Abell 1758N from an optical point of view: new insights on a merging cluster with diffuse radio emission

¹ Fundación Galileo Galilei – INAF (Telescopio Nazionale Galileo), Rambla José Ana Fernández Perez 7, 38712 Breña Baja (La Palma), Canary Islands, Spain
e-mail: boschin@tng.iac.es
² Dipartimento di Fisica dell’Università degli Studi di Trieste – Sezione di Astronomia, via Tiepolo 11, 34143 Trieste, Italy
³ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, 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 Francisco Sánchez s/n, 38205 La Laguna (Tenerife), Canary Islands, Spain

Received: 12 September 2011
Accepted: 27 January 2012

Abstract

Context. The mechanisms producing the diffuse radio emission in galaxy clusters, and in particular their connection with cluster mergers, are still debated.

Aims. We seek to explore the internal dynamics of the cluster Abell 1758N, which has been shown to host a radio halo and two relics, and is known to be a merging bimodal cluster.

Methods. Our analysis is mainly based on new redshift data for 137 galaxies acquired at the Telescopio Nazionale Galileo, only four of which have redshifts previously listed in the literature. We also used photometric data from the Sloan Digital Sky Survey and from the Canada-France-Hawaii Telescope archive. We combined galaxy velocities and positions to select 92 cluster galaxies and analyzed the internal cluster dynamics.

Results. We estimate a cluster redshift of ⟨z⟩ = 0.2782 and quite a high line-of-sight (LOS) velocity dispersion σ_V ~ 1300 km s^-1. Our 2D analysis confirms the presence of a bimodal structure along the NW–SE direction. We add several pieces of information to the previous merging scenario: the two subclusters (here A1758N(NW) and A1758N(SE)) cannot be separated in the velocity analyses and we deduce a small LOS velocity difference (ΔV_rf,LOS ≲ 300 km s^-1 in the cluster rest-frame). The velocity information successfully shows that A1758N is surrounded by two small groups and active galaxies infalling onto, or escaping from, the cluster. Removing the two groups, we estimate σ_V,NW ~ 1000 km s^-1 and σ_V,SE ~ 800 km s^-1 for A1758N(NW) and A1758N(SE), respectively. We find that Abell 1758N is a very massive cluster with a range of M = 2−3 × 10¹⁵ h₇₀^-1 M_⊙, depending on the adopted model.

Conclusions. As expected for clusters that host powerful, extended, diffuse radio emissions, Abell 1758N is a major cluster merger just forming a massive system.