Near-infrared and Mid-infrared Spectroscopy with the Infrared Camera (IRC) for AKARI

Publications- Astronomical Society of Japan (Impact Factor: 2.44). 10/2007; DOI: 10.1093/pasj/59.sp2.S411
Source: arXiv

ABSTRACT The Infrared Camera (IRC) is one of the two instruments on board the AKARI satellite. In addition to deep imaging from 1.8-26.5um for the pointed observation mode of the AKARI, it has a spectroscopic capability in its spectral range. By replacing the imaging filters by transmission-type dispersers on the filter wheels, it provides low-resolution (lambda/d_lambda ~ 20-120) spectroscopy with slits or in a wide imaging field-of-view (approximately 10'X10'). The IRC spectroscopic mode is unique in space infrared missions in that it has the capability to perform sensitive wide-field spectroscopic surveys in the near- and mid-infrared wavelength ranges. This paper describes specifications of the IRC spectrograph and its in-orbit performance.

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    ABSTRACT: We present the result of our analysis about the appearance of 3.3 μm CH4 band in the spectra of brown dwarfs with the Japanese infrared astronomical satellite, AKARI. The CH4 bands at 1.6 and 2.2 μm have been regarded as a key indicative features for classification of T-type brown dwarfs, while the 3.3μm CH4 band has been detected in the dwarfs as early as L5. This implies that the presence of CH4 does not simply rely on the effective temperature, and that we need more investigation to understand how the 3.3 μm CH4 absorption band appears in the infrared spectra of brown dwarfs. We are constructing a comprehensive sample of brown dwarfs in the 2.5-5.0 μm with AKARI. We obtained good quality spectra for 13 objects that enable us to have a better understanding of the atmospheric structure and onset of the CH4 band. We confirm that the 3.3 μm CH4 fundamental band starts appearing at L5. The band is seen in two of four L5-dwarfs in our sample. We derive the physical parameters of the photosphere of the objects by applying the Unified Cloudy Model. We find that the sources with/without 3.3 μm CH4 band have systematically different parameters, i.e. different photospheric structures. We find that the abundance of CH4 depend on the critical temperature (Tcr) and the surface gravity (log g) rather than the effective temperature (Teff). We suspect that the two groups are different in mass and age.
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    ABSTRACT: Near infrared observations of reflection nebulae have set the historical ground for the discovery of interstellar PAHs, but since, space observations have focused on their mid-IR features, and data shortward of 5 μm have remained scarce. The Spitzer/IRAC images in the 3.6 and 4.5 μm channels do show that the near-IR emission from small dust particles is ubiquitous across the Galaxy, but provide no spectroscopic information. To investigate the nature of this near-IR dust emission, we have obtained AKARI spectroscopic observations, over the 2.5-5 μm spectral range, for a set of archetype PDRs mapped with the Spitzer spectrometer at mid-IR wavelengths. These AKARI data supplement earlier observations with the SWS ISO spectrometer, in providing the gain in sensitivity needed to observe low excitation sources, and the spatial information required to spatially correlate near-IR spectroscopic signatures with physical conditions and observed changes in mid-IR spectra. This paper presents the first results of the data analysis, in relation to two open questions on interstellar 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 features longward of the main 3.29 μm PAH feature, and continuum emission. The intensity ratio between the features ascribed to aliphatic CH bonds and the 3.29 μm aromatic band, varies spatially in a way that may be interpreted as evidence for aromatization of the smallest dust particles by photo-processing. The continuum displays a striking step-increase across the 3.29 μm feature. We also present a spectrum of a photodissociation region with a feature at 4.65 μm, which has been speculated to be related to the CD stretch in aliphatic hydrocarbon side-groups on PAHs.
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    ABSTRACT: We present an atlas of 129 spectral energy distributions for nearby galaxies, with wavelength coverage spanning from the UV to the mid-infrared. Our atlas spans a broad range of galaxy types, including ellipticals, spirals, merging galaxies, blue compact dwarfs and luminous infrared galaxies. We have combined ground-based optical drift-scan spectrophotometry with infrared spectroscopy from Spitzer and Akari, with gaps in spectral coverage being filled using MAGPHYS spectral energy distribution models. The spectroscopy and models were normalized, constrained and verified with matched-aperture photometry measured from Swift, GALEX, SDSS, 2MASS, Spitzer and WISE images. The availability of 26 photometric bands allowed us to identify and mitigate systematic errors present in the data. Comparison of our spectral energy distributions with other template libraries and the observed colors of galaxies indicates that we have smaller systematic errors than existing atlases, while spanning a broader range of galaxy types. Relative to the prior literature, our atlas will provide improved K-corrections, photometric redshifts and star-formation rate calibrations.
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