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 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.

Based on observations obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407.