4.2.4. Exceptions

The ``standard'' spectra described above characterize the integrated emission from normal galaxies, but exceptions arise at extreme conditions, and are easiest to detect in specific ISM phases of individual galaxies. At the smallest scales in the most intense parts of HII regions, Contursi (1998), Tran (1998) and Césarsky (1996b) report spectra which depart significantly from the standard ones in the location and strength of features. Similarly distorted spectra are also observed in NGC 4418 (Lu et al. 2000), and close to the Active Nucleus of Cen A (Vigroux et al. 1999). This radical transformation in the spectrum must then result from extremely UV-rich heating radiation, and most likely reflects severe modification of the emitting grains, including the destruction of Aromatic Feature carriers. Significant distortions in the spectrum driven by a rising continuum but without radical changes in the Aromatic Features have also been reported by Lutz et al. (1998) in extremely active galaxies. The mid-infrared spectra of starbursts are relatively ``standard'', whereas galaxies with an active Black Hole nucleus (AGN) have a continuum rising towards longer wavelengths with insignificant AFE. Ultra-Luminous Infrared Galaxies show intermediate spectra, modified in addition by large optical depths (Genzel et al. 1998; Laurent et al. 1999).

At the other end of the heating sequence, Césarsky et al. (1999) report that the 5-9 µm AFE are missing from the emission of the bulge and of the quiet parts of the bright infrared ring in M 31. The 11.3 and 12.7 µm features are present, perhaps a result of their originating primarily on neutral PAHs. The relatively weak and UV-poor heating radiation in these parts of M 31 cannot alone explain the absence of Aromatic Features, for other studies (e.g. Uchida et al. 1998, Uchida et al. 2000) show that photons of a few eV are sufficient to generate the standard spectrum. It would appear that the dust properties in M31 are again modified, this time by prolonged shielding from UV processing in the astration cycle. Additional evidence of abnormal ISM properties in the same regions are found by Pagani et al. (2000) in their study of the broad-band mid-infrared emission.

Another peculiar exception is provided by Sauvage & Thuan (1999) in SBS 0335-052, a blue compact dwarf with a metallicity about 1/40 solar. No AFE are detected here, and the spectrum appears best fit by highly extincted blackbody emission. The authors propose the early chemical age of the system as the most likely reason behind the absence of the AFE, though one cannot rule out the possibility that the UV radiation from the intense star formation episode also plays a role in the destruction of the Aromatic Feature carriers. If this spectrum does indeed characterize all low metallicity objects, one would expect the ``standard'' spectrum to disappear gradually as a function of increasing redshift, thus making it harder to detect the Aromatic signature of dust and distinguish star formation from Black Hole activity as the energy source in the earliest systems.