We present the results of a redshift survey of intercluster galaxies in the central region of the Shapley Concentration supercluster , aimed at determining the distribution of galaxies in between obvious overdensities .
Our sample is formed by 442 new redshifts , mainly in the b _ { J } magnitude range 17 - 18.8 .
Adding the data from our redshift surveys on the A3558 and A3528 complexes , which are close to the geometrical centre of this supercluster , we obtain a total sample of \sim 2000 radial velocities . The average velocity of the observed intercluster galaxies in the Shapley Concentration appears to be a function of their ( \alpha , \delta ) position , and it can be fitted by a plane in the three–dimensional space ( \alpha , \delta , v ) : the distribution of the galaxy distances around the best fit plane is described by a Gaussian with dispersion 3.8 h ^ { -1 } Mpc . Using the 1440 galaxies of our total sample in the magnitude range 17 - 18.8 , we reconstructed the density profile in the central part of the Shapley Concetration ; moreover we detected another significant overdensity at \sim 30000 km/s ( dubbed S300 ) . We estimate the total overdensity in galaxies , the mass and the dynamical state of these structures , and discuss the effect of considering a bias between the galaxy distribution and the underlying matter . The estimated total overdensity in galaxies of these two structures is ( N / \bar { N } ) \sim 11.3 on scale of 10.1 h ^ { -1 } Mpc for the Shapley Concentration and ( N / \bar { N } ) \sim 2.9 on scale of 24.8 h ^ { -1 } Mpc for S300 .
If light traces the mass distribution , the corresponding masses are 1.4 \times 10 ^ { 16 } \Omega _ { o } h ^ { -1 } M _ { \odot } and 5.1 \times 10 ^ { 16 } \Omega _ { o } h ^ { -1 } M _ { \odot } for Shapley Concentration and S300 , respectively . A dynamical analysis suggests that , if light traces mass and \Omega _ { o } = 1 , the Shapley Concentration already reached its turnaround radius and has started to collapse : the final collapse will happen in \sim 3 10 ^ { 9 } h ^ { -1 } yrs . We find an indication that the value of the bias between clusters and galaxies in the Shapley Concentration is higher that that reported in literature , confirming the impression that this supercluster is very rich in clusters . Finally , from the comparison with some theoretical scenarios , we find that the existence of the Shapley Concentration is more consistent with the predictions of the models with a matter density parameter < 1 , such as open CDM and \Lambda CDM .