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We present deep Spitzer/IRS spectra for complete samples of 46 2Jy radiogalaxies (0.05<z<0.7) and 19 3CRR FRII radio galaxies (z<0.1), and use thedetection of polycyclic aromatic hydrocarbon (PAH) features to examine theincidence of contemporaneous star formation and radio-loud AGN activity. Ouranalysis reveals PAH features in only a minority (30%) of the objects with goodIRS spectra. Using the wealth of complementary data available for the 2Jy and3CRR samples we make detailed comparisons between a range of star formationdiagnostics: optical continuum spectroscopy, mid- to far-IR (MFIR) color,far-IR excess and PAH detection. There is good agreement between the variousdiagnostic techniques: most candidates identified to have star formationactivity on the basis of PAH detection are also identified using at least twoof the other techniques. We find that only 35% of the combined 2Jy and 3CRRsample show evidence for recent star formation activity (RSFA) at opticaland/or MFIR wavelengths. This result argues strongly against the idea of aclose link between starburst and powerful radio-loud AGN activity, reinforcingthe view that, although a large fraction of powerful radio galaxies may betriggered in galaxy interactions, only a minority are triggered at the peaks ofstar formation activity in major, gas-rich mergers. However, we find thatcompact radio sources (D < 15 kpc) show a significantly higher incidence ofRSFA (>75%) than their more extended counterparts (=15 -- 25%). We discuss thisresult in the context of a possible bias towards the selection of compact radiosources triggered in gas-rich environments.

Spitzer mid-IR spectroscopy of powerful 2Jy and 3CRR radio galaxies. I. Evidence against a strong starburst-AGN connection in radio-loud AGN