiral galaxies show substructure and signs of interaction with intracluster medium

Abstract

A detailed dynamical analysis of the nearby rich Norma cluster (ACO 3627) is presented. From radial velocities of 296 cluster members, we find a mean velocity of 4871 +/- 54 km/s and a velocity dispersion of 925 km/s. The mean velocity of the E/S0 population (4979 +/- 85 km/s) is offset with respect to that of the S/Irr population (4812 +/- 70 km/s) by `Delta' v = 164 km/s in the cluster rest frame. This offset increases towards the core of the cluster. The E/S0 population is free of any detectable substructure and appears relaxed. Its shape is clearly elongated with a position angle that is aligned along the dominant large-scale structures in this region, the so-called Norma wall. The central cD galaxy has a very large peculiar velocity of 561 km/s which is most probably related to an ongoing merger at the core of the cluster. The spiral/irregular galaxies reveal a large amount of substructure; two dynamically distinct subgroups within the overall spiral-population have been identified, located along the Norma wall elongation. The dynamical mass of the Norma cluster within its Abell radius is 1 - 1.1 x 10^15 h^-1_73 M_Sun. One of the cluster members, the spiral galaxy WKK 6176 which recently was observed to have a 70 kpc X-ray tail, reveals numerous striking low-brightness filaments pointing away from the cluster centre suggesting strong interaction with the intracluster medium.