Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

Abstract

We have analyzed the data from a large-scale CO survey toward the northern region of the Small Magellanic Cloud (SMC) obtained with the Atacama Compact Array (ACA) stand-alone mode of ALMA. The primary aim of this study is to comprehensively understand the behavior of CO as an H₂ tracer in a low-metallicity environment (Z ∼ 0.2 Z_⊙). The total number of mosaic fields is ∼8000, which results in a field coverage of 0.26 deg² (∼2.9 ×10⁵ pc²), corresponding to ∼10% of the area of the galaxy. The sensitive ∼2 pc resolution observations reveal the detailed structure of the molecular clouds previously detected in the single-dish NANTEN survey. We have detected a number of compact CO clouds within lower H₂ column density (∼10²⁰ cm⁻²) regions whose angular scale is similar to the ACA beam size. Most of the clouds in this survey also show peak brightness temperature as low as <1 K, which for optically thick CO emission implies an emission size much smaller than the beam size, leading to beam dilution. The comparison between an available estimation of the total molecular material traced by thermal dust emission and the present CO survey demonstrates that more than ∼90% of H₂ gas cannot be traced by the low-J CO emission. Our processed data cubes and 2D images are publicly available.

DOI： 10.3847/1538-4357/ac1ff4

Other URL： https://iopscience.iop.org/article/10.3847/1538-4357/ac1ff4/pdf

Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/abeb65

Other URL： https://iopscience.iop.org/article/10.3847/1538-4357/abeb65/pdf

Publishing type：Research paper (scientific journal)  

<title>Abstract</title>
While molecular gas mass is usually derived from 12CO(J = 1–0)—the most fundamental line for exploring molecular gas—it is often derived from 12CO(J = 2–1) assuming a constant 12CO(J = 2–1)$/$12CO(J = 1–0) line ratio (R2/1). We present variations of R2/1 and effects of the assumption that R2/1 is a constant in 24 nearby galaxies using 12CO data obtained with the Nobeyama 45 m radio telescope and IRAM 30 m telescope. The median of R2/1 for all galaxies is 0.61, and the weighted mean of R2/1 by 12CO(J = 1–0) integrated intensity is 0.66 with a standard deviation of 0.19. The radial variation of R2/1 shows that it is high (∼0.8) in the inner ∼1 kpc while its median in disks is nearly constant at 0.60 when all galaxies are compiled. In the case that the constant R2/1 of 0.7 is adopted, we found that the total molecular gas mass derived from 12CO(J = 2–1) is underestimated/overestimated by ∼20%, and at most by 35%. The scatter of molecular gas surface density within each galaxy becomes larger by ∼30%, and at most by 120%. Indices of the spatially resolved Kennicutt–Schmidt relation by 12CO(J = 2–1) are underestimated by 10%–20%, at most 39%, in 17 out of 24 galaxies. R2/1 has good positive correlations with star-formation rate and infrared color, and a negative correlation with molecular gas depletion time. There is a clear tendency of increasing R2/1 with increasing kinetic temperature (Tkin). Further, we found that not only Tkin but also pressure of molecular gas is important in understanding variations of R2/1. Special considerations should be made when discussing molecular gas mass and molecular gas properties inferred from 12CO(J = 2–1) instead of 12CO(J = 1–0).

DOI： 10.1093/pasj/psaa119

Publishing type：Research paper (international conference proceedings)  

DOI： 10.1117/12.2560955

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/abb822

Other URL： https://iopscience.iop.org/article/10.3847/1538-4357/abb822

Publishing type：Research paper (scientific journal)  

<title>Abstract</title>
We investigate the molecular gas properties of galaxies across the main sequence of star-forming (SF) galaxies in the local Universe using 12CO(J = 1–0), hereafter 12CO, and 13CO(J = 1–0), hereafter 13CO, mapping data of 147 nearby galaxies obtained in the COMING project, a legacy project of the Nobeyama Radio Observatory. In order to improve the signal-to-noise ratios of both lines, we stack all the pixels where 12CO emission is detected after aligning the line center expected from the first-moment map of 12CO. As a result, 13CO emission is successfully detected in 80 galaxies with a signal-to-noise ratio larger than three. The error-weighted mean of the integrated-intensity ratio of 12CO to 13CO lines (R1213) of the 80 galaxies is 10.9, with a standard deviation of 7.0. We find that (1) R1213 positively correlates to specific star-formation rate (sSFR) with a correlation coefficient of 0.46, and (2) both the flux ratio of IRAS 60 μm to 100 μm (f60/f100) and the inclination-corrected linewidth of 12CO stacked spectra ($\sigma _{ { \rm ^{12}CO},i}$) also correlate with sSFR for galaxies with the R1213 measurement. Our results support the scenario where R1213 variation is mainly caused by changes in molecular gas properties such as temperature and turbulence. The consequent variation of the CO-to-H2 conversion factor across the SF main sequence is not large enough to completely extinguish the known correlations between sSFR and Mmol/Mstar (μmol) or star-formation efficiency (SFE) reported in previous studies, while this variation would strengthen (weaken) the sSFR–SFE (sSFR–μmol) correlation.

DOI： 10.1093/pasj/psaa084

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

We report molecular line and continuum observations toward one of the most<br />
massive giant molecular clouds (GMCs), GMC-16, in M33 using ALMA with an<br />
angular resolution of 0\farcs44 $\times$ 0\farcs27 ($\sim$2 pc $\times$ 1 pc).<br />
We have found that the GMC is composed of several filamentary structures in<br />
$^{12}$CO and $^{13}$CO ($J$ = 2--1). The typical length, width, and total mass<br />
are $\sim$50--70 pc, $\sim$5--6 pc, and $\sim$10$^{5}$ $M_{\odot}$,<br />
respectively, which are consistent with those of giant molecular filaments<br />
(GMFs) as seen in the Galactic GMCs. The elongations of the GMFs are roughly<br />
perpendicular to the direction of the galaxy&#039;s rotation, and several H$\;${\sc<br />
ii} regions are located at the downstream side relative to the filaments with<br />
an offset of $\sim$10--20 pc. These observational results indicate that the<br />
GMFs are considered to be produced by a galactic spiral shock. The 1.3 mm<br />
continuum and C$^{18}$O ($J$ = 2--1) observations detected a dense clump with<br />
the size of $\sim$2 pc at the intersection of several filamentary clouds, which<br />
is referred to as $&quot;$hub-filament.$&quot;$ The first example of protostellar outflow<br />
in M33 also has been identified at the center of the clump. We have<br />
successfully resolved the pc-scale local star formation activity in which the<br />
galactic scale kinematics may induce the formation of the parental filamentary<br />
clouds.

DOI： 10.3847/1538-4357/ab8ad3

Publishing type：Research paper (scientific journal)  

DOI： 10.3847/2041-8213/ab70b7

Other URL： https://iopscience.iop.org/article/10.3847/2041-8213/ab70b7

Publishing type：Research paper (scientific journal)  

<title>Abstract</title>We present the results of $^{12}\textrm{C}$$\textrm{O}$(J = 1–0) and $^{13}\textrm{C}$$\textrm{O}$(J = 1–0) simultaneous mappings toward the nearby barred spiral galaxy NGC 4303 as part of the CO Multi-line Imaging of Nearby Galaxies (COMING) project. Barred spiral galaxies often show lower star-formation efficiency (SFE) in their bar region compared to the spiral arms. In this paper, we examine the relation between the SFEs and the volume densities of molecular gas n(H2) in the eight different regions within the galactic disk with $\textrm{C}$$\textrm{O}$ data combined with archival far-ultraviolet and 24 μm data. We confirmed that SFE in the bar region is lower by 39% than that in the spiral arms. Moreover, velocity-alignment stacking analysis was performed for the spectra in the individual regions. Integrated intensity ratios of $^{12}\textrm{C}$$\textrm{O}$ to $^{13}\textrm{C}$$\textrm{O}$ (R12/13) ranging from 10 to 17 were the results of this stacking. Fixing a kinetic temperature of molecular gas, $n(\rm {H_2})$ was derived from R12/13 via non-local thermodynamic equilibrium (non-LTE) analysis. The density n(H2) in the bar is lower by 31%–37% than that in the arms and there is a rather tight positive correlation between SFEs and n(H2), with a correlation coefficient of ∼0.8. Furthermore, we found a dependence of $n(\rm {H}_2)$ on the velocity dispersion of inter-molecular clouds (ΔV/sin i). Specifically, n(H2) increases as ΔV/sin i increases when ΔV/sin i &amp;lt; 100 km s−1. On the other hand, n(H2) decreases as ΔV/sin i increases when ΔV/sin i &amp;gt; 100 km s−1. These relations indicate that the variations of SFE could be caused by the volume densities of molecular gas, and the volume densities could be governed by the dynamical influence such as cloud–cloud collisions, shear, and enhanced inner-cloud turbulence.

DOI： 10.1093/pasj/psz022

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

<title>Abstract</title>
We examined radial variations in molecular-gas based star formation efficiency (SFE), which is defined as star formation rate per unit molecular gas mass, for 80 galaxies selected from the CO Multi-line Imaging of Nearby Galaxies project (Sorai et al. 2019, PASJ, 71, S14). The radial variations in SFE for individual galaxies are typically a factor of 2–3, which suggests that SFE is nearly constant along the galactocentric radius. We found an averaged SFE in 80 galaxies of (1.69 ± 1.1) × 10−9 yr−1, which is consistent with Leroy et al. (2008, AJ, 136, 2782) if we consider the contribution of helium to the molecular gas mass evaluation and the difference in the assumed initial mass function between the two studies. We compared SFE among different morphological (i.e., SA, SAB, and SB) types, and found that SFE within the inner radii (r/r25 &amp;lt; 0.3, where r25 is the B-band isophotal radius at 25 mag arcsec−2) of SB galaxies is slightly higher than that of SA and SAB galaxies. This trend can be partly explained by the dependence of SFE on global stellar mass, which probably relates to the CO-to-H2 conversion factor through the metallicity. For two representative SB galaxies in our sample, NGC 3367 and NGC 7479, the ellipse of r/r25 = 0.3 seems to cover not only the central region but also the inner part of the disk, mainly the bar. These two galaxies show higher SFE in the bar than in the spiral arms. However, we found an opposite trend in NGC 4303; SFE is lower in the bar than in the spiral arms, which is consistent with earlier studies (e.g., Momose et al. 2010, ApJ, 721, 383). These results suggest a diversity of star formation activities in the bar.

DOI： 10.1093/pasj/psz015

Publishing type：Research paper (scientific journal)  

<title>Abstract</title>Observations of the molecular gas in galaxies are vital to understand the evolution and star-forming histories of galaxies. However, galaxies with molecular gas maps of their whole discs at sufficient resolution to distinguish galactic structures are severely lacking. Millimeter-wavelength studies at a high angular resolution across multiple lines and transitions are particularly needed, severely limiting our ability to infer the universal properties of molecular gas in galaxies. Hence, we conducted a legacy project with the 45 m telescope of the Nobeyama Radio Observatory, called the CO Multi-line Imaging of Nearby Galaxies (COMING), which simultaneously observed 147 galaxies with high far-infrared (FIR) flux in 12CO, 13CO, and C18O J = 1–0 lines. The total molecular gas mass was derived using the standard CO–to–H2 conversion factor and found to be positively correlated with the total stellar mass derived from the WISE 3.4 μm band data. The fraction of the total molecular gas mass to the total stellar mass in galaxies does not depend on their Hubble types nor the existence of a galactic bar, although when galaxies in individual morphological types are investigated separately, the fraction seems to decrease with the total stellar mass in early-type galaxies and vice versa in late-type galaxies. No differences in the distribution of the total molecular gas mass, stellar mass, or the total molecular gas to stellar mass ratio was observed between barred and non-barred galaxies, which is likely the result of our sample selection criteria, in that we prioritized observing FIR bright (and thus molecular gas-rich) galaxies.

DOI： 10.1093/pasj/psz115

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

We report the first evidence for high-mass star formation triggered by<br />
collisions of molecular clouds in M33. Using the Atacama Large<br />
Millimeter/submillimeter Array, we spatially resolved filamentary structures of<br />
giant molecular cloud 37 in M33 using $^{12}$CO($J$ = 2-1), $^{13}$CO($J$ =<br />
2-1), and C$^{18}$O($J$ = 2-1) line emission at a spatial resolution of $\sim$2<br />
pc. There are two individual molecular clouds with a systematic velocity<br />
difference of $\sim$6 km s$^{-1}$. Three continuum sources representing up to<br />
$\sim$10 high-mass stars with the spectral types of B0V-O7.5V are embedded<br />
within the densest parts of molecular clouds bright in the C$^{18}$O($J$ = 2-1)<br />
line emission. The two molecular clouds show a complementary spatial<br />
distribution with a spatial displacement of $\sim$3.5 pc, and show a V-shaped<br />
structure in the position-velocity diagram. These observational features traced<br />
by CO and its isotopes are consistent with those in high-mass star-forming<br />
regions created by cloud-cloud collisions in the Galactic and Magellanic Cloud<br />
HII regions. Our new finding in M33 indicates that the cloud-cloud collision is<br />
a promising process to trigger high-mass star formation in the Local Group.

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

DOI： 10.3847/1538-4357/ab2ade

Other URL： https://iopscience.iop.org/article/10.3847/1538-4357/ab2ade

Publishing type：Research paper (scientific journal)  

The 12CO (J = 1→0) velocity fields of a sample of 20 nearby spiral galaxies, selected from the CO Multi-line Imaging of Nearby Galaxies (COMING) legacy project of Nobeyama Radio Observatory, have been analyzed by Fourier decomposition to determine their basic kinematic properties, such as circular and noncircular velocities. On average, the investigated barred (SAB and SB) galaxies exhibit a ratio of noncircular to circular velocities of molecular gas larger by a factor of 1.5-2 than non-barred (SA) spiral galaxies at radii within the bar semimajor axis ab at 1 kpc resolution, with a maximum at a radius of R/ab ≈ 0.3. Residual velocity field images, created by subtracting model velocity fields from the data, reveal that this trend is caused by kpc-scale streaming motions of molecular gas in the bar region. Applying a new method based on radial velocity reversal, we estimated the corotation radius RCR and bar pattern speed ωb in seven SAB and SB systems. The ratio of the corotation to bar radius is found to be in a range of R≡ RCRab approx 0.8-1.6, suggesting that intermediate (SBb-SBc), luminous barred spiral galaxies host fast and slow rotator bars. Tentative negative correlations are found for ωb vs. ab and ωb vs. total stellar mass M∗, indicating that bars in massive disks are larger and rotate slower, possibly a consequence of angular momentum transfer. The kinematic properties of SAB and SB galaxies, derived from Fourier decomposition, are compared with recent numerical simulations that incorporate various rotation curve models and galaxy interactions.

DOI： 10.1093/pasj/psz004

Kind of work：Joint Work  

Kind of work：Joint Work  

Kind of work：Joint Work  

Kind of work：Joint Work  

Kind of work：Joint Work  

Kind of work：Joint Work