• Publications of The Korean Astronomical Society
• /
• v.27 no.4
• /
• pp.243-248
• /
• 2012

Nearby spiral galaxies M101 and M81 are considered to have undergone a galaxy-galaxy interaction. M101 has experienced HI gas infall due to the interaction. With AKARI far-infrared (IR) photometric observations, we found regions with enhanced star forming activity, which are spatially close to regions affected by the interaction. In addition, the relation between the star formation rate (SFR) and the gas content for such regions shows a significant difference from typical spiral arm regions. We discuss possible explanations for star formation processes on a kiloparsec scale and the association with interaction-triggered star formation. We also observed the compact group of galaxies Stephan's Quintet (SQ) with the AKARI Far-infrared Surveyor (FIS). The SQ shows diffuse intergalactic medium (IGM) due to multiple collisions between the member galaxies and the IGM. The intruder galaxy NGC 7318b is currently colliding with the IGM and causes a large-scale shock. The 160 micron image clearly shows the structure along the shock ridge as seen in warm molecular hydrogen line emission and X-ray emission. The far-IR emission from the shocked region comes from the luminous [CII]$158{\mu}m$ line and cold dust (~ 20 K) that coexist with molecular hydrogen gas. Survival of dust grains is indispensable to form molecular hydrogen gas within the collision age (~ 5 Myr). At the stage of the dusty IGM environment, [CII] and $H_2$ lines rather than X-ray emission are powerful cooling channels to release the collision energy.

• Publications of The Korean Astronomical Society
• /
• v.27 no.4
• /
• pp.237-242
• /
• 2012

We have performed a systematic study of interstellar dust grains in various environments of galaxies. AKARI has revealed the detailed properties of dust grains not only in star-forming regions but also in regions not relevant to star formation, some of which are found not to follow our old empirical knowledge. Because of its unique capabilities, AKARI has provided new knowledge on the processing of large grains and polycyclic aromatic hydrocarbons (PAHs). For example, we detect PAHs from elliptical galaxies, which show unusual spectral features and spatial distributions, demonstrating importance of material processing in the interstellar space. We find that copious amounts of large grains and PAHs are flowing out of starburst galaxies by galactic superwinds, which are being shattered and destroyed in galactic haloes. We discover evidence for graphitization of carbonaceous grains near the center of our Galaxy, providing a clue to understanding the activity of the Galactic center. We review the results obtained from our AKARI program, focusing on the processing of carbonaceous grains in various environments of galaxies.

• Publications of The Korean Astronomical Society
• /
• v.27 no.4
• /
• pp.195-200
• /
• 2012

We show how the rotation emission from isolated interstellar Polycyclic Aromatic Hydrocarbons (PAHs) can explain the so-called anomalous microwave emission (AME). AME has been discovered in the last decade as microwave interstellar emission (10 to 70 GHz) that is in excess compared to the classical emission processes: thermal dust, free-free and synchrotron. The PAHs are the interstellar planar nano-carbons responsible for the near infrared emission bands in the 3 to 15 micron range. Theoretical studies show that under the physical conditions of the interstellar medium (radiation and density) the PAHs adopt supra-thermal rotation velocities, and consequently they are responsible for emission in the microwave range. The first results from the PLANCK mission unexpectedly showed that the AME is not only emitted by specific galactic interstellar clouds, but it is present throughout the galactic plane, and is particularly strong in the cold molecular gas. The comparison of theory and observations shows that the measured emission is fully consistent with rotation emission from interstellar PAHs. We draw the main lines of our PLANCK-AKARI collaborative program which intends to progress on this question by direct comparison of the near infrared (AKARI) and microwave (PLANCK) emissions of the galactic plane.

• Publications of The Korean Astronomical Society
• /
• v.27 no.4
• /
• pp.265-270
• /
• 2012

Many observations have found evidence of the presence of a large number of heavily obscured Active Galactic Nuclei (AGNs). However, the nature of this population is only poorly understood because heavy obscuration by dust prevents one from finding them at optical wavelengths. Mid-infrared AGN searches can overcome this obstacle by penetrating through dust and by detecting direct emission from the dust torus. Thus, we can identify most of the AGN population, including type-2 and buried AGNs. Using the AKARI mid-infrared all-sky survey, we performed an AGN search in the nearby universe. Utilizing the 2MASS photometry, we selected mid-infrared-excess sources and carried out near-infrared spectroscopic observations in the AKARI Phase 3. During these follow-up observations, we have found three galaxies that show strong near-infrared red continuum from hot dust with a temperature of about 500 K, but do not show any AGN features in other wavelengths. The most suitable explanation of near-infrared continuum is the presence of central AGNs. Therefore, we conclude that they are AGNs obscured by dust. We performed X-ray observations of the two galaxies with SUZAKU. No detections in the 0.4-10 keV suggest that the column density may be much higher than $N_H=10^{23.5}cm^{-2}$. Comparing the masses of the host galaxies with those of the SDSS AGNs, we find that the host galaxies of the dusty AGNs discovered with AKARI are less massive populations than those of optically selected AGNs.

• Publications of The Korean Astronomical Society
• /
• v.32 no.1
• /
• pp.67-71
• /
• 2017

Debris disks are circumstellar dust disks around main-sequence stars. They are important observational clues to understanding the planetary system formation. The zodiacal light is the thermal emission from the dust disk in our Solar system. For a comprehensive understanding of the nature and the evolution of dust disks around main-sequence stars, we try a comparative study of debris disks and the zodiacal light. We search for debris disks using the AKARI mid-infrared all-sky point source catalog. By applying accurate flux estimate of the photospheric emission based on the follow-up near-infrared observations with IRSF, we have improved the detection rate of debris disks. For a detailed study of the structure and grain properties in the zodiacal dust cloud, as an example of dust disks around main-sequence stars, we analyze the AKARI mid-infrared all-sky diffuse maps. As a result of the debris disks search, we found old (>1 Gyr) debris disks which have large excess emission compared to their age, which cannot be explained simply by the conventional steady-state evolution model. From the zodiacal light analysis, we find the possibility that the dust grains trapped in the Earth's resonance orbits have increased by a factor of ~3 in the past ~20 years. Combining these results, we discuss the non-steady processes in debris disks and the zodiacal light.

• Publications of The Korean Astronomical Society
• /
• v.32 no.1
• /
• pp.163-167
• /
• 2017

Utilizing a unique capability of AKARI that allows deep spectroscopy at $2.5-5.0{\mu}m$, we performed a spectroscopy study of more than 200 quasars through one of the AKARI mission programs, QSONG (Quasar Spectroscopic Observation with NIR Grism). QSONG targeted 155 high redshift (3.3 < z < 6.42) quasars and 90 low redshift active galactic nuclei (0.002 < z < 0.48). In order to provide black hole mass estimates based on the rest-frame optical spectra, the high redshift part of QSONG is designed to detect the $H{\alpha}$ line and the rest-frame optical spectra of quasars at z > 3.3. The low redshift part of QSONG is geared to uncover the rest-frame $2.5-5.0{\mu}m$ spectral features of active galactic nuclei to gain useful information such as the dust-extinction-free black hole mass estimators based on the Brackett lines and the temperatures of the hot dust torus. We outline the program strategy, and present some of the scientific highlights from QSONG, including the detection of the $H{\alpha}$ line from a quasar at z > 4.5 which indicates a rigorous growth of black holes in the early universe, and the $Br{\beta}$-based black hole mass estimators and the hot dust temperatures (~ 1100 K) of low redshift AGNs.

• Publications of The Korean Astronomical Society
• /
• v.32 no.1
• /
• pp.97-99
• /
• 2017

Our understanding of dust emission, interaction, and evolution, is evolving. In recent years, electric dipole emission by spinning dust has been suggested to explain the anomalous microwave excess (AME), appearing between 10 and 90 Ghz. The observed frequencies suggest that spinning grains should be on the order of 10nm in size, hinting at polycyclic aromatic hydrocarbon molecules (PAHs). We present data from the AKARI/Infrared Camera (IRC) due to its high sensitivity to the PAH bands. By inspecting the IRC data for a few AME regions, we find a preliminary indication that regions well-fitted by a spinning-dust model have a higher $9{\mu}m$ than $18{\mu}m$ intensity vs. non-spinning-dust regions. Ongoing efforts to improve the analysis by using DustEM and including data from the AKARI Far Infrared Surveyor (FIS), IRAS, and Planck High Frequency Instrument (HFI) are described.

• Publications of The Korean Astronomical Society
• /
• v.32 no.1
• /
• pp.257-261
• /
• 2017

We built a $8{\mu}m$ selected sample of galaxies in the NEP-AKARI field by defining 4 redshift bins with the four AKARI bands at 11, 15, 18 and 24 microns (0.15 < z < 0.49, 0.75 < z < 1.34, 1.34 < z < 1.7 and 1.7 < z < 2.05). Our sample contains 4079 sources, 599 are securely detected with Herschel/PACS. Also adding ultraviolet (UV) data from GALEX, we fit the spectral energy distributions using the physically motivated code CIGALE to extract the star formation rate, stellar mass, dust attenuation and the AGN contribution to the total infrared luminosity ($L_{IR}$). We discuss the impact of the adopted attenuation curve and that of the wavelength coverage to estimate these physical parameters. We focus on galaxies with a luminosity close the characteristic $L^*_{IR}$ in the different redshift bins to study the evolution with redshift of the dust attenuation in these galaxies.

• Publications of The Korean Astronomical Society
• /
• v.32 no.1
• /
• pp.135-139
• /
• 2017

We have performed systematic studies of the properties of dust in various environments of nearby galaxies with AKARI. The unique capabilities of AKARI, such as near-infrared (near-IR) spectroscopy combined with all-sky coverage in the mid- and far-IR, enable us to study processing of dust, particularly carbonaceous grains includings polycyclic aromatic hydrocarbons (PAHs), for unbiased samples of nearby galaxies. In this paper, we first review our recent results on individual galaxies, highlighting the uniqueness of AKARI data for studies of nearby galaxies. Then we present results of our systematic studies on nearby starburst and early-type galaxies. From the former study based on the near-IR spectroscopy and mid-IR all-sky survey data, we find that the properties of PAHs change systematically from IR galaxies to ultraluminous IR galaxies, depending on the IR luminosity of a galaxy or galaxy population. From the latter study based on the mid- and far-IR all-sky survey data, we find that there is a global correlation between the amounts of dust and old stars in early-type galaxies, giving an observational constraint on the origin of the dust.

• Publications of The Korean Astronomical Society
• /
• v.32 no.1
• /
• pp.235-237
• /
• 2017

The AKARI NEP Deep Field Survey is an international multiwavelength survey over $0.4deg^2$ of the sky. This is the deepest survey made by the InfraRed Camera (IRC) of the infrared astronomical satellite AKARI with 9 filters continuously covering the $2-25{\mu}m$ range, including three filters in the Spitzer gap between the IRAC and MIPS coverages. This enabled us to make sensitive MIR detection of AGN candidates at z~ 1, based on hot dust emission in the AGN torus. It is also efficient in detecting highly obscured Compton-thick AGN population. In this article, we report the first results of X-ray observations on this field. The field was covered by 15 overlapping Chandra ACIS-I observations with a total exposure of ~300 ks, detecting ${\approx}450$ X-ray sources. We utilize rest-frame stacking analysis of the MIR AGN candidates that are not detected individually. Our preliminary analysis shows a marginal detection of the rest-frame stacked Fe $k{\alpha}$ line from our strong Compton-thick candidates.