Evolution of the most massive galaxies to z ∼ 0.6-II. The link between radio AGN activity and star formation

We analyse the optical spectra of massive (log M-*/M-circle dot > 11.4) radio-loud galaxies at z similar to 0.2 and z similar to 0.6. Our samples are generated by cross-matching the Sloan Digital Sky Survey DR7 and Baryon Oscillation Spectroscopic Survey spectroscopic galaxy catalogues with the Faint Images of the Radio Sky at Twenty centimetres and NVSS radio continuum surveys. By comparing stellar population parameters of these radio-loud samples with radio-quiet control samples matched in stellar mass, velocity dispersion and redshift, we investigate how the presence of a radio-emitting jet relates to the recent star formation history of the host galaxy. We also investigate how the emission-line properties of the radio galaxies evolve with redshift by stacking their spectra. Our main results are the following. (1) Both at low and at high redshift, half as many radio-loud as radio-quiet galaxies have experienced significant star formation in the past Gyr. This difference in star formation history is independent of the luminosity of the radio AGN, except at radio luminosities greater than 10(25.5) W Hz(-1), where it disappears. (2) The Balmer absorption-line properties of massive galaxies that have experienced recent star formation show that star formation occurred as a burst in many of these systems. (3) Both the radio and the emission-line luminosity of radio AGN evolve significantly with redshift. The average [O III] rest equivalent width increases by 1 dex from z = 0.2 to z = 0.6, and emission-line ratios change from LINER-like at low redshift to Seyfert-like at high redshift. However, radio galaxies with similar stellar population parameters have similar emission-line properties both at high and at low redshift. These results suggest that massive galaxies experience cyclical episodes of gas accretion, star formation and black hole growth, followed by the production of a radio jet that shuts down further activity. The behaviour of galaxies with log M-*/M-circle dot > 11.4 is the same at z = 0.6 as it is at z = 0.2, except that higher redshift galaxies experience more star formation and black hole growth and produce more luminous radio jets during each accretion cycle.