HerMES: SPIRE/Sub-millimetre Emission from Radio Selected AGN

TL;DR

This study investigates the far-infrared emission of radio-loud AGN across different redshifts, revealing an increase in star formation rates with redshift and estimating the contribution of AGN to cosmic star formation.

Contribution

It provides new measurements of star formation rates in radio-selected AGN across redshifts and constrains their evolution and duty cycle using Herschel and Spitzer data.

Findings

Star formation rates increase with redshift, following ~(1+z)^Q with Q=4.2+/-0.8.

Radio-loud AGN contribute a small fraction (0.1-0.5%) to the total SFR density at high redshift.

No significant correlation between radio luminosity and mean SFR within redshift bins.

Abstract

We examine the rest-frame far-infrared emission from powerful radio sources with 1.4GHz luminosity densities of 25<=log(L_1.4/WHz^-1)<=26.5 in the extragalactic Spitzer First Look Survey field. We combine Herschel/SPIRE flux densities with Spitzer/IRAC and MIPS infrared data to obtain total (8-1000um) infrared luminosities for these radio sources. We separate our sources into a moderate, 0.4<z<0.9, and a high, 1.2<z<3.0, redshift sub-sample and we use Spitzer observations of a z<0.1 3CRR sample as a local comparison. By comparison to numbers from the SKA Simulated Skies we find that our moderate redshift sample is complete and our high redshift sample is 14per cent complete. We constrain the ranges of mean star formation rates (SFRs) to be 3.4-4.2, 18-41 and 80-581Msun/yr for the local, moderate and high redshift samples respectively. Hence, we observe an increase in the mean SFR with increasing redshift which we can parameterise as ~(1+z)^Q, where Q=4.2+/-0.8. However we observe no trends of mean SFR with radio luminosity within the moderate or high redshift bins. We estimate that radio-loud AGN in the high redshift sample contribute 0.1-0.5per cent to the total SFR density at that epoch. Hence, if all luminous starbursts host radio-loud AGN we infer a radio-loud phase duty cycle of 0.001-0.005.