Internal dynamics of Abell 2254: a merging galaxy cluster with a clumpy, diffuse radio emission

¹ Dipartimento di Fisica dell’Università degli Studi di Trieste - Sezione di Astronomia, via Tiepolo 11, 34143 Trieste, Italy
e-mail: mgirardi@oats.inaf.it
² INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
³ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Instituto de Astrofísica de Canarias, C/vía Láctea s/n, 38205 La Laguna (Tenerife), Canary Islands, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, Av. del Astrofísico Franciso Sánchez s/n, 38205 La Laguna (Tenerife), Canary Islands, Spain
⁶ Fundación Galileo Galilei – INAF, Rambla José Ana Fernández Perez 7, 38712 Breña Baja (La Palma), Canary Islands, Spain
⁷ INAF-IASF Milano, via Bassini 15, 20133 Milano, Italy
⁸ Department of Physics and Astronomy, University of California at Irvine, 4129, Frederick Reines Hall, Irvine, CA, 92697-4575, USA

Received: 24 May 2011
Accepted: 25 September 2011

Abstract

Context. The mechanisms giving rise to diffuse radio emission in galaxy clusters and, in particular, their connection with cluster mergers are still being debated.

Aims. We explore the internal dynamics of Abell 2254, which has been shown to host a very clumpy and irregular radio halo.

Methods. Our analysis is mainly based on redshift data for 128 galaxies acquired at the Telescopio Nazionale Galileo. We combined galaxy velocities and positions to select 110 cluster galaxies and analyze its internal dynamics. We also used new (g′, r′, i′) photometric data acquired at the Isaac Newton Telescope, and (V, i′) photometric data available in the Subaru Archive. X-ray data from the XMM-Newton Science Archive were analyzed to study the hot gas component.

Results. We estimate the cluster redshift ⟨z⟩ = 0.177, a high line-of-sight (LOS) velocity dispersion, σ_V ~ 1350 km s^-1, and the X-ray temperature kT ~ 6.4 keV. Both our optical and X-ray analyses reveal complex dynamical activity. The analysis of the 2D galaxy distribution reveals the presence of two density peaks, one to the east and the other to the west (E and W peaks). Using the full 3D information we detect a high-velocity (ΔV_rf,LOS ~ 3000 km s^-1), low-mass (σ_V ~ 200–500 km s^-1) group at the position of the 2D E peak. For the main system we compute a velocity dispersion σ_V ~ 1000–1200 km s^-1. In the assumption of a bimodal system we estimate a mass M_sys = 1.5–2.9 . The X-ray morphological analysis, which is based on power ratios, centroid shifts, and concentration parameter, confirms that Abell 2254 is a dynamically disturbed cluster. The X-ray isophotes are elongated in the east direction, in agreement with a merger in the post core-crossing phase. A simple bimodal model finds that data are consistent with a bound, outgoing subcluster observed a few fractions of Gyr after the core crossing. However, both optical and X-ray analyses suggest that the main system is, in turn, a nonrelaxed structure, indicating north-south as a possible direction for a past accretion.

Conclusions. We conclude that Abell 2254, for its mass and merging structure, fits well among the typical clusters with radio halos. We briefly discuss how the particular irregularity of the radio halo might be linked to the complexity of the Abell 2254 structure.