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

TL;DR

This study clarifies the true infrared luminosity of quasar 3C318 by disentangling its emission from a nearby star-forming galaxy pair, revealing a lower star formation rate than previously thought.

Contribution

It provides a refined analysis of 3C318's infrared emission, correcting earlier overestimations by accounting for contamination from a neighboring galaxy pair.

Findings

Most IR flux from 3C318 is from a nearby galaxy pair, not the quasar itself.

The host galaxy's star formation rate is three times lower than earlier estimates.

3C318's star formation activity aligns with other similar high-redshift radio galaxies.

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.