  We present a 5-8 micron analysis of the Spitzer-IRS 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 (SB) as the
power source of the extreme infrared emission. Around 5 micron an AGN is much
brighter (by a factor 30) than a starburst 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.
