3C 318 , 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 3C 318 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 3C 318 ’ 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 .