  We use deep Herschel Space Observatory observations of a 90% complete sample
of 32 intermediate-redshift 2Jy radio galaxies (0.05 < z < 0.7) to estimate the
dust masses of their host galaxies and thereby investigate the triggering
mechanisms for their quasar-like AGN. The dust masses derived for the radio
galaxies (7.2x10^5 < M_d < 2.6x10^8 M_sun) are intermediate between those of
quiescent elliptical galaxies on the one hand, and ultra luminous infrared
galaxies (ULIRGs) on the other. Consistent with simple models for the
co-evolution of supermassive black holes and their host galaxies, these results
suggest that most of the radio galaxies represent the late time re-triggering
of AGN activity via mergers between the host giant elliptical galaxies and
companion galaxies with relatively low gas masses. However, a minority of the
radio galaxies in our sample (~20%) have high, ULIRG-like dust masses, along
with evidence for prodigious star formation activity. The latter objects are
more likely to have been triggered in major, gas-rich mergers that represent a
rapid growth phase for both their host galaxies and their supermassive black
holes.
