The uncertainty surrounding the nature of the heating mechanism for the dust that emits at mid- to far-IR (MFIR) wavelengths in active galaxies limits our understanding of the links between active galactic nuclei (AGN) and galaxy evolution, as well as our ability to interpret the prodigious infrared and submm emission of some of the most distant galaxies in the Universe. Here we report deep Spitzer observations of a complete sample of powerful, intermediate redshift (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 (24μm) and the far-IR (70μm) luminosities, however, with increased scatter in the 70μm correlation. A major cause of this increased scatter is a group of objects that falls above the main correlation and displays evidence for prodigious recent star formation activity at optical wavelengths, along with relatively cool MFIR colours. These results provide evidence that illumination by the AGN is the primary heating mechanism for the dust emitting at both 24 and 70μm, with starbursts dominating the heating of the cool dust in only 20 – 30% of objects. This implies that powerful AGN are not always accompanied by the type of luminous starbursts that are characteristic of the peak of activity in major gas-rich mergers.

Publication Date



This is the pre-print of an article published by the American Astronomical Society. The final, published version is available here: https://doi.org/10.1086/518421

© 2007 The American Astronomical Society.

Also archived in: arXiv:astro-ph/0703790 v1 Mar 2007

Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works in February 2014.

Document Type


Department, Program, or Center

School of Physics and Astronomy (COS)


RIT – Main Campus