Is Abell 2261 a fossil galaxy cluster in a transitional dynamical state?

TL;DR

This study uses new spectroscopic data to analyze Abell 2261's dynamical state, suggesting it is moderately relaxed but possibly recently transitioned from fossil status due to group infall and filamentary accretion.

Contribution

It provides a detailed dynamical analysis of Abell 2261 with improved data, revealing evidence of recent group infall and filamentary structure affecting its fossil status.

Findings

A2261 is moderately relaxed based on dynamical indicators.

Presence of a group candidate near the virial radius suggests recent infall.

Identification of a filamentary structure connecting to the cluster outskirts.

Abstract

Abell 2261 (A2661) is a well-studied fossil cluster, but previous studies give contradictory results on its dynamical states, such as its X-ray central entropy and magnitude gap. To improve our understanding of its dynamical state, we conduct multi-object spectroscopic observations with Hectospec on the MMT, covering an area out to 5 virial radii from the cluster center, and get improved completeness and membership. Using this new data, we calculate multiple dynamical indicators including gaussianity, distance offset, and velocity offset. These indicators suggest that A2261 is moderately relaxed. However, Dressler-Shectman test reveals a group candidate to the south, at a projected distance that is near the virial radius, and that overlaps with an X-ray tail-like feature. One of the galaxies associated with that group would be sufficiently bright to reduce the fossil magnitude gap. This raises the possibility that A2261 could have recently transit in fossil status, if the group had previously crossed the cluster and is only now found outside. In the cluster outskirts, we see an extended feature of galaxies located on the opposite side of the cluster to the group candidate. On even larger scales, we find this feature connects, both on the sky and in velocity space, with a long (~4.4Mpc) filamentary structure in the Sloan Digital Sky Survey data. This could support the idea that a group was fed into the cluster through the filament, temporarily breaking the fossil status, and resulting in a minor merger that weakly disturbed the intracluster medium of the cluster.