2022ApJ...934...27S

We present the results of a stacking analysis performed on Spitzer/Infrared Spectrograph high-resolution mid-infrared (mid-IR) spectra of luminous infrared galaxies (LIRGs) in the Great Observatories All-Sky LIRG Survey. By binning in relation to mid-IR active galactic nucleus (AGN) fraction and stacking spectra, we detect bright emission lines [Ne II] and [Ne III], which trace star formation, and fainter emission lines [Ne V] and [O IV], which trace AGN activity, throughout the sample. We find that the [Ne II] luminosity is fairly constant across all AGN fraction bins, while the [O IV] and [Ne V] luminosities increase by over an order of magnitude. Our measured average line ratios, [Ne V]/[Ne II] and [O IV]/[Ne II], at low AGN fraction are similar to H II galaxies, while the line ratios at high AGN fraction are similar to LINERs and Seyferts. We decompose the [O IV] luminosity into star formation and AGN components by fitting the [O IV] luminosity as a function of the [Ne II] luminosity and the mid-IR AGN fraction. The [O IV] luminosity in LIRGs is dominated by star formation for mid-IR AGN fractions ≲0.3. With the corrected [O IV] luminosity, we calculate black hole accretion rates (BHARs) ranging from 10^–5M_⊙ yr^–1 at low AGN fractions to 0.2 M_⊙ yr^–1 at the highest AGN fractions. We find that using the [O IV] luminosity, without correcting for star formation, can lead to overestimation of the BHAR by up to a factor of 30 in starburst-dominated LIRGs. Finally, we show that the BHAR/star formation rate ratio increases by more than three orders of magnitude as a function of mid-IR AGN fraction in LIRGs.