Cosmic downsizing of powerful radio galaxies to low radio luminosities

E.E.R. acknowledges financial support from NWO (grant number: NWO-TOP LOFAR 614.001.006). E.E.R. and J.A. thank the Leiden/ESA astrophysics program for summer students (LEAPS) which supported J.A. in Leiden.Aims. At bright radio powers (P1.4 GHz > 1025 W/Hz) the space density of the most powerful sources peaks at higher redshift than that of their weaker counterparts. This paper establishes whether this luminosity-dependent evolution persists for sources an order of magnitude fainter than those previously studied, by measuring the steep-spectrum radio luminosity function (RLF) across the range 1024 1026 W/Hz the redshift of the peak space density increases with luminosity, whilst at lower radio luminosities the position of the peak remains constant within the uncertainties. This cosmic downsizing behaviour is found to be similar to that seen at optical wavelengths for quasars, and is interpreted as representing the transition from radiatively efficient to inefficient accretion modes in the steep-spectrum population. This conclusion is supported by constructing simple models for the space density evolution of these two different radio galaxy classes; these models are able to successfully reproduce the observed variation in peak redshift.Publisher PDFPeer reviewe