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 ( 7.2 \times 10 ^ { 5 } < M _ { d } < 2.6 \times 10 ^ { 8 } Â M _ { \odot } ) 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 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 ( \sim 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 .