In the evolutionary path of interstellar medium inquiry, many new species of interstellar dust have been modeled and discovered. The modes by which these species interact and evolve are beginning to be understood, but in recent years a peculiar new f
eature has appeared in microwave surveys. Anomalous microwave emission (AME), appearing between 10 and 90 GHz, has been correlated with thermal dust emission, leading to the popular suggestion that this anomaly is electric dipole emission from spinning dust. The observed frequencies suggest that spinning grains should be on the order of 1 nm in size, hinting at poly-cyclic aromatic hydrocarbon molecules. We present data from AKARI/Infrared Camera (IRC), due to the effective PAH/Unidentified Infrared Band (UIR) coverage of its 9 micron survey to investigate their role within a few regions showing strong AME in the Planck low frequency data. We include the well studied Perseus and rho Ophiuchi clouds . We use the IRAS/IRIS 100 micron data to account for the overall dust temperature. We present our results as abundance maps for dust emitting around 9 micron, and 100 micron. Part of the AME in these regions may actually be attributed to thermal dust emission, or the star forming nature of these targets is masking the vibrational modes of PAHs which should be present there, suggesting further investigation for various galactic environments.
Using a large collection of near-infrared spectra (2.5-5.4 um) of Galactic HII regions and HII 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 directers time program. In addition to the well-known 3.3-3.6 um features, most spectra show a relatively-weak emission feature at 5.22 um with sufficient signal-to-noise ratios, which we identify as the PAH 5.25 um band previously reported. By careful analysis, we find good correlations between the 5.25 um band and both the aromatic hydrocarbon feature at 3.3 um and the aliphatic ones at around 3.4-3.6 um. The present results give us convincing evidence that the astronomical 5.25 um 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)/I(3.3) decreases against the ratio of the 3.7 um continuum intensity to the 3.3 um band, I(3.7 cont)/I(3.3), which is an indicator of the ionization fraction of PAHs. The mid-infrared color of I(9)/I(18) also declines steeply against the ratio of the hydrogen recombination line Bralpha at 4.05 um to the 3.3 um band, I(Bralpha)/I(3.3). These facts indicate possible dust processing inside or at the boundary of ionized gas.
We report the results of a search for emission features from interstellar deuterated polycyclic aromatic hydrocarbons (PAHs) in the 4um region with the Infrared Camera (IRC) onboard AKARI. No significant excess emission is seen in 4.3-4.7um in the sp
ectra 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 4.65um, but the ratio of their intensity to that of the band emission from PAHs at 3.3-3.5um 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 4um region, if present, is about 2% of the PAH band emission in the 3um 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.
Using mid-infrared (MIR) images of four photometric bands of the Infrared Camera (IRC) onboard the AKARI satellite, S7 (7 um), S11 (11 um), L15 (15 um), and L24 (24 um), we investigate the interstellar dust properties of the nearby pair of galaxies M
51 with respect to its spiral arm structure. The arm and interarm regions being defined based on a spatially filtered stellar component model image, we measure the arm-to-interarm contrast for each band. The contrast is lowest in the S11 image, which is interpreted as that among the four AKARI MIR bands the S11 image best correlates with the spatial distribution of dust grains including colder components, while the L24 image with the highest contrast traces warmer dust heated by star forming activities. The surface brightness ratio between the bands, i.e. color, is measured over the disk of the main galaxy, M51a, at 300 pc resolution. We find that the distribution of S7/S11 is smooth and well traces the global spiral arm pattern while L15/S11 and L24/S11 peak at individual HII regions. This result indicates that the ionization state of PAHs is related to the spiral structure. Comparison with observational data and dust models also supports the importance of the variation in the PAH ionization state within the M51a disk. However, the mechanism driving this variation is not yet clear from currently available data sets. Another suggestion from the comparison with the models is that the PAH fraction to the total dust mass is higher than previously estimated.
We investigate the polycyclic aromatic hydrocarbon features in the young Galactic planetary nebula PN G095.2+00.7 based on mid-infrared observations. The near- to mid-infrared spectra obtained with the AKARI/IRC and the Spitzer/IRS show the PAH featu
res as well as the broad emission feature at 12 {mu}m usually seen in proto-planetary nebulae (pPNe). The spatially resolved spectra obtained with Subaru/COMICS suggest that the broad emission around 12 {mu}m is distributed in a shell-like structure, but the unidentified infrared band at 11.3 {mu}m is selectively enhanced at the southern part of the nebula. The variation can be explained by a difference in the amount of the UV radiation to excite PAHs, and does not necessarily require the chemical processing of dust grains and PAHs. It suggests that the UV self-extinction is important to understand the mid-infrared spectral features. We propose a mechanism which accounts for the evolutionary sequence of the mid-infrared dust features seen in a transition from pPNe to PNe.
We present the results of near- to mid-infrared slit spectroscopic observations (2.55--13.4 um) of the diffuse emission toward nine positions in the Large Magellanic Cloud with the Infrared Camera (IRC) on board AKARI. The target positions are select
ed 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 um 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 HII regions. In Group A, the band ratios of I(3.3)/I(11.3), I(6.2)/I(11.3), I(7.7)/$I(11.3) and $I(8.6)/$I(11.3) 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 um 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)/I(11.3) v.s. $I(7.7)/$I(11.3) as an efficient diagnostic tool to infer the physical conditions of the interstellar medium.
Mid-infrared images frequently suffer artifacts and extended point spread functions (PSFs). We investigate the characteristics of the artifacts and the PSFs in images obtained with the Infrared Camera (IRC) onboard AKARI at four mid-infrared bands of
the S7 (7{mu}m), S11 (11{mu}m), L15 (15{mu}m), and L24 (24 {mu}m). Removal of the artifacts significantly improves the reliability of the ref- erence data for flat-fielding at the L15 and L24 bands. A set of models of the IRC PSFs is also constructed from on-orbit data. These PSFs have extended components that come from diffraction and scattering within the detector arrays. We estimate the aperture correction factors for point sources and the surface brightness correction factors for diffuse sources. We conclude that the surface brightness correction factors range from 0.95 to 0.8, taking account of the extended component of the PSFs. To correct for the extended PSF effects for the study of faint structures, we also develop an image reconstruction method, which consists of the deconvolution with the PSF and the convolution with an appropriate Gaussian. The appropriate removal of the artifacts, improved flat-fielding, and image reconstruction with the extended PSFs enable us to investigate de- tailed structures of extended sources in IRC mid-infrared images.
Spitzer and AKARI observations have found that polycyclic aromatic hydrocarbons (PAHs) are present in nearby elliptical galaxies, but their spatial distributions are still unknown. In order to investigate their distributions, we performed deep spectr
al mapping observations of the PAH-detected elliptical galaxy NGC4589, a merger remnant with a minor-axis optical dust lane. As a result, we obtain clear evidence that the PAH 11.3 um emission comes predominantly from the dust lane of the galaxy. We also detect molecular hydrogen line emissions from the dust lane. The PAH 17 um emission is distributed differently from the PAH 11.3 um emission, and more similarly to the dust continuum emission. From their distinctive distributions, we suggest that the PAHs responsible for the 11.3 um feature are secondary products through the evolution of the ISM brought in by the merger.
