The far-infrared emission of the radio-loud quasar 3C318

Abstract

3C318, a radio-loud quasar at z=1.574, is a subgalactic-sized radio source, and a good test-bed for the interplay between black hole and galaxy growth in the high-z Universe. Based on its IRAS, ISO, and SCUBA detections, it has long been considered as one of the most intrinsically luminous (L${}_{\mathrm{IR}}$ > 10${}^{13}$ L${}_{\odot }$) infrared sources in the Universe. Recent far-infrared data from the Herschel Space Observatory reveal that most of the flux associated with 3C318 measured with earlier instruments in fact comes from a bright nearby source. Optical imaging and spectroscopy show that this infrared-bright source is a strongly star-forming pair of interacting galaxies at z=0.35. Adding existing Spitzer and SDSS photometry, we perform a spectral energy distribution analysis of the pair, and find that it has a combined infrared luminosity of L${}_{\mathrm{IR}}$ = 1.5 $\times$ 10${}^{12}$ L${}_{\odot }$, comparable to other intermediate-redshift ultra-luminous infrared galaxies studied with Herschel. Isolating the emission from 3C318's host, we robustly constrain the level of star formation to a value a factor of three lower than that published earlier, which is more in line with the star formation activity found in other Herschel-detected 3CR objects at similar redshift.

4 pages, 3 figures, 2 tables. Accepted for publication in A&A

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• Disentangling the AGN and Star-Formation Contributions to the Radio-X-ray Emission of Radio-Loud Quasars at 1<z<2