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,
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 H2 rotational
emission lines in the range 5--38 microns, atomic lines, and prominent PAH
features. We investigate the nature and origin of the PAH emission,
characterized by unusually low 6 -- 9/11.3 microns inter-band ratios. After the
subtraction of a passive early type galaxy template, we find that the 7 -- 9
microns 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 microns spectra seem to be dominated
by large and neutral PAH emission, responsible for the low 6 -- 9/11.3 microns
ratios, plus two broad dust emission features peaking at 8.2 microns and 12
microns. Theses 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 microns region. We propose that the
PAH molecules in our ETGs arise from fresh carbonaceous material which is
continuously released by a population of carbon stars, formed in a rejuvenation
episode which 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 ETG.
