No clear submillimetre signature of suppressed star formation amongst X-ray luminous AGNs

TL;DR

This study investigates whether luminous AGNs at redshifts 1-3 suppress star formation, finding no clear evidence of suppression and suggesting previous results were due to limited data.

Contribution

The paper extends Herschel-SPIRE observations to larger samples, challenging prior claims of star formation suppression in luminous AGNs at high redshift.

Findings

No strong evidence for suppressed star formation in luminous AGNs.

Mean star formation rates are consistent with typical star-forming galaxies at z~2.

Previous results likely affected by low number statistics.

Abstract

Many theoretical models require powerful active galactic nuclei (AGNs) to suppress star formation in distant galaxies and reproduce the observed properties of today's massive galaxies. A recent study based on Herschel-SPIRE submillimetre observations claimed to provide direct support for this picture, reporting a significant decrease in the mean star-formation rates (SFRs) of the most luminous AGNs (Lx>10^44 erg/s) at z=1-3 in the Chandra Deep Field-North (CDF-N). In this letter we extend these results using Herschel-SPIRE 250um data in the COSMOS and CDF-S fields to achieve an order of magnitude improvement in the number of sources at Lx>10^44 erg/s. On the basis of our analysis, we find no strong evidence for suppressed star formation in Lx>10^44 erg/s AGNs at z=1-3. The mean SFRs of the AGNs are constant over the broad X-ray luminosity range of Lx~10^43-10^45 erg/s (with mean SFRs consistent with typical star-forming galaxies at z~2; <SFRs>~100-200 Msol/yr). We suggest that the previous CDF-N results were likely due to low number statistics. We discuss our results in the context of current theoretical models.