Fossil group origins XIV: The radial orbits of A267

TL;DR

This study analyzes the orbital distribution of galaxies in the fossil cluster A267, confirming that fossil groups have more radial orbits than typical clusters, which may influence their large magnitude gap.

Contribution

First to determine the orbital distribution of galaxies in an individual fossil group, linking radial orbits to the group's unique properties.

Findings

Galaxies near the center have tangential orbits, while outer galaxies are more radial.

Fossil groups host galaxies on more radial orbits than average clusters.

The orbital distribution may contribute to the large magnitude gap in fossil groups.

Abstract

Fossil groups (FGs) are groups or clusters of galaxies with a single, massive, central galaxy and with a clear lack of L* galaxies. The physical reason for their large magnitude gap (dm12) may arise from early FG formation, which allowed all L galaxies to merge with the central one, and/or it could be related to the fact that galaxies accreting on the FGs move on radial orbits, shortening their merging timescales. The latter properties could be linked with the peculiar position of FGs within the cosmic web. We determine the velocity anisotropy profile beta(r) of the fossil cluster A267, which is related to the orbital distribution of cluster galaxies. This is the first individual FG for which the orbital distribution of galaxies is determined. We aim to confirm previous findings based on stack samples that indicate that FGs, on average, host galaxies on more radial orbits than normal clusters. We started with a sample of 2315 redshifts in the field of A267 and we determined the membership for 329 of them. Of these, 174 are located within r200. We used them as tracers of the gravitational potential of the cluster to solve the Jeans equation using the MAMPOSSt algorithm. We thus obtained the cluster mass M(r) and beta(r) profiles. We also estimated M(r) from the X-ray data. A comparison of the MAMPOSSt and X-ray-determined M(r)s allows us to estimate the cluster hydrostatic mass bias, that is consistent with previous findings. The anisotropy parameter beta(r) indicates tangential orbits for the galaxies near the cluster centre and increasingly radial orbits in the external regions. We therefore confirm that FGs are characterised by more radial orbits for their member galaxies than the average cluster population. We speculate that this different orbital distribution might be an important element in creating a large dm12.