Detection of PAH emission features from nearby elliptical galaxies with the Spitzer infrared spectrograph

According to the current understanding, the presence of a considerable amount of dust in elliptical galaxies is quite common. Recent studies with ISO and Spitzer even suggest the presence of polycyclic aromatic hydrocarbon (PAH) emission features in the spectral energy distributions of several elliptical galaxies. Hot ionized gas filling the interstellar space of elliptical galaxies, however, is expected to easily destroy such very small grains through sputtering by plasma ions. Here we present the results of mid-IR spectroscopic observations of four elliptical galaxies with the Spitzer Infrared Spectrograph (IRS). We succeeded in detecting PAH emission features from elliptical galaxies. The observed spectra seem to be quite unusual; the PAH features at 6.2, 7.7, and 8.6 mm are very faint or even absent, in contrast to prominent emission features at 11.3 and 12.7 mu m, which may reflect peculiar physical conditions of the interstellar medium. The detection of the PAHs provides strong constraints on evolution scenarios for the interstellar medium of elliptical galaxies.