• Publications of The Korean Astronomical Society
• /
• v.32 no.1
• /
• pp.175-177
• /
• 2017

In order to reveal physical conditions of molecular gas in active galaxies (active galaxies mean both starbursts and AGNs in this paper), we carried out systematic observations (R = 19 ~ 120) of CO fundamental band at $4.7{\mu}m$ in absorption with AKARI. We also made follow-up CO absorption observations at higher spectral resolution (R = 5000 ~ 1000) with Subaru. Recently, Herschel made extensive observations of highly-excited CO lines in emission in the far-infrared. The two data sets (absorption and emission) sometimes provide us with apparently inconsistent results. One case is starburst galaxies: Subaru observations showed low temperature of molecular gas toward the starburst NGC 253, while Herschel detected highly excited CO lines in the starburst. This suggests that warm molecular clouds are more deeply embedded than newly formed star clusters. The other case is obscured AGNs; Herschel detected highly excited CO lines in emission in nearby AGNs, while AKARI and Subaru observations showed CO absorption only in some of the obscured AGNs. This could reflect the difference of nature of molecular tori in these AGNs. We propose the combination of the absorption and emission observations as an effective tool to reveal geometry of warm molecular clouds in active galaxies.

• Publications of The Korean Astronomical Society
• /
• v.27 no.4
• /
• pp.111-116
• /
• 2012

Far-infrared observations provide crucial data for the investigation and characterisation of the properties of dusty material in the Interstellar Medium (ISM), since most of its energy is emitted between ~ 100 and $200{\mu}m$. We present the first all-sky image from a sensitive all-sky survey using the Japanese AKARI satellite, in the wavelength range $50-180{\mu}m$. Covering > 99% of the sky in four photometric bands with four filters centred at $65{\mu}m$, $90{\mu}m$, $140{\mu}m$, and $160{\mu}m$ wavelengths, this achieved spatial resolutions from 1 to 2 arcmin and a detection limit of < 10 MJy $sr^{-1}$, with absolute and relative photometric accuracies of < 20%. All-sky images of the Galactic dust continuum emission enable astronomers to map the large-scale distribution of the diffuse ISM cirrus, to study its thermal dust temperature, emissivity and column density, and to measure the interaction of the Galactic radiation field and embedded objects with the surrounding ISM. In addition to the point source population of stars, protostars, star-forming regions, and galaxies, the high Galactic latitude sky is shown to be covered with a diffuse filamentary-web of dusty emission that traces the potential sites of high latitude star formation. We show that the temperature of dust particles in thermal equilibrium with the ambient interstellar radiation field can be estimated by using $90{\mu}m$, $140{\mu}m$, and $160{\mu}m$ data. The FIR AKARI full-sky maps provide a rich new data set within which astronomers can investigate the distribution of interstellar matter throughout our Galaxy, and beyond.

• Publications of The Korean Astronomical Society
• /
• v.27 no.4
• /
• pp.257-258
• /
• 2012

With AKARI, we carried out near-infrared spectroscopy of the nearby barred spiral galaxy, NGC 1097, categorized as Seyfert 1 with a circumnuclear starburst ring. Our observations mapped the galactic center region. As a result, we obtain the spatial distributions of the polycyclic aromatic hydrocarbon $3.3{\mu}m$ and the aliphatic hydrocarbon $3.4-3.6{\mu}m$ emission. The former is detected from all the observed regions and the latter is enhanced near the bar connecting the ring with the nucleus. In addition, we detect absorption features due to $H_2O$ ice and CO/SiO at the ring and the galactic center, while we detect the hydrogen recombination line $Br{\alpha}$ only from the ring. Hence the observed spectra change dramatically within the central 1 kpc region.

• Publications of The Korean Astronomical Society
• /
• v.32 no.1
• /
• pp.169-173
• /
• 2017

We provide a new physical insight on the hot molecular clouds near the nucleus of the obscured AGNs. We performed near-infrared spectroscopic observations of heavily obscured AGNs in order to reveal physical characteristics of molecular clouds, especially focused on the CO fundamental ro-vibrational absorption around $4.7{\mu}m$. We have made systematic moderate-resolution spectroscopic observations toward 30 representative (U)LIRGs using the AKARI/IRC, and some of the ULIRGs showed the strong CO absorption feature. For three bright (U)LIRGs that show a steep red continuum with the deep CO absorption feature, IRAS 08572+3915, UGC 05101, and IRAS 01250+2832, we have also made high-resolution spectroscopic observations using the Subaru/IRCS. We have successfully detected many absorption lines up to highly excited rotational levels, and these lines are very deep and extremely broad. The derived physical conditions of molecular clouds are extreme; the gas temperature is as high as several 100 to a 1000 K, the $H_2$ column density is larger than $10^{22}cm^{-2}$, and the gas density is greater than $10^7cm^{-3}$. Such hot and dense molecular clouds must exist around the central engine of the AGN.

• Publications of The Korean Astronomical Society
• /
• v.27 no.4
• /
• pp.263-264
• /
• 2012

We present the properties of dust and the near-infrared spectral features in nearby early-type galaxies. The properties of dust are obtained from the AKARI far-infrared all-sky survey diffuse map. The AKARI/IRC is used for the near-infrared spectra. We improve spectral data with the new dark subtraction method on the basis of the knowledge acquired in our laboratory experiments of the engineering-model detector for the IRC. We have succeeded in fitting the continuum by a power-law function and detecting CO and SiO absorption features in early-type galaxy spectra. Comparing the properties of dust and near-infrared spectral features, we find that the power-law slope depends on dust temperature, but not on the dust mass, which suggests that low-luminosity AGNs may contribute to the changes in the power-law slope and dust temperature.

• Publications of The Korean Astronomical Society
• /
• v.32 no.1
• /
• pp.141-145
• /
• 2017

We study $CO_2/H_2O$ ice abundance ratios in nearby galaxies using AKARI near-infrared slit spectroscopy. Past studies of the ices intensively examined $CO_2/H_2O$ ratios mainly in our Galaxy, and found that there were considerable variations in the $CO_2/H_2O$ ratios from object to object. The cause of the variations is, however, still under debate. As a result of the analysis of our sample that includes 1031 regions in 158 galaxies, the $CO_2/H_2O$ ratios are in a range of 0.05-0.30. In the dataset, we find that the $CO_2/H_2O$ ratios positively correlate with the $Br{\alpha}/PAH$ $3.3{\mu}m$ ratios which reflect the massive star formation activity. Furthermore, we find that the $CO_2/H_2O$ ratios positively correlate with the specific star formation rates of the galaxies where the ices are detected, that reflect the evolutionary stage of a galaxy. These results suggest that the $CO_2/H_2O$ ratios are enhanced in active star-forming regions and young galaxies.

• Journal of The Korean Astronomical Society
• /
• v.36 no.3
• /
• pp.167-175
• /
• 2003

There is accumulating evidence for a strong link between nuclear starbursts and AGN. Molecular gas in the central regions of galaxies plays a critical role in fueling nuclear starburst activity and feeding central AGN. The dense molecular ISM is accreted to the nuclear regions by stellar bars and galactic interactions. Here we describe recent observational results for the OB star forming regions in M51 and the nuclear star burst in Arp 220 - both of which have approximately the same rate of star formation per unit mass of ISM. We suggest that the maximum efficiency for forming young stars is an Eddington-like limit imposed by the radiation pressure of newly formed stars acting on the interstellar dust. This limit corresponds to approximately 500 $L_{\bigodot} / M_{\bigodot}$ for optically thick regions in which the radiation has been degraded to the NIR. Interestingly, we note that some of the same considerations can be important in AGN where the source of fuel is provided by stellar evolution mass-loss or ISM accretion. Most of the stellar mass-loss occurs from evolving red giant stars and whether their mass-loss can be accreted to a central AGN or not depends on the radiative opacity of the mass-loss material. The latter depends on whether the dust survives or is sublimated (due to radiative heating). This, in turn, is determined by the AGN luminosity and the distance of the mass-loss stars from the AGN. Several AGN phenomena such as the broad emission and absorption lines may arise in this stellar mass-loss material. The same radiation pressure limit to the accretion may arise if the AGN fuel is from the ISM since the ISM dust-to-gas ratio is the same as that of stellar mass-loss.

• Publications of The Korean Astronomical Society
• /
• v.32 no.1
• /
• pp.11-15
• /
• 2017

We present the AKARI far-infrared (FIR) all-sky maps and describe its characteristics, calibration accuracy and scientific capabilities. The AKARI FIR survey has covered 97% of the whole sky in four photometric bands, which cover continuously 50-180 micron with band central wavelengths of 65, 90, 140, and 160 microns. The data have been publicly released in 2014 (Doi et al., 2015) with improved data quality that have been achieved since the last internal data release (Doi et al., 2012). The accuracy of the absolute intensity is ${\leq}10%$ for the brighter regions. Quantitative analysis of the relative intensity accuracy and its dependence upon spatial scan numbers has been carried out. The data for the first time reveal the whole sky distribution of interstellar matter with arcminute-scale spatial resolutions at the peak of dust continuum emission, enabling us to investigate large-scale distribution of interstellar medium in great detail. The filamentary structure covering the whole sky is well traced by the all-sky maps. We describe advantages of the AKARI FIR all-sky maps for the study of interstellar matter comparing to other observational data.

• Publications of The Korean Astronomical Society
• /
• v.27 no.4
• /
• pp.209-212
• /
• 2012

We present the results of the near-infrared (NIR) to mid-infrared (MIR) slit spectroscopic observations of the diffuse emission toward nine positions in the nearby irregular galaxy Large Magellanic Cloud (LMC) with the Infrared Camera (IRC) on board AKARI. The unique characteristic of AKARI/IRC provides a great opportunity to analyze variations in the unidentified infrared (UIR) bands based on continuous spectra from 2.5 to $13.4{\mu}m$ of the same slit area. The observed variation of $I_{3.3}/I_{11.3}$ suggests destruction of small-sized UIR band carriers, polycyclic aromatic hydrocarbons (PAHs) in harsh environments. This result demonstrates that the UIR $3.3{\mu}m$ band provides us powerful information on the excitation conditions and/or the size distribution of PAHs, which is of importance for understanding the evolutionary process of hydrocarbon grains in the Universe. It also suggests a new diagnostic diagram of two band ratios, such as $I_{3.3}/I_{11.3}$ versus $I_{7.7}/I_{11.3}$, for the interstellar radiation conditions. We discuss on the applicability of the diagnostic diagram to other astronomical objects, comparing the LMC results with those observed in other galaxies such as NGC 6946, NGC 1313, and M51.

• Publications of The Korean Astronomical Society
• /
• v.32 no.1
• /
• pp.25-27
• /
• 2017

We are creating all-sky diffuse maps from the AKARI mid-infrared survey data with the two photometric bands centered at wavelengths of 9 and $18{\mu}m$. The AKARI mid-infrared diffuse maps achieve higher spatial resolution and higher sensitivity than the IRAS maps. In particular, the $9{\mu}m$ data are unique resources as an all-sky tracer of the emission of polycyclic aromatic hydrocarbons (PAHs). However, the original data suffer many artifacts. Thus, we have been developing correction methods. Among them, we have recently improved correction methods for the non-linearity and the reset anomaly of the detector response. These corrections successfully reduce the artifact level down to $0.1MJy\;sr^{-1}$ on average, which is essential for discussion on faint extended emission (e.g., the Galactic PAH emission). We have also made progress in the subtraction of the scattered light caused in the camera optics. We plan to release the improved diffuse maps to the public within a year.