The role of nuclear activity as the power source of ultraluminous
  infrared galaxies
  We present the results of a 5-8 micron spectral analysis performed on the
largest sample of local ultraluminous infrared galaxies (ULIRGs) selected so
far, consisting of 164 objects up to a redshift of ~0.35. The unprecedented
sensitivity of the Infrared Spectrograph onboard Spitzer allowed us to develop
an effective diagnostic method to disentangle the active galactic nucleus (AGN)
and starburst (SB) contribution to this class of objects. The intrinsic
bolometric corrections are estimated for both the components, in order to
obtain the relative AGN/SB contribution to the total luminosity of each source.
Our main results are the following: 1) The AGN detection rate among local
ULIRGs amounts up to 70 per cent, with 113/164 convincing detections within our
sample, while the global AGN/SB power balance is ~1/3. 2) A general agreement
is found with optical classification; however, among the objects with no
spectral signatures of nuclear activity, our IR diagnostics find a subclass of
elusive, highly obscured AGN. 3) We analyse the correlation between nuclear
activity and IR luminosity, recovering the well-known trend of growing AGN
significance as a function of the overall energy output of the system: the
average AGN contribution rises from ~10 to ~60 per cent across the ULIRG
luminosity range. 4) We confirm that the AGN content is larger in compact
systems, but the link between activity and evolutionary stage is rather loose.
5) By analysing a control sample of IR-luminous galaxies around z ~ 1, we find
evidence for only minor changes with redshift of the large-scale spectral
properties of the AGN and SB components. This underlines the potential of our
method as a straightforward and quantitative AGN/SB diagnostic tool for
ULIRG-like systems at high redshift as well.
