AKARI near-infrared spectroscopy of Galactic H II regions

We performed 2.5-5.5 μm near-infrared slit-spectroscopic observations for 36 Galactic H II regions with the infrared camera onboard the AKARI satellite. In addition to the well-known 3.3-3.6 μm features, a relatively weak emission feature is detected at around 5.0-5.4 μm with sufficient signal-to-noise ratios, which we identify as the previously reported PAH 5.25 μm band. The band correlates well with both the aromatic 3.3 μm feature and the aliphatic 3.4-3.6 μm features, which supports that the 5.25 μm band comes from C-H vibration modes of PAHs. The analysis also shows the systematic variation of the ratio of I(3.4-3.5)/I(3.3) against the ratio of I(3.7, cont)/I(3.3) and the decrease of the AKARI mid-infrared color of 9 μm to 18 μm with the transition from photodissociation region (PDR)-dominated to ionized gas-dominated regions. These facts may possibly reflect some dust processing inside the ionized-gas or its boundary.