We report the results of a search for emission features from interstellar deuterated polycyclic aromatic hydrocarbons (PAHs) in the 4 µm region with the Infrared Camera (IRC) onboard AKARI. No significant excess emission is seen in 4.3−4.7 µm in the spectra toward the Orion Bar and M17 after the subtraction of line emission from the ionized gas. A small excess of emission remains at around 4.4 and 4.65 µm, but the ratio of their intensity to that of the band emission from PAHs at 3.3−3.5 µm is estimated as 2−3%. This is an order of magnitude smaller than the values previously reported and also those predicted by the model of deuterium depletion onto PAHs. Since the subtraction of the ionized gas emission introduces an uncertainty, the deuterated PAH features are also searched for in the reflection nebula GN 18.14.0, which does not show emission lines from ionized gas. We obtain a similar result that excess emission in the 4 µm region, if present, is about 2% of the PAH band emission in the 3 µm region. The present study does not find evidence for the presence of the large amount of deuterated PAHs that the depletion model predicts. The results are discussed in the context of deuterium depletion in the interstellar medium.

Aims. The amount of deuterium locked up in polycyclic aromatic hydrocarbons (PAHs) has to date been an uncertain value. We present a near-infrared (NIR) spectroscopic survey of Hii regions in the Milky Way, Large Magellanic Cloud, and Small Magellanic Cloud obtained with AKARI, which aims to search for features indicative of deuterated PAHs (PAD or D n -PAH) to better constrain the D/H ratio of PAHs. Methods. Fifty-three Hii regions were observed in the NIR (2.5-5 μm), using the Infrared Camera (IRC) on board the AKARI satellite. Through comparison of the observed spectra with a theoretical model of deuterated PAH vibrational modes, the aromatic and (a)symmetric aliphatic C-D stretch modes were identified. Results. We see emission features between 4.4-4.8 μm, which could be unambiguously attributed to deuterated PAHs in only six of the observed sources, all of which are located in the Milky Way. In all cases, the aromatic C-D stretching feature is weaker than the aliphatic C-D stretching feature, and, in the case of M17b, this feature is not observed at all. Based on the weak or absent PAD features in most of the observed spectra, it is suggested that the mechanism for PAH deuteration in the ISM is uncommon.

We present the results of near-to mid-infrared slit spectroscopic observations (2.55-13.4 µm) of the diffuse emission toward nine positions in the Large Magellanic Cloud with the Infrared Camera (IRC) on board AKARI. The target positions are selected to cover a wide range of the intensity of the incident radiation field. The unidentified infrared bands at 3.3, 6.2, 7.7, 8.6 and 11.3 µm are detected toward all the targets, and ionized gas signatures: hydrogen recombination lines and ionic forbidden lines toward three of them. We classify the targets into two groups: those without the ionized gas signatures (Group A) and those with the ionized signatures (Group B). Group A includes molecular clouds and photo-dissociation regions, whereas Group B consists of H II regions.In Group A, the band ratios of I 3.3 µm /I 11.3 µm , I 6.2 µm /I 11.3 µm , I 7.7 µm /I 11.3 µm and I 8.6 µm /I 11.3 µm show positive correlation with the IRAS and AKARI colors, but those of Group B do not follow the correlation. We discuss the results in terms of the polycyclic aromatic hydrocarbon (PAH) model and attribute the difference to the destruction of small PAHs and an increase in the recombination due to the high electron density in Group B. In the present study, the 3.3 µm band provides crucial information on the size distribution and/or the excitation conditions of PAHs and plays a key role in the distinction of Group A from B. The results suggest the possibility of the diagram of I 3.3 µm /I 11.3 µm v.s. I 7.7 µm /I 11.3 µm as an efficient diagnostic tool to infer the physical conditions of the interstellar medium.Subject headings: dust, extinction -Magellanic Clouds -galaxies: ISM -infrared: galaxies -infrared: ISMRecent laboratory experiments and quantum chemical calculations suggest that the properties of the UIR bands (e.g., shapes, center wavelengths, interband ratios, etc.) reflect the chemical and physical properties of PAHs (e.g., molecular structure, size distribution, ionization state, temperature, etc.), which may be altered in interstellar and circumstellar environments (Tielens 2008). Therefore the UIR bands have a great potential to be used as efficient diagnostic tools to infer the physical condition of the ISM even in remote galaxies.

Using a large collection of near-infrared spectra (2.5-5.4 µm) of Galactic H II regions and H II region-like objects, we perform a systematic investigation of the astronomical polycyclic aromatic hydrocarbon (PAH) features. 36 objects were observed by the use of the infrared camera onboard the AKARI satellite as a part of a directer's time program. In addition to the well-known 3.3-3.6 µm features, most spectra show a relatively-weak emission feature at 5.22 µm with sufficient signal-to-noise ratios, which we identify as the PAH 5.25 µm band previously reported. By careful analysis, we find good correlations between the 5.25 µm band and both the aromatic hydrocarbon feature at 3.3 µm and the aliphatic ones at around 3.4-3.6 µm. The present results give us convincing evidence that the astronomical 5.25 µm band is associated with C-H vibrations as suggested by previous studies and show its potential to probe the PAH size distribution. The analysis also shows that the aliphatic to aromatic ratio of I 3.4−3.6 µm /I 3.3 µm decreases against the ratio of the 3.7 µm continuum intensity to the 3.3 µm band, I cont,3.7 µm /I 3.3 µm , which is an indicator of the ionization fraction of PAHs. The mid-infrared color of I 9 µm /I 18 µm also declines steeply against the ratio of the hydrogen recombination line Brα at 4.05 µm to the 3.3 µm band, I Brα /I 3.3 µm .These facts indicate possible dust processing inside or at the boundary of ionized gas. Subject headings: dust, extinction -infrared: ISM 1. Introduction H II regions are luminous gas clouds ionized by embedded massive O-type or B-type stars. Considering their extreme physical conditions (intense radiation fields and stellar winds from the central stars), H II regions are a good laboratory for investigating the evolution of materials in the I 9 µm /I 18 µm .Section 2 describes the observations and data reduction. Spectral analysis is presented in Section 3, and its results and discussion in Section 4. Summary and conclusion are given in Section 5. Observations NIR slit spectroscopy with the AKARI/IRCThe present studies are mainly based on the slit spectroscopic data obtained by the AKARI/IRC in the framework of director's time program during the AKARI post-helium mission phase (Phase 3), during which only NIR channel (2.0-5.5 µm) was in operation. All of the

We report near-infrared K', L', and M' band imaging observations of the nearby merging galaxy NGC 6240 with the Infrared Camera and Spectrograph on the Subaru telescope. The observations were performed with the assistance of the Subaru Adaptive Optics System, and the achieved spatial resolutions were around 0.10-0.20 ′′ . We also obtained new mid-infrared imaging in the Si-2 filter band (8.7 µm) and N-band (7.5-13 µm) spectroscopy of this galaxy with the CanariCam on the Gran Telescopio Canarias with a spatial resolution of 0.4-0.5 ′′ . In the K' band image the two nuclei of the galaxy each show a double-peak suggesting the complex geometry of the source, while the L', M', and Si-2 band images show single compact structures in each of the two nuclei. Assuming 1 that the center core observed at wavelengths longer than 3.8 µm is associated with dust heated by the buried AGN, we re-evaluated the spectral energy distributions (SEDs) of the southern nucleus from 2 to 30 µm with the additional literature values, and performed the SED+spectroscopy fitting using the clumpy torus models of Nenkova et al. (2008) and a Bayesian fitting approach. The model fit suggests that the high covering factor torus emission in the southern nucleus is also obscured by foreground dust in the host galaxy. The estimated AGN bolometric luminosity of the southern nucleus, L bol (AGN) ∼ 1 × 10 45 erg·s −1 , accounts for approximately 40% of the whole luminosity of the system.