Direct Spectroscopic Evidence for Ionized Polycyclic Aromatic Hydrocarbons in the Interstellar Medium

Long-slit 8-13 μm spectroscopy of the nebula around NGC 1333 SVS 3 reveals spatial variations in the strength and shape of emission features that are probably produced by polycyclic aromatic hydrocarbons (PAHs). Close to SVS 3, the 11.2 μm feature develops an excess at ~10.8-11.0 μm, and a feature appears at ~10 μm. These features disappear with increasing distance from the central source, and they show striking similarities to recent laboratory data of PAH cations, providing the first identification of emission features arising specifically from ionized PAHs in the interstellar medium.