2004ApJ...613..770M

We present new spectroscopic observations of the 3.4 µm absorption feature in the Seyfert galaxies NGC 1068 and NGC 7674 and the ultraluminous infrared galaxy IRAS 08572+3915. A signature of CH bonds in aliphatic hydrocarbons, the 3.4 µm feature indicates the presence of organic material in Galactic and extragalactic dust. Here we compare the 3.4 µm feature in all the galaxies in which it has been detected. In several cases, the signal-to-noise ratio and spectral resolution permit a detailed examination of the feature profile, something which has rarely been attempted in extragalactic lines of sight. The 3.4 µm band in these galaxies closely resembles that seen in the Galactic diffuse interstellar medium (ISM) and in newly formed dust in a protoplanetary nebula. The similarity implies a common carrier for the carbonaceous component of dust, and one which is resistant to processing in the interstellar and/or circumnuclear medium. We also examine the mid-IR spectrum of NGC 1068, because absorption bands in the 5-8 µm region further constrain the chemistry of the 3.4 µm band carrier. While weak features like those present in the mid-IR spectrum of diffuse dust toward the Galactic center would be undetectable in NGC 1068, the strong bands found in the spectra of many proposed dust analog materials are clearly absent, eliminating certain candidates and production mechanisms for the carrier. The absence of strong absorption features at 5-8 µm is also consistent with the interpretation that the similarity in the 3.4 µm feature in NGC 1068 to that in Galactic lines of sight reflects real chemical similarity in the carbonaceous dust.