The active galactic nuclei/starburst content in high-redshift ultraluminous infrared galaxies

Abstract

We apply a simple model, tested on local ultraluminous infrared galaxies (ULIRGs), to disentangle the active galactic nuclei (AGN) and starburst contributions in submillimetre and 24 μm-selected ULIRGs observed with the Spitzer-Infrared Spectrograph spectrometer. We quantitatively estimate the average AGN contribution to the stacked 6-8 μm rest-frame spectra of these sources in different luminosity and redshift ranges and, under the assumption of similar infrared-to-bolometric ratios as in local ULIRGs, the relative AGN/starburst contributions to the total infrared luminosity. Though the starburst component is always dominant in submillimetre-selected ULIRGs, we find a significant increase of the AGN contribution at redshift z > 2.3 with respect to lower z objects. Finally, we quantitatively confirm that the mid-infrared emission of 24 μm-selected ULIRGs is dominated by the AGN component, but the starburst component contributes significantly to the bolometric luminosity. © 2009 The Authors. Journal compilation © 2009 RAS.