Local luminous infrared galaxies. II. Active galactic nucleus activity from spitzer/infrared spectrograph spectra

We quantify the active galactic nucleus (AGN) contribution to the mid-infrared (mid-IR) and the total infrared (IR, 8-1000 μm) emission in a complete volume-limited sample of 53 local luminous infrared galaxies (LIRGs, L _IR = 10¹¹-10¹² L _⊙). We decompose the Spitzer Infrared Spectrograph low-resolution 5-38 μm 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) = (0.4-50) × 10 ⁴³ erg s^-1. 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 (L _IR = 10 ¹²-10¹³ L _⊙), 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 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 Ω^AGN _IR = 3 × 10⁵ L _⊙ Mpc^-3 in LIRGs.