arXiv subjects

Mitsuyoshi Yamagishi

Publications and source records attributed to Mitsuyoshi Yamagishi.

At least 19 recordsLinked to original sources

arXiv2023

Cosmic-ray-driven enhancement of the C$^0$/CO abundance ratio in W51C

We examine spatial variations of the C$^0$/CO abundance ratio ($X_{\mathrm{C/CO}}$) in the vicinity of the $γ$-ray supernova remnant W51C, based on [CI] ($^3P_1$-$^3P_0$), $^{12}$CO(1-0), and $^{13}$CO(1-0) observations with the ASTE and Nobeyama 45-m telescopes. We find that $X_{\mathrm{C/CO}}$ varies in a range of 0.02-0.16 (0.05 in median) inside the molecular clouds of $A_V>$100 mag, where photodissociation of CO by the interstellar UV is negligible. Furthermore, $X_{\mathrm{C/CO}}$ is locally enhanced up to by a factor of four near the W51C center, depending on the projected distance from the W51C center. In high-$A_V$ molecular clouds, $X_{\mathrm{C/CO}}$ is determined by the ratio of the cosmic-ray (CR) ionization rate to the H$_2$ density, and we find no clear spatial variation of the H$_2$ density against the projected distance. Hence, the high CR ionization rate may locally enhance $X_{\mathrm{C/CO}}$ near the W51C center. We also find that the observed spatial extent of the enhanced $X_{\mathrm{C/CO}}$ ($\sim$17 pc) is consistent with the diffusion distance of CRs with the energy of 100 MeV. The fact suggests that the low-energy CRs accelerated in W51C enhance $X_{\mathrm{C/CO}}$. The CR ionization rate at the $X_{\mathrm{C/CO}}$-enhanced cloud is estimated to be 3$\times$10$^{-16}$ s$^{-1}$ on the basis of time-dependent PDR simulations of $X_{\mathrm{C/CO}}$, the value of which is 30 times higher than that in the standard Galactic environment. These results demonstrate that [CI] is a powerful probe to investigate the interaction between CRs and the interstellar medium for a wide area in the vicinity of supernova remnants.

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arXiv2022

ALCHEMI finds a "shocking" carbon footprint in the starburst galaxy NGC~253

Centers of starburst galaxies may be characterized by a specific gas and ice chemistry due to their gas dynamics and the presence of various ice desorption mechanisms. This may result in a peculiar observable composition. We analyze abundances of $CO_2$, a reliable tracer of ice chemistry, from data collected as part of the ALMA large program ALCHEMI, a wide-frequency spectral scan toward the starburst galaxy NGC~253 with an angular resolution of 1.6$''$. We constrain the $CO_2$ abundances in the gas phase using its protonated form $HOCO^+$. The distribution of $HOCO^+$ is similar to that of methanol, which suggests that $HOCO^+$ is indeed produced from the protonation of $CO_2$ sublimated from ice. The $HOCO^+$ fractional abundances are found to be $(1-2)\times10^{-9}$ at the outer part of the central molecular zone (CMZ), while they are lower ($\sim10^{-10}$) near the kinematic center. This peak fractional abundance at the outer CMZ is comparable to that in the Milky Way CMZ, and orders of magnitude higher than that in Galactic disk star-forming regions. From the range of $HOCO^+/CO_2$ ratios suggested from chemical models, the gas-phase $CO_2$ fractional abundance is estimated to be $(1-20)\times10^{-7}$ at the outer CMZ, and orders of magnitude lower near the center. We estimate the $CO_2$ ice fractional abundances at the outer CMZ to be $(2-5)\times10^{-6}$ from the literature. A comparison between the ice and gas $CO_2$ abundances suggests an efficient sublimation mechanism. This sublimation is attributed to large-scale shocks at the orbital intersections of the bar and CMZ.

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arXiv2021

ALMA view of the $ρ$ Ophiuchi A PDR with a 360-au beam: the [CI] emission originates from the plane-parallel PDR and extended gas

We present the results of data analysis of the [CI] ($^{3}P_{1}$-$^{3}P_{0}$) emission from the $ρ$ Ophiuchi A photon-dominated region (PDR) obtained in the ALMA ACA stand-alone mode with a spatial resolution of 2.''6 (360 au). The [CI] emission shows filamentary structures with a width of $\sim$1000 au, which are adjacent to the shell structure seen in the 4.5 $μ$m map. We found that the 4.5 $μ$m emission, C$^0$, and CO are distributed in this order from the excitation star (S1) in a complementary pattern. These results indicate that [CI] is emitted from a thin layer in the PDR generated by the excitation star, as predicted in the plane-parallel PDR model. In addition, extended [CI] emission was also detected, which shows nearly uniform integrated intensity over the entire field-of-view (1.'6$\times$1.'6). The line profile of the extended component is different from that of the above shell component. The column density ratio of C$^0$ to CO in the extended component was $\sim$2, which is significantly higher than those of Galactic massive star-forming regions (0.1-0.2). These results suggest that [CI] is emitted also from the extended gas with a density of $n_\mathrm{H_2} \sim 10^3$ cm$^{-3}$, which is not greatly affected by the excitation star.

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arXiv2019

Nobeyama 45-m Cygnus X CO Survey: II Physical Properties of $\mathrm{C^{18}O}$ Clumps

We report the statistical physical properties of the C$^{18}$O($J=1-0$) clumps present in a prominent cluster-forming region, Cygnus X, using the dataset obtained by the Nobeyama 45-m radio telescope. This survey covers 9 deg$^2$ of the north and south regions of Cygnus X, and totally 174 C$^{18}$O clumps are identified using the dendrogram method. Assuming a distance of 1.4 kpc, these clumps have radii of 0.2-1 pc, velocity dispersions of $<2.2~\mathrm{km~s^{-1}}$, gas masses of 30-3000 $M_\odot$, and H$_2$ densities of (0.2-5.5)$\times10^4~\mathrm{cm^{-3}}$. We confirm that the C$^{18}$O clumps in the north region have a higher H$_2$ density than those in the south region, supporting the existence of a difference in the evolution stages, consistent with the star formation activity of these regions. The difference in the clump properties of the star-forming and starless clumps is also confirmed by the radius, velocity dispersion, gas mass, and H$_2$ density. The average virial ratio of 0.3 supports that these clumps are gravitationally bound. The C$^{18}$O clump mass function shows two spectral index components, $α=-1.4$ in 55-140 $M_\odot$ and $α=-2.1$ in $>140~M_\odot$, which are consistent with the low- and intermediate-mass parts of the Kroupa's initial mass function. The spectral index in the star-forming clumps in $>140~M_\odot$ is consistent with that of the starless clumps in 55-140 $M_\odot$, suggesting that the latter will evolve into star-forming clumps while retaining the gas accretion. Assuming a typical star formation efficiency of molecular clumps (10%), about ten C$^{18}$O clumps having a gas mass of $>10^3~M_\odot$ will evolve into open clusters containing one or more OB stars.

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arXiv2019

A systematic study of ULIRGs using near-infrared absorption bands reveals a strong UV environment in their star-forming regions

We present a systematic study of the 3.0 um H2O ice and the 3.4 um aliphatic carbon absorption features toward 48 local ultraluminous infrared galaxies (ULIRGs) using spectra obtained by the AKARI Infrared Camera to investigate the UV environment in their star-forming regions. All the ULIRGs in our sample exhibit a ratio of optical depth of H2O ice to silicate dust (tau3.0/tau9.7) that is lower than that in the Taurus dark cloud. This implies that ULIRGs cannot be described as an ensemble of low-mass star-forming regions and that a significant amount of high-mass star-forming regions contribute to star-forming clouds in local ULIRGs. The results also show that the ratios of optical depth of aliphatic carbon to silicate dust, tau3.4/tau9.7, exhibit diverse values. We investigate two effects that can affect this ratio: the geometric temperature gradient (which increases the ratio) and the intense UV environment (which decreases it). The geometric temperature gradient is typically considered as a sign of active galactic nuclei (AGN). ULIRGs with AGN signs (optical classification, NIR color, and a PAH emission strength of 3.3 um) indeed tend to exhibit a large tau3.4/tau9.7 ratio. However, we find that the presence of buried AGN is not the only cause of the geometric temperature gradient, because the enhancement of the ratio is also evident in pure starburst-like ULIRGs without these AGN signs. Regarding the intense UV environment in star-forming regions, the correlation between the aliphatic carbon ratio and the ratio of the [C II] 158 um line luminosity to the far-infrared luminosity (L[CII]/LFIR), which represents the UV environment in photodissociation regions, implies that the intense UV environment causes the decrease of the aliphatic carbon ratio. We find that an intense UV environment (G/nH > 3) in star-forming regions is needed for the aliphatic carbon ratio to be suppressed.

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arXiv2019

Massive star formation in W51A triggered by cloud-cloud collisions

W51A is one of the most active star-forming region in our Galaxy, which contains giant molecular clouds with a total mass of 10^6 Msun. The molecular clouds have multiple velocity components over ~20 km/s, and interactions between these components have been discussed as the mechanism which triggered the massive star formation in W51A. In this paper, we report an observational study of the molecular clouds in W51A using the new 12CO, 13CO, and C18O (J=1-0) data covering a 1.4x1.0 degree region of W51A obtained with the Nobeyama 45-m telescope at 20" resolution. Our CO data resolved the four discrete velocity clouds at 50, 56, 60, and 68 km/s with sizes and masses of ~30 pc and 1.0-1.9x10^5 Msun. Toward the central part of the HII region complex G49.5-0.4, we identified four C18O clumps having sizes of ~1 pc and column densities of higher than 10^23 cm^-3, which are each embedded within the four velocity clouds. These four clumps are distributed close to each others within a small distance of 5 pc, showing a complementary distribution on the sky. In the position-velocity diagram, these clumps are connected with each others by bridge features with intermediate intensities. The high intensity ratios of 13CO (J=3-2/J=1-0) also indicates that these four clouds are associated with the HII regions. We also found these features in other HII regions in W51A. The timescales of the collisions are estimated to be several 0.1 Myrs as a crossing time of the clouds, which are consistent with the ages of the HII regions measured from the size of the HII regions in the 21 cm continuum emissions. We discuss the cloud-cloud collision scenario and massive star formation in W51A by comparing with the recent observational and theoretical studies of cloud-cloud collision.

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arXiv2018

Molecular gas in a Spitzer bubble N4: possible evidence for cloud-cloud collisions as a trigger of massive star formation

Herein, we present the 12CO (J=1-0) and 13CO (J=1-0) emission line observations via the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) toward a Spitzer bubble N4. We observed clouds of three discrete velocities: 16, 19, and 25 km/s. Their masses were 0.1x10^4 Msun, 0.3x10^4 Msun, and 1.4x10^4 Msun, respectively. The distribution of the 25-km/s cloud likely traces the ring-like structure observed at mid-infrared wavelength. We could not find clear expanding motion of the molecular gas in N4. On the contrary, we found a bridge feature and a complementary distribution, which are discussed as observational signatures of a cloud-cloud collision, between the 16- and 25-km/s clouds. We proposed a possible scenario wherein the formation of a massive star in N4 was triggered by a collision between the two clouds; however whereas the 19-km/s cloud is possibly not a part of the interaction with N4. The time scale of collision is estimated to be 0.2-0.3 Myr, which is comparable to the estimated dynamical age of the HII region of ~0.4 Myr. In N4W, a star-forming clump located west of N4, we observed molecular outflows from young stellar objects and the observational signature of a cloud-cloud collision. Thus, we also proposed a possible scenario in which massive- or intermediate-mass star formation was triggered via a cloud-cloud collision in N4W.

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arXiv2018

AKARI mid-infrared slit-less spectroscopic catalogue

AKARI/IRC has a capability of the slit-less spectroscopy in the mid-infrared (5--13 $μ$m) over a 10 arcmin$\times$10 arcmin area with a spectral resolution of 50, which is suitable for serendipitous surveys. The data reduction is, however, rather complicated by the confusion of nearby sources after dispersing the spectra. To make efficient and reliable data reduction, we first compiled a point-source list from the reference image in each field-of-view and checked the overlaps of the spectra using their relative positions and fluxes. Applying this procedure to 886 mid-infrared slit-less spectroscopic data taken in the cryogenic phase, we obtained 862 mid-infrared spectra from 604 individual non-overlapping sources brighter than 1.5 mJy. We find a variety of objects in the spectroscopic catalogue, ranging from stars to galaxies. We also obtained a by-product catalogue of 9 $μ$m point sources containing 42,387 objects brighter than 0.3 mJy. The spectroscopic and point-source catalogues are available online.

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arXiv2018

Revised Wavelength and Spectral Response Calibrations for AKARI Near-Infrared Grism Spectroscopy: Post-Cryogenic Phase

We present a new calibration for the second-order light contamination in the near-infrared grism spectroscopy with the Infrared Camera aboard AKARI, specifically for the post-cryogenic phase of the satellite (Phase 3). Following our previous work on the cryogenic phase (Phases 1 and 2), the wavelength and spectral response calibrations were revised. Unlike Phases 1 and 2, during Phase 3, the temperature of the instrument was not stable and gradually increased from 40 to 47 K. To assess the effect of the temperature increase, we divided Phase 3 into three sub-phases and performed the calibrations separately. As in Phases 1 and 2, we confirmed that there was contamination due to the wavelength dependence of the refractive index of the grism material in every sub-phase. The wavelength calibration curves for the three sub-phases coincided with each other and did not show any significant temperature dependence. The response decreased with temperature by ~10% from the beginning to the end of Phase 3. We approximated the temperature dependence of the response as a linear relation and derived a correction factor as a function of temperature. The relative fraction of the second-order light contamination to the first-order light was found to be by 25% smaller than in Phases 1 and 2.

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arXiv2018

Enhanced dust emissivity power-law index along the western H$α$ filament of NGC 1569

We used a data set from AKARI and Herschel images at wavelengths from 7 $μ$m to 500 $μ$m to catch the evidence of dust processing in galactic winds in NGC 1569. Images show a diffuse infrared (IR) emission extending from the galactic disk into the halo region. The most prominent filamentary structure seen in the diffuse IR emission is spatially in good agreement with the western H$α$ filament (western arm). The spatial distribution of the $F_\mathrm{350}/F_\mathrm{500}$ map shows high values in regions around the super-star clusters (SSCs) and towards the western arm, which are not found in the $F_\mathrm{250}/F_\mathrm{350}$ map. The color-color diagram of $F_\mathrm{250}/F_\mathrm{350}$-$F_\mathrm{350}/F_\mathrm{500}$ indicates high values of the emissivity power-law index ($β_\mathrm{c}$) of the cold dust component in those regions. From a spectral decomposition analysis on a pixel-by-pixel basis, a $β_\mathrm{c}$ map shows values ranging from $\sim1$ to $\sim2$ over the whole galaxy. In particular, high $β_\mathrm{c}$ values of $\sim2$ are only observed in the regions indicated by the color-color diagram. Since the average cold dust temperature in NGC 1569 is $\sim30$ K, $β_\mathrm{c}<2.0$ in the far-IR and sub-mm region theoretically suggests emission from amorphous grains, while $β_\mathrm{c}=2.0$ suggests that from crystal grains. Given that the enhanced $β_\mathrm{c}$ regions are spatially confined by the HI ridge that is considered to be a birthplace of the SSCs, the spatial coincidences may indicate that dust grains around the SSCs are grains of relatively high crystallinity injected by massive stars originating from starburst activities and that those grains are blown away along the HI ridge and thus the western arm.

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arXiv2018

RCW 36 in the Vela Molecular Ridge: Evidence for a high-mass star cluster formation triggered by Cloud-Cloud Collision

A collision between two molecular clouds is one possible candidate for high-mass star formation. The HII region RCW 36, located in the Vela molecular ridge, contains a young star cluster with two O-type stars. We present new CO observations of RCW 36 with NANTEN2, Mopra, and ASTE using $^{12}$CO($J$ = 1-0, 2-1, 3-2) and $^{13}$CO($J$ = 2-1) line emissions. We have discovered two molecular clouds lying at the velocities $V_\mathrm{LSR} \sim$5.5 and 9 km s$^{-1}$. Both clouds are likely to be physically associated with the star cluster, as verified by the good spatial correspondence among the two clouds, infrared filaments, and the star cluster. We also found a high intensity ratio of $\sim$0.6-1.2 for CO $J$ = 3-2 / 1-0 toward both clouds, indicating that the gas temperature has been increased due to heating by the O-type stars. We propose that the O-type stars in RCW 36 were formed by a collision between the two clouds, with a relative velocity separation of 5 km s$^{-1}$. The complementary spatial distributions and the velocity separation of the two clouds are in good agreement with observational signatures expected for O-type star formation triggered by a cloud-cloud collision. We also found a displacement between the complementary spatial distributions of the two clouds, which we estimate to be 0.3 pc assuming the collision angle to be 45$^{\circ}$ relative to the line-of-sight. We estimate the collision timescale to be $\sim$10$^5$ yr. It is probable that the cluster age by Ellerbroek et al. (2013b) is dominated by the low-mass members which were not formed under the triggering by cloud-cloud collision, and that the O-type stars in the center of the cluster are explained by the collisional triggering independently from the low-mass star formation.

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arXiv2017

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) : Molecular clouds toward W33 ; possible evidence for a cloud-cloud collision triggering O star formation

We observed molecular clouds in the W33 high-mass star-forming region associated with compact and extended HII regions using the NANTEN2 telescope as well as the Nobeyama 45-m telescope in the $J=$1-0 transitions of $^{12}$CO, $^{13}$CO, and C$^{18}$O as a part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) legacy survey. We detected three velocity components at 35 km s$^{-1}$, 45 km s$^{-1}$, and 58 km s$^{-1}$. The 35 km s$^{-1}$ and 58 km s$^{-1}$ clouds are likely to be physically associated with W33 because of the enhanced $^{12}$CO $J=$ 3-2 to $J=$1-0 intensity ratio as $R_{\rm 3-2/1-0} > 1.0$ due to the ultraviolet irradiation by OB stars, and morphological correspondence between the distributions of molecular gas and the infrared and radio continuum emissions excited by high-mass stars. The two clouds show complementary distributions around W33. The velocity separation is too large to be gravitationally bound, and yet not explained by expanding motion by stellar feedback. Therefore, we discuss that a cloud-cloud collision scenario likely explains the high-mass star formation in W33.

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arXiv2017

A Relationship of Polycyclic Aromatic Hydrocarbon Features with Galaxy Merger in Star-forming Galaxies at $z<0.2$

Using the AKARI, Wide-field Infrared Survey Explorer (WISE), Infrared Astronomical Satellite (IRAS), Sloan Digital Sky Survey (SDSS) and Hubble Space Telescope (HST) data, we investigated the relation of polycyclic aromatic hydrocarbon (PAH) mass ($M_{\rm PAH}$), very small grain mass ($M_{\rm VSG}$), big grain mass ($M_{\rm BG}$) and stellar mass ($M_{\rm star}$) with galaxy merger for 55 star-forming galaxies at redshift $z<0.2$. Using the SDSS image at $z<0.1$ and the HST image at $z>0.1$, we divided the galaxies into merger galaxies and non-merger galaxies with the morphological parameter asymmetry $A$, and quantified merging stages of galaxies based on the morphological indicators, the second-order momentum of the brightest 20$\%$ region $M_{20}$ and the Gini coefficient. We find that $M_{\rm PAH}/M_{\rm BG}$ of merger galaxies tend to be lower than that of non-merger galaxies and there are no systematic differences of $M_{\rm VSG}/M_{\rm BG}$ and $M_{\rm BG}/M_{\rm star}$ between merger galaxies and non-merger galaxies. We find that galaxies with very low $M_{\rm PAH}/M_{\rm BG}$ seem to be merger galaxies at late stages. These results suggest that PAHs are partly destroyed at late stages of merging processes. Furthermore, we investigated $M_{\rm PAH}/M_{\rm BG}$ variations in radiation field intensity strength $G_0$ and the emission line ratio of $[{\rm O\,{\scriptsize I}}]λ6300/{\rm H}α$ which is a shock tracer for merger galaxies and find that $M_{\rm PAH}/M_{\rm BG}$ decreases with increasing both $G_0$ and $[{\rm O\,{\scriptsize I}}]/{\rm H}α$. PAH destruction is likely to be caused by two processes; strong radiation fields and large-scale shocks during merging processes of galaxies.

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arXiv2017

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) I: Project Overview and Initial Results

The FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) project is one of the legacy projects using the new multi-beam FOREST receiver installed on the Nobeyama 45-m telescope. This project aims to investigate the distribution, kinematics, and physical properties of both diffuse and dense molecular gas in the Galaxy at once by observing 12CO, 13CO, and C18O J=1-0 lines simultaneously. The mapping regions are a part of the 1st quadrant (10d < l < 50d, |b| < 1d) and the 3rd quadrant (198d < l <236d, |b| < 1d) of the Galaxy, where spiral arms, bar structure, and the molecular gas ring are included. This survey achieves the highest angular resolution to date (~20") for the Galactic plane survey in the CO J=1-0 lines, which makes it possible to find dense clumps located farther away than the previous surveys. FUGIN will provide us with an invaluable dataset for investigating the physics of the galactic interstellar medium (ISM), particularly the evolution of interstellar gas covering galactic scale structures to the internal structures of giant molecular clouds, such as small filament/clump/core. We present an overview of the FUGIN project, observation plan, and initial results, which reveal wide-field and detailed structures of molecular clouds, such as entangled filaments that have not been obvious in previous surveys, and large-scale kinematics of molecular gas such as spiral arms.

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arXiv2017

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) 2: Possible evidence for formation of NGC~6618 cluster in M17 by cloud-cloud collision

We present $^{12}$CO $J=$1--0, $^{13}$CO $J=$1--0 and C$^{18}$O $J=$1--0 images of the M17 giant molecular clouds obtained as part of FUGIN (FOREST Ultra-wide Galactic Plane Survey InNobeyama) project. The observations cover the entire area of M17 SW and M17 N clouds at the highest angular resolution ($\sim$19$"$) to date which corresponds to $\sim$ 0.15 pc at the distance of 2.0 kpc. We find that the region consists of four different velocity components: very low velocity (VLV) clump, low velocity component (LVC), main velocity component (MVC), and high velocity component (HVC). The LVC and the HVC have cavities. UV photons radiated from NGC 6618 cluster penetrate into the N cloud up to $\sim$ 5 pc through the cavities and interact with molecular gas. This interaction is correlated with the distribution of YSOs in the N cloud. The LVC and the HVC are distributed complementary after that the HVC is displaced by 0.8 pc toward the east-southeast direction, suggesting that collision of the LVC and the HVC create the cavities in both clouds. The collision velocity and timescale are estimated to be 9.9 km s$^{-1}$ and $1.1 \times 10^{5}$ yr, respectively. The high collision velocity can provide the mass accretion rate up to 10$^{-3}$ $M_{\solar}$ yr$^{-1}$, and the high column density ($4 \times 10^{23}$ cm$^{-2}$) might result in massive cluster formation. The scenario of cloud-cloud collision likely well explains the stellar population and its formation history of NGC 6618 cluster proposed by Hoffmeister et al. (2008).

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arXiv2017

A star formation study of the ATLAS3D early-type galaxies with the AKARI all-sky survey

The star formation properties of early-type galaxies (ETGs) are currently the subject of considerable interest, particularly whether they differ from those of gas-rich spirals. We perform a systematic study of star formation in a large sample of local ETGs using polycyclic aromatic hydrocarbon (PAH) and dust emission, focusing on the galaxies' star formation rates (SFRs) and star formation efficiencies (SFEs). Our sample is composed of the 260 ETGs from the ATLAS3D survey, from which we use the cold gas measurements (HI and CO). The SFRs are estimated from stellar, PAH and dust fits to spectral energy distributions created from new AKARI measurements and literature data from WISE and 2MASS. The mid-infrared luminosities of non-CO-detected galaxies are well correlated with their stellar luminosities, showing that they trace (circum)stellar dust emission. CO-detected galaxies show an excess above these correlations, uncorrelated with their stellar luminosities, indicating that they likely contain PAHs and dust of interstellar origin. PAH and dust luminosities of CO-detected galaxies show tight correlations with their molecular gas masses, and the derived current SFRs are typically 0.01-1 Msun/yr. These SFRs systematically decrease with stellar age at fixed stellar mass, while they correlate nearly linearly with stellar mass at fixed age. The majority of local ETGs follow the same star-formation law as local star-forming galaxies, and their current SFEs do not depend on either stellar mass or age. Our results clearly indicate that molecular gas is fueling current star formation in local ETGs, that appear to acquire this gas via mechanisms regulated primarily by stellar mass. The current SFEs of local ETGs are similar to those of local star-forming galaxies, indicating that their low SFRs are likely due to smaller cold gas fractions rather than a suppression of star formation.

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arXiv2016

Mid- and far-infrared properties of Spitzer Galactic bubbles revealed by the AKARI all-sky surveys

We have carried out a statistical study on the mid- and far-infrared (IR) properties of Galactic IR bubbles observed by Spitzer. Using the Spitzer 8 $μ{\rm m}$ images, we estimated the radii and covering fractions of their shells, and categorized them into closed, broken and unclassified bubbles with our data analysis method. Then, using the AKARI all-sky images at wavelengths of 9, 18, 65, 90, 140 and 160 $μ{\rm m}$, we obtained the spatial distributions and the luminosities of polycyclic aromatic hydrocarbon (PAH), warm and cold dust components by decomposing 6-band spectral energy distributions with model fitting. As a result, 180 sample bubbles show a wide range of the total IR luminosities corresponding to the bolometric luminosities of a single B-type star to many O-type stars. For all the bubbles, we investigated relationships between the radius, luminosities and luminosity ratios, and found that there are overall similarities in the IR properties among the bubbles regardless of their morphological types. In particular, they follow a power-law relation with an index of $\sim$3 between the total IR luminosity and radius, as expected from the conventional picture of the Str$\rm{\ddot{o}}$mgren sphere. The exceptions are large broken bubbles; they indicate higher total IR luminosities, lower fractional luminosities of the PAH emission, and dust heating sources located nearer to the shells. We discuss the implications of those differences for a massive star-formation scenario.

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arXiv2016

A quality check of the $AKARI$ mid-infrared all-sky diffuse map toward the massive star-forming regions NGC 6334 and NGC 6357

We present a comparative study of CO and polycyclic aromatic hydrocarbon (PAH) emission toward a region including the massive star-forming regions of NGC 6334 and NGC 6357. We use the NANTEN $^{12}$CO($J$ = 1--0) data and the $AKARI$ 9 $μ$m All-Sky diffuse map in order to evaluate the calibration accuracy of the $AKARI$ data. We confirm that the overall CO distribution shows a good spatial correspondence with the PAH emission, and their intensities exhibit a good power-law correlation with a spatial resolution down to 4$'$ over the region of 10$^\circ$$\times$10$^\circ$. We also reveal poorer correlation for small scale structures between the two quantities toward NGC 6357, due to strong UV radiation from local sources. Larger scatter in the correlation toward NGC 6357 indicates higher ionization degree and/or PAH excitation than that of NGC 6334.

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