The Heating Mechanism for the Warm/Cool Dust in Powerful, Radio-loud Active Galactic Nuclei

The uncertainty surrounding the nature of the heating mechanism for the dustthat emits at mid- to far-IR (MFIR) wavelengths in active galaxies limits ourunderstanding of the links between active galactic nuclei (AGN) and galaxyevolution, as well as our ability to interpret the prodigious infrared andsub-mm emission of some of the most distant galaxies in the Universe. Here wereport deep Spitzer observations of a complete sample of powerful, intermediateredshift (0.05 < z < 0.7) radio galaxies and quasars. We show that AGN power,as traced by [OIII]5007 emission, is strongly correlated with both the mid-IR(24 micron) and the far-IR (70 micron) luminosities, however, with increasedscatter in the 70 micron correlation. A major cause of this increased scatteris a group of objects that falls above the main correlation and displaysevidence for prodigious recent star formation activity at optical wavelengths,along with relatively cool MFIR colours. These results provide evidence thatillumination by the AGN is the primary heating mechanism for the dust emittingat both 24 and 70 microns, with starbursts dominating the heating of the cooldust in only 20 -- 30% of objects. This implies that powerful AGN are notalways accompanied by the type of luminous starbursts that are characteristicof the peak of activity in major gas-rich mergers.Comment: 13 pages, 3 figures. Accepted for publication in astrophysical journal letter