2007ApJ...669..810D

We explore the relationships between the polycyclic aromatic hydrocarbon (PAH) feature strengths, mid-infrared continuum luminosities, far-infrared spectral slopes, optical spectroscopic classifications, and silicate optical depths in a sample of 107 ULIRGs observed with the Infrared Spectrograph on the Spitzer Space Telescope. The detected 6.2 µm PAH equivalent widths (EWs) in the sample span more than 2 orders of magnitude (∼0.006-0.8 µm), and ULIRGs with H II-like optical spectra or steep far-infrared spectral slopes (S₂₅/S₆₀<0.2) typically have 6.2 µm PAH EWs that are half that of lower luminosity starbursts. A significant fraction (∼40%-60%) of H II-like, LINER-like, and cold ULIRGs have very weak PAH EWs. Many of these ULIRGs also have large (τ_9.7>2.3) silicate optical depths. The far-infrared spectral slope is strongly correlated with PAH EW, but not with silicate optical depth. In addition, the PAH EW decreases with increasing rest-frame 24 µm luminosity. We argue that this trend results primarily from dilution of the PAH EW by continuum emission from dust heated by a compact central source, probably an AGN. High-luminosity, high-redshift sources studied with Spitzer appear to have a much larger range in PAH EW than seen in local ULIRGs, which is consistent with extremely luminous starburst systems being absent at low redshift, but present at early epochs.