Galaxy evolution in Abell 85 I. Cluster substructure and environmental effects on the blue galaxy population

In this series of papers we explore the evolution of late-type galaxies in the rich cluster Abell 85. In this first paper we revisit the complex dynamical state of A85 by using independent methods. First, we analyze the galaxy redshift distribution towards A85 in the whole range 0-40 000 km s(-1), and determine the mean redshifts of the background clusters A87 and A89, very close in projection to A85. Then we search for substructures in A85 by considering the 2D galaxy distribution of its members (13 000-20 000 km s(-1)) and by applying the kinematical 3D Delta-test to both projected positions and radial velocities. This clearly reveals several substructures: one close to the cluster core and three more projected towards the southeast, along the region where an X-ray filament has been extensively studied. We also analyse the distribution of the brightest blue galaxies across a major fraction of the cluster volume, considering if they are gas-rich or poor. We report a very asymmetric distribution of the blue member galaxies, with most of them to the east and southeast, namely in the region joining the core of A85 to its farthest substructure in this direction - dubbed the SE clump. By matching our sample of bright blue member galaxies with Hi detections reported in the literature, we identify gas-rich and gas-poor ones. As expected, the Hi-rich blue galaxies follow the same trend as the parent sample, with most of them projected on the eastern side of the cluster as well. Interestingly no blue objects have been detected in Hi up to a projected radius of 2 Mpc in this zone. We finally estimate the ram pressure stripping exerted by the intra-cluster medium as a function of the projected distance from A85, in order to quantify how important this mechanism might be in sweeping the gas out of the infalling spirals.