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

Abstract

The uncertainty surrounding the nature of the heating mechanism for the dust\nthat emits at mid- to far-IR (MFIR) wavelengths in active galaxies limits our\nunderstanding of the links between active galactic nuclei (AGN) and galaxy\nevolution, as well as our ability to interpret the prodigious infrared and\nsub-mm emission of some of the most distant galaxies in the Universe. Here we\nreport deep Spitzer observations of a complete sample of powerful, intermediate\nredshift (0.05 < z < 0.7) radio galaxies and quasars. We show that AGN power,\nas traced by [OIII]5007 emission, is strongly correlated with both the mid-IR\n(24 micron) and the far-IR (70 micron) luminosities, however, with increased\nscatter in the 70 micron correlation. A major cause of this increased scatter\nis a group of objects that falls above the main correlation and displays\nevidence for prodigious recent star formation activity at optical wavelengths,\nalong with relatively cool MFIR colours. These results provide evidence that\nillumination by the AGN is the primary heating mechanism for the dust emitting\nat both 24 and 70 microns, with starbursts dominating the heating of the cool\ndust in only 20 -- 30% of objects. This implies that powerful AGN are not\nalways accompanied by the type of luminous starbursts that are characteristic\nof the peak of activity in major gas-rich mergers.\n