Structure of Abell 1995 from optical and X-ray data: a galaxy cluster with an elongated radio halo⋆
Fundación Galileo Galilei – INAF (Telescopio Nazionale Galileo), Rambla
José Ana Fernández Perez 7, 38712 Breña Baja (La Palma)
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: 30 April 2012
Accepted: 27 September 2012
Context. Abell 1995 is a puzzling galaxy cluster hosting a powerful radio halo, but it has not yet been recognized as a obvious cluster merger, as usually expected for clusters with diffuse radio emission.
Aims. We aim at an exhaustive analysis of the internal structure of Abell 1995 to verify that this cluster is really dynamically relaxed, as reported in previous studies.
Methods. We base our analysis on new and archival spectroscopic and photometric data for 126 galaxies in the field of Abell 1995. The study of the hot intracluster medium was performed on X-ray archival data.
Results. Based on 87 fiducial cluster members, we have computed the average cluster redshift ⟨z⟩ = 0.322 and the global radial velocity dispersion σV ~ 1300 km s-1. We detect two main optical subclusters separated by 1.5′that cause the known NE-SW elongation of the galaxy distribution and a significant velocity gradient in the same direction. As for the X-ray analysis, we confirm that the intracluster medium is mildly elongated, but we also detect three X-ray peaks. Two X-ray peaks are offset with respect to the two galaxy peaks and lie between them, thus suggesting a bimodal merger caught in a phase of post core-core passage. The third X-ray peak lies between the NE galaxy peak and a third, minor galaxy peak suggesting a more complex merger. The difficulty of separating the two main systems leads to a large uncertainty on the line-of-sight (LOS) velocity separation and the system mass: ΔVrf,LOS = 600–2000 km s-1and Msys = 2–5×1015 h70-1 M⊙, respectively. Simple analytical arguments suggest a merging scenario for Abell 1995, where two main subsystems are seen just after the collision with an intermediate projection angle.
Conclusions. The high mass of Abell 1995 and the evidence of merging suggest it is not atypical among clusters with known radio halos. Interestingly, our findings reinforce the previous evidence for the peculiar dichotomy between the dark matter and galaxy distributions observed in this cluster.
Key words: galaxies: clusters: individual: Abell 1995 / galaxies: clusters: general / galaxies: kinematics and dynamics / X-rays: galaxies: clusters
Table 1 is available in electronic form at http://www.aanda.org
© ESO, 2012