We present the results of a study of the spectral properties of galaxies in the central 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 \leq b _ { J } \leq 18.8 ) and contains \sim 1300 spectra of galaxies at the same distance , covering an area of \sim 26 deg ^ { 2 } .
These characteristics allowed an accurate spectral classification , that we performed using a Principal Components Analysis technique . This spectral classification , together with the [ OII ] equivalent widths and the star formation 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 field environment . No significant differences are present between samples at low density regimes ( i.e .
intercluster and field galaxies ) .
Cluster galaxies , instead , not only have values significantly 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 complexes , 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 , finding again a decreasing trend ( at \sim 3 \sigma significance level ) .
Our analysis is consistent with the claim of Balogh et al .
( 1998 ) that the star formation in clusters is depressed .