PAH emission from ultraluminous infrared galaxies

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 mu m PAH equivalent widths (EWs) in the sample span more than 2 orders of magnitude (similar to 0.006-0.8 mu m), and ULIRGs with H II - like optical spectra or steep far-infrared spectral slopes (S-25/S-60 < 0.2) typically have 6.2 mu m PAH EWs that are half that of lower luminosity starbursts. A significant fraction (similar to 40% - 60%) of H II - like, LINER-like, and cold ULIRGs have very weak PAH EWs. Many of these ULIRGs also have large (T-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 mu m 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.