Near-Infrared Spectroscopy of Interstellar Dust

Abstract

Near infrared observations of reflection nebulae have set the historical ground for the discovery of interstellar PAHs, but since, space observations have focused on their mid-IR features, and data shortward of 5   μ m have remained scarce. The Spitzer/IRAC images in the 3.6 and 4.5   μ m channels do show that the near-IR emission from small dust particles is ubiquitous across the Galaxy, but provide no spectroscopic information. To investigate the nature of this near-IR dust emission, we have obtained AKARI spectroscopic observations, over the 2.5−5   μ m spectral range, for a set of archetype PDRs mapped with the Spitzer spectrometer at mid-IR wavelengths. These AKARI data supplement earlier observations with the SWS ISO spectrometer, in providing the gain in sensitivity needed to observe low excitation sources, and the spatial information required to spatially correlate near-IR spectroscopic signatures with physical conditions and observed changes in mid-IR spectra. This paper presents the first results of the data analysis, in relation to two open questions on interstellar PAHs. (1) Is there an evolutionary link from aliphatic carbon dust to PAHs? (2) What is the origin of the near-IR dust continuum? The AKARI spectra display features longward of the main 3.29   μ m PAH feature, and continuum emission. The intensity ratio between the features ascribed to aliphatic CH bonds and the 3.29   μ m aromatic band, varies spatially in a way that may be interpreted as evidence for aromatization of the smallest dust particles by photo-processing. The continuum displays a striking step-increase across the 3.29   μ m feature. We also present a spectrum of a photodissociation region with a feature at 4.65   μ m, which has been speculated to be related to the CD stretch in aliphatic hydrocarbon side-groups on PAHs.