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We present a 5–8 μm analysis of the  Spitzer  spectra of 71 ultraluminous infrared galaxies (ULIRGs) with redshift   z  &lt; 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 ∼5 μm, an AGN is much brighter (by a factor of ≈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 ∼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 (∼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.

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