Near-infrared spectroscopy of diffuse galactic radiation with AKARI/IRC

We present the results of near-infrared (NIR: 2.5-5 μm) spectroscopy of diffuse Galactic sources on the Galactic plane obtained with the Infrared Camera (IRC) on board AKARI. The spectral region of 2.5-5 μm is rich in various emission and absorption features. In this report, we focus on the H₂O and CO₂ ice absorption features at 3.0 and 4.3 μm, respectively, and report a search for features of deuterated polycyclic aromatic hydrocarbons (PAHs) in 4.4-4.6 μm. The column densities of CO₂ and H₂O ices show a correlation in agreement with that obtained with ISO observations. The correlation nearly crosses the origin, suggesting that H₂O and CO₂ ices form in tandem for a wide range of physical conditions. The ratio of the ice column densities in AFGL2591 along the slit is relatively constant over an area of 30", also supporting the tandem formation of H₂O and CO₂ ices. The H₂O ice column density shows a weak trend with AV estimated from HI recombination lines, which is in contrast to the clear correlation seen toward quiescent clouds. The weak correlation may be attributed partly to the uncertainty in AV and/or a range of the environmental conditions in the present targets. Only weak excess emission is seen in 4.4-4.6 μm in the spectra of the Orion bar, M17, and a reflection nebula. From these spectra, the ratio of deuterated PAHs to undeuterated PAHs is estimated as 3% at most. This is significantly smaller than the previously reported value and suggests that missing deuterium must reside in large PAHs that do not emit the 3 μm bands, if it is depleted into PAHs.