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