Star formation and active galactic nuclei in the core of the Shapley supercluster: A very large array survey of A3556, A3558, SC 1327-312, SC 1329-313, and A3562

The core of the Shapley supercluster (A3556, A3558, SC 1327-312, SC 1329-313, and A3562) is an ideal region in which to study the effects of cluster mergers on the activity of individual galaxies. This paper presents the most comprehensive radio continuum investigation of the region, relying on a 63 pointing mosaic obtained with the Very Large Array yielding an areal coverage of nearly 7 deg(2). The mosaic provides a typical sensitivity of about 80 mu Jy at a resolution of 1600, enabling the detection of galaxies with star formation rates as low as 1M(circle dot) yr(-1). The radio data are complemented by optical imaging in the B and R bands, producing a catalog of 210 radio-detected galaxies with m(R) <= 17: 36 (M-R <= -19). At least 104 of these radio-detected galaxies are members of the supercluster on the basis of public velocity measurements. Across the entire core of the supercluster, there appears to be a significant deficit of radio galaxies at intermediate optical magnitudes (-21 >= M-R > -22). This deficit is offset somewhat by an increase in the frequency with which brighter galaxies (M-R <= -22) host radio sources. More dramatic is the highly significant increase in the probability for fainter galaxies (-20 >= M-R > -21) in the vicinity of A3562 and SC 1329-313 to be associated with radio emission. The radio and optical data for these sources strongly suggest that these active galaxies are powered by star formation. In conjunction with recent X-ray analysis, this is interpreted as young starbursts related to the recent merger of SC 1329-313 with A3562 and the rest of the supercluster.