A study of the core of the Shapley Concentration — VI. Spectral properties of galaxies

ABSTRACT We present the results of a study of the spectral properties of galaxies in the cen-tral part of the Shapley Concentration, covering an extremely wide range of densities,from the rich cluster cores to the underlying supercluster environment.Our sample is homogeneous, in a well defined magnitude range (17 ≤ b J ≤ 18.8) andcontains ∼ 1300 spectra of galaxies at the same distance, covering an area of ∼ 26deg 2 . These characteristics allowed an accurate spectral classification, that we per-formed using a Principal Components Analysis technique.This spectral classification, together with the [OII] equivalent widths and the star for-mation rates, has been used to study the properties of galaxies at different densities:cluster, intercluster (i.e. galaxies in the supercluster but outside clusters) and fieldenvironment.No significant differences are present between samples at low density regimes (i.e.intercluster and field galaxies). Cluster galaxies, instead, not only have values signifi-cantly different from the field ones, but also show a dependence on the local density.Moreover, a well defined morphology-density relation is present in the cluster com-plexes, although these structures are known to be involved in major merging events.Also the mean equivalent width of [OII] shows a trend with the local environment,decreasing at increasing densities, even if it is probably induced by the morphology-density relation.Finally we analyzed the mean star formation rate as a function of the density, findingagain a decreasing trend (at ∼ 3σ significance level). Our analysis is consistent withthe claim of Balogh et al. (1998) that the star formation in clusters is depressed.Key words: galaxies: distances and redshifts – galaxies: spectra and morphology –galaxies: clusters: general – galaxies: clusters: individuals: A3528 - A3530 - A3532 -A3535 - A3556 - A3558 - A3562 –