No Submillimetre Signature of Star Formation Suppression Among X-Ray Luminous AGN

Abstract

In this chapter we aim to answer the questions: is there any evidence that X-ray luminous active galactic nuclei (AGN) have suppressed star-formation rates compared to the galaxy population as a whole? Many theoretical models require powerful active galactic nuclei (AGN) to suppress star formation in distant galaxies in order to reproduce the observed properties of today’s massive galaxies. The observational work that has been investigating the star formation rates of powerful AGN has provided conflicting results. In particular, a recent study based on Herschel-SPIRE observations claimed toect support for star formation suppression, reporting a significant decrease in the mean star-formation rates (SFRs) of the most luminous AGN (\(L_\mathrm{X}>10^{44}\,{\text {erg}\,{\text {s}}^{-1}}\)) at \(z\approx \) 1–3. However, this study was based on a small number of targets. In this chapter we extend these results using Herschel-SPIRE 250 \({\upmu {\text {m}}}\) data to achieve an order of magnitude improvement in the number of sources at \(L_\mathrm{X}>10^{44}\,{\text {erg}\,{\text {s}}^{-1}}\) in the same redshift range. By using these data to measure the star formation rates as a function of AGN power, we find no evidence for suppressed star formation in the most luminous AGN. The mean SFRs of the AGN are constant over the broad X-ray luminosity range of \(L_\mathrm{X}\approx 10^{43}\)–\(10^{45}\,{\text {erg}\,{\text {s}}^{-1}}\). The star formation rates are also consistent with the mean SFRs of the general population of star-forming galaxies at these redshifts (i.e., \(\langle {\text {SFRs}}\rangle \approx \) 100–200 \(\mathrel {\text {M}}_{\odot }\,{\text {year}}^{-1}\)). We suggest that the previous results were likely due to low number statistics. We discuss our results in the context of current theoretical models. We suggest that these observations may not show any evidence for the predicted suppressed star formation rates because of the variable and short timescales on which luminous AGN activity is observed and the longer/delayed timescales that it takes for this activity to impact upon the star formation in the host galaxies.