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

D. Dicken, C. Tadhunter, D. Axon, R. Morganti, A. Robinson, M. B.N. Kouwenhoven, H. Spoon, P. Kharb, K. J. Inskip, J. Holt, C. Ramos Almeida, N. P.H. Nesvadba

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Abstract

We present deep Spitzer/Infrared Spectrograph (IRS) spectra for complete samples of 46 2Jy radio galaxies (0.05 <z <0.7) and 19 3CRR FRII radio galaxies (z <0.1), and use the detection of polycyclic aromatic hydrocarbon (PAH) features to examine the incidence of contemporaneous star formation and radio-loud active galactic nucleus (AGN) activity. Our analysis reveals PAH features in only a minority (30%) of the objects with good IRS spectra. Using the wealth of complementary data available for the 2Jy and 3CRR samples we make detailed comparisons between a range of star formation diagnostics: optical continuum spectroscopy, mid- to far-IR (MFIR) color, far-IR excess and PAH detection. There is good agreement between the various diagnostic techniques: most candidates identified to have star formation activity on the basis of PAH detection are also identified using at least two of the other techniques. We find that only 35% of the combined 2Jy and 3CRR sample show evidence for recent star formation activity (RSFA) at optical and/or MFIR wavelengths. This result argues strongly against the idea of a close link between starburst and powerful radio-loud AGN activity, reinforcing the view that, although a large fraction of powerful radio galaxies may be triggered in galaxy interactions, only a minority are triggered at the peaks of star formation activity in major, gas-rich mergers. However, we find that compact radio sources (D <15kpc) show a significantly higher incidence of RSFA (>75%) than their more extended counterparts (≈15%-25%). We discuss this result in the context of a possible bias toward the selection of compact radio sources triggered in gas-rich environments.
Original languageEnglish
JournalAstrophysical Journal
Volume745
Issue number2
DOIs
Publication statusPublished - 1 Feb 2012

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