Near-Infrared Spectroscopy of Interstellar Dust

Abstract

Near infrared observations of reflection nebulae have set the historical ground for thediscovery of interstellar PAHs, but since, space observations have focused on their mid-IRfeatures, and data shortward of 5   μm have remained scarce. TheSpitzer/IRAC images in the 3.6 and 4.5   μm channels do show that thenear-IR emission from small dust particles is ubiquitous across the Galaxy, but provide nospectroscopic information. To investigate the nature of this near-IR dust emission, wehave obtained AKARI spectroscopic observations, over the 2.5−5   μmspectral range, for a set of archetype PDRs mapped with the Spitzer spectrometer at mid-IRwavelengths. These AKARI data supplement earlier observations with the SWS ISOspectrometer, in providing the gain in sensitivity needed to observe low excitationsources, and the spatial information required to spatially correlate near-IR spectroscopicsignatures with physical conditions and observed changes in mid-IR spectra. This paperpresents the first results of the data analysis, in relation to two open questions oninterstellar 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 featureslongward of the main 3.29   μm PAH feature, and continuum emission. Theintensity ratio between the features ascribed to aliphatic CH bonds and the3.29   μm aromatic band, varies spatially in a way that may beinterpreted as evidence for aromatization of the smallest dust particles byphoto-processing. The continuum displays a striking step-increase across the3.29   μm feature. We also present a spectrum of a photodissociationregion with a feature at 4.65   μm, which has been speculated to berelated to the CD stretch in aliphatic hydrocarbon side-groups on PAHs.