AKARI Near-Infrared Spectroscopy of 3 Micron PAH and 4 Micron PAD Features

Near-infrared (NIR; 2.5–5  μ m) low-resolution ( λ /Δ λ  ~ 100) spectra were obtained for a number of Galactic and extragalactic objects with the Infrared Camera (IRC) in the AKARI warm mission. These data provide us with the first opportunity to make a systematic study of the 3.3–3.5  μ m PAH features in a galactic scale as well as within an object. Whereas the 3.3  μ m band is well resolved in most spectra, the 3.5  μ m band is not clearly separated from the 3.4  μ m band in the IRC spectrum. The intensity ratio of the summation of the 3.4 and 3.5  μ m bands to the 3.3  μ m band shows a tendency to increase towards the Galactic center, although a large variation in the ratio is also seen in a local scale. A search for deuterated PAH features in the 4  μ m region is carried out in IRC NIR spectra. Emission lines originating from the ionized gas together with the detector anomaly hamper an accurate search at certain wavelengths, but little convincing evidence has so far been obtained for the presence of significant features in 4.2–4.7  μ m. A conservative upper limit of a few percents is obtained for the integrated intensity ratio of the 4.4–4.7  μ m possible features to the 3.3–3.5  μ m PAH features in the spectra so far obtained.