Local Luminous Infrared Galaxies. II. AGN Activity from Spitzer/IRS
  spectra
  We quantify the active galactic nucleus (AGN) contribution to the
mid-infrared (mid-IR) and the total infrared (IR, 8-1000micron) emission in a
complete volume-limited sample of 53 local luminous infrared galaxies (LIRGs).
We decompose the Spitzer Infrared Spectrograph (IRS) low-resolution 5-38micron
spectra of the LIRGs into AGN and starburst components using clumpy torus
models and star-forming galaxy templates, respectively. We find that 50%
(25/50) of local LIRGs have an AGN component detected with this method. There
is good agreement between these AGN detections through mid-IR spectral
decomposition and other AGN indicators, such as the optical spectral class,
mid-IR spectral features and X-ray properties. Taking all the AGN indicators
together, the AGN detection rate in the individual nuclei of LIRGs is ~62%. The
derived AGN bolometric luminosities are in the range L_bol(AGN)=0.4 -50x10^{43}
erg/s. The AGN bolometric contribution to the IR luminosities of the galaxies
is generally small, with 70% of LIRGs having L_bol(AGN)/L_IR<0.05. Only ~8% of
local LIRGs have a significant AGN bolometric contribution L_bol(AGN)/L_IR >
0.25. From the comparison of our results with literature results of
ultraluminous infrared galaxies, we confirm that in the local universe the AGN
bolometric contribution to the IR luminosity increases with the IR luminosity
of the galaxy/system. If we add up the AGN bolometric luminosities we find that
AGNs only account for 5%^{+8%}_{-3%} of the total IR luminosity produced by
local LIRGs (with and without AGN detections). This proves that the bulk of the
IR luminosity of local LIRGs is due to star formation activity. Taking the
newly determined IR luminosity density of LIRGs in the local universe, we then
estimate an AGN IR luminosity density of Omega_IR(AGN) = 3x10^5 L_sun Mpc^{-3}$
in LIRGs.
