Spectral decomposition of starbursts and active galactic nuclei in 5-8 μm Spitzer-IRS spectra of local ultraluminous infrared galaxies

Abstract

We present an analysis of the 5-8 μm Spitzer-IRS spectra of a sample of 68 local ultraluminous infrared galaxies (ULIRGs). Our diagnostic technique allows a clear separation of the active galactic nucleus (AGN) and starburst (SB) components in the observed mid-infrared emission, and a simple analytic model provides a quantitative estimate of the AGN/SB contribution to the bolometric luminosity. We show that AGN are ~30 times brighter at 6 μm than SBs with the same bolometric luminosity, so that even faint AGN can be detected. Star formation events are confirmed as the dominant power source for extreme IR activity, since ~85 per cent of ULIRG luminosity arises from the SB component. None the less an AGN is present in the majority (46/68) of our sources. © 2008 The Authors. Journal compilation © 2008 RAS.