Exploring the active galactic nucleus and starburst content of local ultraluminous infrared galaxies through 5-8 μm spectroscopy

We present a 5-8 mu m analysis of the Spitzer spectra of 71 ultraluminous infrared galaxies (ULIRGs) with redshift z < 0.15, devoted to the study of the role of active galactic nuclei (AGN) and starbursts (SBs) as the power source of the extreme infrared emission. Around similar to 5 mu m, an AGN is much brighter (by a factor of approximate to 30) than an SB of equal bolometric luminosity. This allows us to detect the presence of even faint accretion-driven cores inside ULIRGs: signatures of AGN activity are found in similar to 70 per cent of our sample (50/71 sources). Through a simple analytical model, we are also able to obtain a quantitative estimate of the AGN/SB contribution to the overall energy output of each source. Although the main fraction of ULIRG luminosity is confirmed to arise from star formation events, the AGN contribution is non-negligible (similar to 23 per cent) and is shown to increase with luminosity. The existence of a rather heterogeneous pattern in the composition and geometrical structure of the dust among ULIRGs is newly supported by the comparison between individual absorption features and continuum extinction.