Testing the starburst/AGN connection with SWIRE X-ray/70 μm sources

We explore the nature of X-ray sources with 70 mu m counterparts selected in the Spitzer Wide-Area Infrared Extragalactic Survey (SWIRE) fields: ELAIS-N1, Lockman Hole and Chandra Deep Field South, for which Chandra X-ray data are available. A total of 28 X-ray/70 mu m sources in the redshift interval 0.5 < z < 1.3 are selected. The X-ray luminosities and the shape of the X-ray spectra show that these sources are active galactic nuclei (AGN). Modelling of the optical to far-infrared (IR) spectral energy distribution indicates that most of them (27/28) have a strong starburst component (> 50 M-circle dot yr-1) that dominates in the IR. It is found that the X-ray and IR luminosities of the sample sources are broadly correlated, consistent with a link between AGN activity and star formation. Contrary to the predictions of some models for the co-evolution of AGN and galaxies, the X-ray/70 mu m sources in the sample are not more obscured at X-ray wavelengths compared to the overall X-ray population. It is also found that the X-ray/70 mu m sources have lower specific star formation rates compared to the general 70 mu m population, consistent with AGN feedback moderating the star formation in the host galaxies.