2006A&A...455...45G

We conduct an intensive study of the rich, X-ray luminous, and hot galaxy cluster Abell 697 (at z=0.282), likely containing a diffuse radio-emission, to determine its dynamical status.Our analysis is based on new spectroscopic data obtained at the TNG telescope for 93 galaxies and on new photometric data obtained at the INT telescope in a large field. We combine galaxy velocity and position information to select 68 cluster members (out to ∼1.3 h₇₀^–1Mpc from the cD galaxy), determine global dynamical properties, and detect possible substructures. The investigation of the dynamical status is also performed by using X-ray data stored in the Chandra archive.We compute the line-of-sight (LOS) velocity dispersion of galaxies, σ_v=1334_–95⁺¹¹⁴km/s, in agreement with the high average X-ray temperature T_X=(10.2±0.8)keV recovered from Chandra data, as expected in the case of energy-density equipartition between galaxies and gas. Assuming that the cluster is in dynamical equilibrium and mass follows the galaxy distribution, we find that A697 is a very massive cluster obtaining M(<R_max=0.75 h₇₀^–1Mpc)=9.5^+1.8_–1.5x10¹⁴ h₇₀^–1M_☉ and M(<R_vir=3.85h₇₀^–1Mpc)=4.5_–1.3^+1.4x10¹⁵h₇₀^–1M_☉ for the region well sampled by the spectroscopic data and for the entire virialized region, respectively. Further investigations find that A697 is not fully relaxed, as shown by the non Gaussianity of the velocity distribution, the elongation of the X-ray emission, and the presence of small-size substructures in the central region. Our results suggest that we are observing a cluster that has undergone a complex cluster merger occurring mainly along the LOS, with a transverse component in the SSE-NNW direction. The importance and the phase of the merging event is discussed. The spatial correlation between the (likely) radio halo and the optical and X-ray cluster structure supports the hypothesis of a relation between extended radio emission and merging phenomena.