2009ApJ...699L...1R

We have performed Spitzer InfraRed Spectrograph (IRS) low-resolution 5-12 µm spectroscopy on a sample of galaxies selected to be at three distinct poststarburst evolutionary stages based on their optical spectral indices. The resulting IRS spectra show distinctive polycyclic aromatic hydrocarbon (PAH) emission line structures at 6.2, 7.7, 8.6, and 11.3 µm and little silicate absorption, indicative of ongoing star formation. However, the PAH interline ratios, in particular the 11.3/6.2 µm and 7.7/6.2 µm ratio, show large variations. These variations are found to correlate with both time since the most recent starburst and active galactic nucleus (AGN) activity. We speculate that the evolution observed in these PAH ratios is related to an increase in AGN activity with time since starburst.