Suppressed radio emission in supercluster galaxies: enhanced ram pressure in merging clusters?

Abstract

Context. Continuum radio emission of galaxies is related to AGN activity and to starbursts, both of which require a supply of gas, respectively to the central black hole and to molecular clouds. Aims. The environmental influence on the 21 cm (1.4 GHz) continuum radio emission of galaxies is analyzed in a 600 deg 2 region of the local Universe containing the Shapley Supercluster (SSC), whose core is thought to be the site of cluster-cluster merging. Methods. Galaxies in the FLASH and 6dFGS optical/NIR redshift surveys are cross-identified with NVSS radio sources, selected in a subsample doubly complete in volume and luminosity, and classified as starbursts or AGN according to their radio luminosity. We study radio luminosities as well as radio loudness (luminosities normalized by stellar mass) RK . Environmental effects are studied through a smoothed density field (normalized to that obtained from random catalogs with the same survey edges and redshift selection function) and through the projected distance to the nearest cluster (in units of its virial radius, whose relation to the aperture velocity dispersion is quantified). Results. The fraction of high RK galaxies in the dense 10 Mpc Abell 3558 cluster complex at the core of the SSC (SSC-CR) is half as large than elsewhere. Moreover, radio loudness in the SSC-CR is anti-correlated with the density of the large-scale environment and correlated with clustercentric radius: central brightest cluster galaxies (BCGs) in the SSC-CR are an order of magnitude less radioloud than BCGs elsewhere, with signs of suppressed radio loudness also present beyond the BCGs, out to at least 0.3r200. The gradual suppression of radio loudness from inner cluster regions to the cluster centers highlights a significant correlation of radio loudness with clustercentric radius, not seen outside the SSC-CR. This correlation is nearly as strong as the tight correlation of K-band luminosity, LK, with clustercentric radius (K-luminosity segregation), inside the SSC-CR, with a mild K-luminosity segregation outside the SSC-CR. Conclusions. The suppression of radio loudness in SSC-CR BCGs can be attributed to major cluster-cluster mergers that destroy the cool core and thus the supply of gas to the central AGN. We analytically demonstrate that the low radio loudness of non-BCG galaxies within SSC-CR clusters cannot be explained by direct major galaxy mergers or rapid galaxy flyby collisions, but by the loss of gas supply through the enhanced ram pressure felt when these galaxies cross the shock front between the two merging clusters and are afterwards subjected to the stronger wind from the second cluster.