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We present the analysis of Spitzer-IRS spectra of four early-type galaxies,NGC 1297, NGC 5044, NGC 6868, and NGC 7079, all classified as LINERs in theoptical bands. Their IRS spectra present the full series of H2 rotationalemission lines in the range 5--38 microns, atomic lines, and prominent PAHfeatures. We investigate the nature and origin of the PAH emission,characterized by unusually low 6 -- 9/11.3 microns inter-band ratios. After thesubtraction of a passive early type galaxy template, we find that the 7 -- 9microns spectral region requires dust features not normally present in starforming galaxies. Each spectrum is then analyzed with the aim of identifyingtheir components and origin. In contrast to normal star forming galaxies, wherecationic PAH emission prevails, our 6--14 microns spectra seem to be dominatedby large and neutral PAH emission, responsible for the low 6 -- 9/11.3 micronsratios, plus two broad dust emission features peaking at 8.2 microns and 12microns. Theses broad components, observed until now mainly in evolved carbonstars and usually attributed to pristine material, contribute approximately30-50% of the total PAH flux in the 6--14 microns region. We propose that thePAH molecules in our ETGs arise from fresh carbonaceous material which iscontinuously released by a population of carbon stars, formed in a rejuvenationepisode which occurred within the last few Gyr. The analysis of the MIR spectraallows us to infer that, in order to maintain the peculiar size and chargedistributions biased to large and neutral PAHs, this material must be shocked,and excited by the weak UV interstellar radiation field of our ETG.

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