Unusual PAH Emission in Nearby Early-type Galaxies: A Signature of an Intermediate-age Stellar Population?

We present the analysis of Spitzer-IRS spectra of four early-type galaxies (ETGs), NGC 1297, NGC 5044, NGC 6868, and NGC 7079, all classified as LINERs in the optical bands. Their IRS spectra present the full series of H-2 rotational emission lines in the range 5-38 mu m, atomic lines, and prominent polycyclic aromatic hydrocarbon (PAH) features. We investigate the nature and origin of the PAH emission, characterized by unusually low 6-9/11.3 mu m interband ratios. After the subtraction of a passive ETG template, we find that the 7-9 mu m spectral region requires dust features not normally present in star-forming galaxies. Each spectrum is then analyzed with the aim of identifying their components and origin. In contrast to normal star-forming galaxies, where cationic PAH emission prevails, our 6-14 mu m spectra seem to be dominated by large and neutral PAH emission, responsible for the low 6-9/11.3 mu m ratios, plus two broad dust emission features peaking at 8.2 mu m and 12 mu m. These broad components, observed until now mainly in evolved carbon stars and usually attributed to pristine material, contribute approximately 30%-50% of the total PAH flux in the 6-14 mu m region. We propose that the PAH molecules in our ETGs arise from fresh carbonaceous material that is continuously released by a population of carbon stars, formed in a rejuvenation episode that occurred within the last few Gyr. The analysis of the MIR spectra allows us to infer that, in order to maintain the peculiar size and charge distributions biased to large and neutral PAHs, this material must be shocked and excited by the weak UV interstellar radiation field of our ETGs.