The dynamical status of the galaxy cluster Abell 115

Aims. We present the results of a new spectroscopic and photometric survey of the hot, binary X-ray cluster A115 at z = 0.193, containing a radio relic. Methods. Our analysis is based on new spectroscopic data obtained at the Telescopio Nazionale Galileo for 115 galaxies and on new photometric data obtained at the Isaac Newton Telescope in a large field. We combine galaxy velocity and position information to select 85 galaxies recognized as cluster members, determine global dynamical properties and detect substructures. Results. We find that A115 appears as a well isolated peak in the redshift space, with a global line-of-sight ( LOS) velocity dispersion sigma(v) = 1362(-108)(+126) km s(-1). Our analysis confirms the presence of two structures of cluster-type well recognizable in the plane of the sky and shows that they differ of similar to 2000 km s(-1) in the LOS velocity. The northern, high velocity subcluster ( A115N) is likely centred on the second brightest cluster galaxy ( BCM-A, coincident with radio source 3C28) and the northern X-ray peak. The southern, low velocity subcluster ( A115S) is likely centred on the first brightest cluster galaxy ( BCM-B) and the southern X-ray peak. We estimate that A115S is slightly dynamically more important than A115N having sigma(nu) = 900-1100 km s(-1) vs. sigma(nu) = 750-850 km s(-1). Moreover, we find evidence for two small groups at low velocities. We estimate a global cluster virial mass of 2.2- 3.5 x 10(15) h(70)(-1) M-circle dot. Conclusions. Our results agree with a pre- merging scenario where A115N and A115S are colliding with a LOS impact velocity. Delta nu(rf) similar to 1600 km s(-1). The most likely solution to the two- body problem suggests that the merging axis lies at similar to 20 degrees from the plane of the sky and that the cores will cross after similar to 0.1 Gyr. The radio relic with its largest dimension perpendicular to the merging axis is likely connected to this merger.