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

We explore the nature of X-ray sources with 70 micron counterparts selected in the 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 micron 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 AGN. Modelling of the optical to far-infrared Spectral Energy Distribution indicates that most of them (27/28) have a strong starburst component (>50 solar masses per year) that dominates in the infrared. It is found that the X-ray and infrared 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 micron 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 micron sources have lower specific star-formation rates compared to the general 70 micron population, consistent with AGN feedback moderating the star-formation in the host galaxies.

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