Tidally induced star formation in Abell 1367

Our principal aim is to compare global star formation rates between cluster galaxies and field galaxies in order to clarify environmental influence on star formation. We use an objective prism technique to survey over 200 Zwicky catalogue (CGCG) galaxies within similar to 2.degrees 5 of Abell 1367 for H alpha emission. After a brief discussion of the survey characteristics, we consider first the dependence of H alpha detection on Hubble type, galaxy disturbance and the presence of a bar. As expected, we rarely detect early-type galaxies and consequently restrict further discussion to spirals (type S0/a and later), of which we detect similar to 35 per cent in H alpha. We find that an extremely valuable distinction to make is between galaxies with diffuse H alpha and galaxies with compact H alpha. There is a very significant tendency for galaxies with compact H alpha emission to be disturbed, and there may be a weak tendency for them to be barred. Neither of these tendencies is found for galaxies with diffuse H alpha emission. We infer that compact emission results from tidally induced star formation, while diffuse emission results from more normal disc star formation. After considering field contamination, we adopt as a 'predominantly cluster' sample the spiral population inside 0.5 r(A); a 'predominantly field' sample outside 0.5 r(A); and a 'pure field' sample outside 1.5 r(A). We consistently find a much larger fraction of spirals detected with compact H alpha in the cluster sample compared to the field samples (e.g. 38 versus 0 per cent detected in cluster and 'pure field' samples, chi(2) significance 3.6 sigma). This increased fraction detected in the cluster is found for early-, mid- and late-type spirals separately. No such cluster/field differences are found for galaxies with diffuse H alpha emission. We conclude that tidal perturbations are more common in the cluster than in the field, leading to a higher incidence of compact tidally triggered star formation. By combining information on galaxy disturbance, galaxy companions, and the location of galaxies within the cluster, we have tried to identify the origin of the tidal perturbations. We find strong evidence that near-neighbour interaction plays a significant role in triggering star formation. However, we also find candidate objects near the cluster core which may be perturbed by the overall cluster tidal field, and candidate objects which may be influenced by a higher speed 'harassment' interaction between galaxies.