Spitzer mid-IR spectroscopy of powerful 2Jy and 3CRR radio galaxies. I.
  Evidence against a strong starburst-AGN connection in radio-loud AGN
  We present deep Spitzer/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 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 < 15 kpc) 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 towards the selection of compact radio
sources triggered in gas-rich environments.
