A study of the core of the Shapley Concentration - IV. Distribution of intercluster galaxies and supercluster properties*

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 ~2000 radial velocities. The average velocity of the observed intercluster galaxies in the Shapley Concentration appears to be a function of their (α, δ) position, and it can be fitted by a plane in the three-dimensional space (α, δ, v): the distribution of the galaxy distances around the best-fitting plane is described by a Gaussian with dispersion 3.8h^-1Mpc. Using the 1440 galaxies of our total sample in the magnitude range 17-18.8, we reconstruct the density profile in the central part of the Shapley Concentration; moreover, we detect another significant overdensity at ~30000kms^-1 (dubbed S300). We estimate the total overdensity of galaxies, and the mass and 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 of galaxies of these two structures is on a scale of 10.1h^-1Mpc for the Shapley Concentration, and on a scale of 24.8h^-1Mpc for S300. If light traces the mass distribution, the corresponding masses are 1.4x10¹⁶Ω_oh^-1M_solar and 5.1x10¹⁶Ω_oh^-1M_solar for the Shapley Concentration and S300, respectively. A dynamical analysis suggests that, if light traces mass and Ω_o=1, the Shapley Concentration has already reached its turnaround radius and has started to collapse: the final collapse will happen in ~3x10⁹h^-1yr. 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 ΛCDM.