arXiv subjects

Naomasa Nakai

Publications and source records attributed to Naomasa Nakai.

At least 19 recordsLinked to original sources

arXiv2026

High-Resolution [CI] $({^3\mathrm{P}_2}\rightarrow{^3\mathrm{P}_1})$ and CO $(J=7\rightarrow6)$ Observations of Circumnuclear Disks in Nearby Seyfert Galaxies

We present [CI] $({^3\mathrm{P}_2}\mathrm{-}{^3\mathrm{P}_1})$, CO $(J=7\mathrm{-}6)$, and 800-GHz continuum observations of the circumnuclear disks (CNDs) in the Seyfert galaxies NGC 613 and NGC 1808 with the Atacama Large Millimeter/submillimeter Array. The images reveal the distributions of neutral gas and dust in nearby galaxy nuclei at a resolution of $0.2''$ (10 pc) which is unprecedented at these frequencies. Both lines and continuum exhibit peaks at the position of molecular tori. The maximum [CI]$({^3\mathrm{P}_2}\mathrm{-}{^3\mathrm{P}_1})/({^3\mathrm{P}_1}\mathrm{-}{^3\mathrm{P}_0})$ integrated-intensity ratio is $\approx1.1$ $(\mathrm{K~km~s}^{-1}$ scale) in the region of the torus, yielding an excitation temperature of $T_\mathrm{ex}\approx60~\mathrm{K}$ under the approximation of local thermodynamic equilibrium (LTE) and optically thin emission. A non-LTE radiative transfer analysis of the [CI] line ratio in the CND constrains the density and kinetic temperature of molecular gas to $n_\mathrm{H_2}>10^3~\mathrm{cm}^{-3}$ and $T_\mathrm{k}>50~\mathrm{K}$, respectively. To estimate the C/CO abundance ratio and physical conditions, we simultaneously modeled the [CI] and multiple CO lines, and obtained solutions with densities and temperatures of $n_\mathrm{H_2}\sim10^{3\mathrm{-}4}~\mathrm{cm}^{-3}$ and $T_\mathrm{k}\sim80\mathrm{-}250~\mathrm{K}$ in the central 63 pc. The C/CO ratios are $\approx2$ in NGC 613 and $\approx0.6$ in NGC 1808. C/CO tends to increase with $T_\mathrm{k}$ and decrease with $n_\mathrm{H_2}$ in the CND, which is consistent with models of photodissociation regions. Based on the kinematics of gas traced by [CI] $({^3\mathrm{P}_2}\mathrm{-}{^3\mathrm{P}_1})$ and CO $(J=7\mathrm{-}6)$, the mass of the supermassive black hole at the center of NGC 613 is estimated to be $\sim2\times10^7~M_\odot$.

astro-ph.GA↗

arXiv2026

ATT12: The Antarctic 12-m Terahertz Telescope for Studies of Dusty Galaxies. I. Instrument Sensitivity and Science Forecasts

We present a feasibility study of the Antarctic 12m Terahertz Telescope (ATT12), a next-generation facility to be constructed at New Dome Fuji in Antarctica, designed to open up the FIR and THz windows for extragalactic astronomy. While ATT12 will enable a wide range of Galactic and extragalactic science, this paper focuses on its potential for studies of dusty star-forming galaxies (DSFGs) across cosmic time. Using realistic atmospheric transmission models and the planned instrumental specifications of heterodyne spectrometers and wide-field multi-color continuum cameras, we assess the expected sensitivity and scientific capabilities. We show that spectroscopic observations will enable detections of [CII]158um from galaxies with log(LIR/Lsun)>12 out to z~7, while [OIII]88um will remain observable for HyLIRG-class systems up to z~10. Line ratios including [OIII]52/88um, [NII]122/205um, and [OIII]/[NIII] will provide unique diagnostics of electron density and O/N abundance at z~4-8. Wide-field continuum surveys with the continuum cameras (KIDS-1/2; 300-850 GHz) will reach confusion-limited depths of ~1-2 mJy over ~10,000 deg$^2$, detecting of order $10^{6}$-$10^{7}$ DSFGs with log(LIR/Lsun)>12 at z<5 and $\lesssim10^{3}$--$10^{4}$ HyLIRGs up to z~7 or higher. Higher-frequency cameras (KIDS-3/4; >850 GHz) are designed for targeted follow-up observations and to extend coverage toward the THz regime. Taken together, ATT12 will provide the first statistically representative samples of DSFGs across cosmic time and, through synergy with ALMA, JWST, and the proposed FIR Probe PRIMA, will establish a multi-wavelength framework in which ATT12 discovers large samples through wide-area surveys, ALMA provides high-resolution follow-up of gas and ISM structure, JWST probes stellar populations and metallicity in the rest-frame optical/NIR, and PRIMA delivers ultra-sensitive FIR spectroscopy.

astro-ph.GA↗

arXiv2025

Water Maser Disk and a Supermassive Black Hole at the Nucleus of the Active Galaxy NGC 7738

We present the results of very long baseline interferometry (VLBI) observations of water vapor masers in the nucleus of the LINER galaxy NGC 7738. The red- and blue-shifted and newly detected systemic maser features show an almost edge-on disk located at a distance of ${0.031}\mbox{-}{0.222}$ pc from the galactic center and rotating with a velocity of $324\mbox{-}454$ km s$^{-1}$ . The velocity field of the disk indicates sub-Keplerian rotation, suggesting a non-negligible disk mass. The Mestel disk model reveals the central and disk masses to be $(1.2 \pm 0.4) \times 10^6$ $M_{\odot}$ and $(4.7 \pm 1.5) \times10^6$ $M_{\odot}$, respectively. The mean volume density within the inner radius of the disk [$(1.2 \pm 0.5) \times 10^{10}$ $M_{\odot}$ $\mathrm{pc^{-3}}$] strongly suggests the existence of a supermassive black hole at the center.

astro-ph.GA↗

arXiv2023

Probing Jet-Torus Interaction in the Radio Galaxy NGC 1052 by Sulfur-Bearing Molecules

The radio galaxy NGC 1052 casts absorption features of sulfur-bearing molecules, H$_2$S, SO, SO$_2$, and CS toward the radio continuum emission from the core and jets. Using ALMA, we have measured the equivalent widths of SO absorption features in multiple transitions and determined the temperatures of $344 \pm 43$ K and $26 \pm 4$ K in sub-millimeter and millimeter wavelengths, respectively. Since sub-mm and mm continuum represents the core and jets, the high and low temperatures of the absorbers imply warm environment in the molecular torus and cooler downstream flows. The high temperature in the torus is consistent with the presence of 22-GHz H$_2$O maser emission, vibrationally excited HCN and HCO$^+$ absorption lines, and sulfur-bearing molecules in gas phase released from dust. The origin of the sulfur-bearing gas is ascribed to evaporation of icy dust component through jet-torus interaction. Shock heating is the sole plausible mechanism to maintain such high temperature of gas and dust in the torus. Implication of jet-torus interaction also supports collimation of the sub-relativistic jets by gas pressure of the torus.

astro-ph.GA↗

arXiv2022

High-sensitivity VLBI Observations of the Water Masers in the Seyfert Galaxy NGC 1068

We present observational results of water vapor maser emission with our high-sensitivity 22 GHz VLBI imaging of the Seyfert galaxy NGC 1068. In this galaxy, there are the following four nuclear radio sources; NE, C, S1, and S2. Among them, the S1 component has been identified as the nucleus while the C component has been considered as attributed to the radio jet. In our VLBI observation, we find the following two types of the water maser emission at the S1 component. One is the linearly aligned component that is considered as an edge-on disk with the inner radius of 0.62 pc. The dynamical mass enclosed within the inner radius was estimated to be $1.5\times10^7 M_{\odot}$ by assuming the circular Keplerian motion. Note, however, that the best fit rotation curve shows a sub-Keplerian rotation ($v\propto r^{-0.24\pm0.10}$). The other is the water maser emission distributed around the rotating disk component up to 1.5 pc from the S1 component, suggesting the bipolar outflow from the S1 component. Further, we detected the water maser emission in the C component for the first time with VLBI, and discovered a ring-like distribution of the water maser emission. It is known that a molecular cloud is associated with the C component (both HCN and HCO$^+$ emission lines are detected by ALMA). Therefore, the ring-like maser emission can be explained by the jet collision to the molecular cloud. However, if these ring-like water masing clouds constitute a rotating ring around the C component, it is likely that the C component also has a supermassive black hole with the mass of $\sim 10^6 M_{\odot}$ that could be supplied from a past minor merger of a nucleated satellite galaxy.

astro-ph.GA↗

arXiv2021

A simple numerical experiment on the dust temperature bias for Lyman break galaxies at $z\gtrsim 5$

Some studies suggest that the dust temperatures ($T_\mathrm{d}$) in high-redshift ($z\gtrsim 5$) Lyman break galaxies (LBGs) are high. However, possible observational bias in $T_\mathrm{d}$ is yet to be understood. Thus, we perform a simple test using random realizations of LBGs with various stellar masses, dust temperatures, and dust-to-stellar mass ratios, and examine how the sample detected by ALMA is biased in terms of $T_\mathrm{d}$. We show that ALMA tends to miss high-$T_\mathrm{d}$ objects even at total dust luminosity. LBGs are, however, basically selected by the stellar UV luminosity. The dust-temperature bias in a UV-selected sample is complicated because of the competing effects between high $T_\mathrm{d}$ and low dust abundance. For ALMA Band 6, there is no tendency of high-$T_\mathrm{d}$ LBGs being more easily detected in our experiment. Thus, we suggest that the observed trend of high $T_\mathrm{d}$ in $z\gtrsim 5$ LBGs is real. We also propose that the 450 $μ$m band is useful in further clarifying the dust temperatures. To overcome the current shallowness of 450 $μ$m observations, we examine a future Antarctic 30-m class telescope with a suitable atmospheric condition for wavelengths $\lesssim 450~μ$m, where the detection is not confusion-limited. We find that, with this telescope, an $L_\mathrm{IR}$-selected sample with $\log(L_\mathrm{IR}/\mathrm{L}_{\odot})>11$ is constructed for $z\gtrsim 5$, and detection in the intermediate-$M_\star$ (stellar mass) range [$9<\log (M_\star /M_{\odot})<9.5$] is much improved, especially at high $T_\mathrm{d}$.

astro-ph.GA↗

arXiv2021

Atomic Carbon [CI]$(^3P_1-^3P_0)$ Mapping of the Nearby Galaxy M83

Atomic carbon (CI) has been proposed to be a global tracer of the molecular gas as a substitute for CO, however, its utility remains unproven. To evaluate the suitability of CI as the tracer, we performed [CI]$(^3P_1-^3P_0)$ (hereinafter [CI](1-0)) mapping observations of the northern part of the nearby spiral galaxy M83 with the ASTE telescope and compared the distributions of [CI](1-0) with CO lines (CO(1-0), CO(3-2), and $^{13}$CO(1-0)), HI, and infrared (IR) emission (70, 160, and 250$ μ$m). The [CI](1-0) distribution in the central region is similar to that of the CO lines, whereas [CI](1-0) in the arm region is distributed outside the CO. We examined the dust temperature, $T_{\rm dust}$, and dust mass surface density, $Σ_{\rm dust}$, by fitting the IR continuum-spectrum distribution with a single-temperature modified blackbody. The distribution of $Σ_{\rm dust}$ shows a much better consistency with the integrated intensity of CO(1-0) than with that of [CI](1-0), indicating that CO(1-0) is a good tracer of the cold molecular gas. The spatial distribution of the [CI] excitation temperature, $T_{\rm ex}$, was examined using the intensity ratio of the two [CI] transitions. An appropriate $T_{\rm ex}$ at the central, bar, arm, and inter-arm regions yields a constant [C]/[H$_2$] abundance ratio of $\sim7 \times 10^{-5}$ within a range of 0.1 dex in all regions. We successfully detected weak [CI](1-0) emission, even in the inter-arm region, in addition to the central, arm, and bar regions, using spectral stacking analysis. The stacked intensity of [CI](1-0) is found to be strongly correlated with $T_{\rm dust}$. Our results indicate that the atomic carbon is a photodissociation product of CO, and consequently, compared to CO(1-0), [CI](1-0) is less reliable in tracing the bulk of "cold" molecular gas in the galactic disk.

astro-ph.GA↗

arXiv2020

Molecular Gas Outflow in the Starburst Galaxy NGC 1482

Galactic winds are essential to regulation of star formation in galaxies. To study the distribution and dynamics of molecular gas in a wind, we imaged the nearby starburst galaxy NGC 1482 in CO ($J=1\rightarrow0$) at a resolution of 1'' ($\approx100$ pc) using the Atacama Large Millimeter/submillimeter Array. Molecular gas is detected in a nearly edge-on disk with a radius of 3 kpc and a biconical outflow emerging from the central 1 kpc starburst and extending to at least 1.5 kpc perpendicular to the disk. In the outflow, CO gas is distributed approximately as a cylindrically symmetrical envelope surrounding the warm and hot ionized gas traced by H$α$ and soft X-rays. The velocity, mass outflow rate, and kinetic energy of the molecular outflow are $v_\mathrm{w}\sim100~\mathrm{km~s^{-1}}$, $\dot{M}_\mathrm{w}\sim7~M_\odot~\mathrm{yr}^{-1}$, and $E_\mathrm{w}\sim7\times10^{54}~\mathrm{erg}$, respectively. $\dot{M}_\mathrm{w}$ is comparable to the star formation rate ($\dot{M}_\mathrm{w}/\mathrm{SFR}\sim2$) and $E_\mathrm{w}$ is $\sim1\%$ of the total energy released by stellar feedback in the past $1\times10^7~\mathrm{yr}$, which is the dynamical timescale of the outflow. The results indicate that the wind is starburst driven.

astro-ph.GA↗

arXiv2020

ALMA Observations of Multiple-CO and C Lines Toward the Active Galactic Nucleus of NGC 7469: X-Ray-dominated Region Caught in the Act

We used the Atacama Large Millimeter/submillimeter Array (ALMA) to map $^{12}$CO($J$ = 1-0), $^{12}$CO($J$ = 2-1), $^{12}$CO($J$ = 3-2), $^{13}$CO($J$ = 2-1), and [CI]($^3P_1$-$^3P_0$) emission lines around the type 1 active galactic nucleus (AGN) of NGC 7469 ($z = 0.0164$) at $\sim 100$ pc resolutions. The CO lines are bright both in the circumnuclear disk (central $\sim 300$ pc) and the surrounding starburst (SB) ring ($\sim 1$ kpc diameter), with two bright peaks on either side of the AGN. By contrast, the [CI]($^3P_1$-$^3P_0$) line is strongly peaked on the AGN. Consequently, the brightness temperature ratio of [CI]($^3P_1$-$^3P_0$) to $^{13}$CO(2-1) is $\sim 20$ at the AGN, as compared to $\sim 2$ in the SB ring. Our local thermodynamic equilibrium (LTE) and non-LTE models indicate that the enhanced line ratios (or CI enhancement) are due to an elevated C$^0$/CO abundance ratio ($\sim 3-10$) and temperature ($\sim 100-500$ K) around the AGN as compared to the SB ring (abundance ratio $\sim 1$, temperature $\lesssim 100$ K), which accords with the picture of the X-ray-dominated Region (XDR). Based on dynamical modelings, we also provide CO(1-0)-to- and [CI]($^3P_1$-$^3P_0$)-to-molecular mass conversion factors at the central $\sim 100$ pc of this AGN as $α_{\rm CO} = 4.1$ and $α_{\rm CI} = 4.4~M_\odot$ (K km s$^{-1}$ pc$^2$)$^{-1}$, respectively. Our results suggest that the CI enhancement is potentially a good marker of AGNs that could be used in a new submillimeter diagnostic method toward dusty environments.

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arXiv2020

A Massive Molecular Torus inside a Gas-Poor Cirnumnuclear Disk in the Radio Galaxy NGC 1052 Discovered with ALMA

We report ALMA observations of NGC 1052 to quest mass accretion in a gas-poor active galactic nucleus (AGN). We detected CO emission representing a rotating ring-like circumnuclear disk (CND) seen edge-on with the gas mass of $5.3 \times 10^{5}$ M$_{\odot}$. The CND has smaller gas mass than that in typical Seyfert galaxies with circumnuclear star formation and is too gas-poor to drive mass accretion onto the central engine. The continuum emission casts molecular absorption features of CO, HCN, HCO$^+$, SO, SO$_2$, CS, CN, and H$_2$O, with H$^{13}$CN and HC$^{15}$N and vibrationally-excited (v$_2 = 1$) HCN and HCO$^+$. Broader absorption line widths than CND emission line widths imply presence of a geometrically thick molecular torus with a radius of $2.4 \pm 1.3$ pc and a thickness ratio of $0.7 \pm 0.3$. We estimate the H$_2$ column density of $(3.3 \pm 0.7) \times 10^{25}$ cm$^{-2}$ using H$^{12}$CN, H$^{13}$CN, and HCO$^{+}$ absorption features and adopting abundance ratio of $^{12}$C-to-$^{13}$C and a HCO$^{+}$-to-H$_2$, and derived the torus gas mass of $(1.3 \pm 0.3) \times 10^7$ M$_{\odot}$, which is $\sim 9$\% of the central black-hole mass. The molecular gas in the torus is clumpy with the estimated covering factor of $0.17^{+0.06}_{-0.03}$. The gas density of clumps inside the torus is inferred to be $(6.4 \pm 1.3) \times 10^7$ cm$^{-3}$, which meets the excitation conditions of H$_2$O maser. Specific angular momentum in the torus exceeds a flat-rotation curve extrapolated from that of the CND, indicating a Keplerian rotation inside a 14.4-pc sphere of influence.

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arXiv2019

ALMA Observations of Atomic Carbon [C I] (${^3\mathrm{P}}_1\rightarrow{^3\mathrm{P}}_0$) and Low-$J$ CO Lines in the Starburst Galaxy NGC 1808

We present [C I] (${^3\mathrm{P}}_1\rightarrow{^3\mathrm{P}}_0$), $^{12}$CO, $^{13}$CO, and C$^{18}$O ($J=2\rightarrow1$) observations of the central region (radius 1 kpc) of the starburst galaxy NGC 1808 at 30-50 pc resolution conducted with Atacama Large Millimeter/submillimeter Array. Radiative transfer analysis of multiline data indicates warm ($T_\mathrm{k}\sim40\mathrm{-}80$ K) and dense ($n_\mathrm{H_2}\sim10^{3\mathrm{-}4}$ cm$^{-3}$) molecular gas with high column density of atomic carbon ($N_\mathrm{CI}\sim3\times10^{18}$ cm$^{-2}$) in the circumnuclear disk (central 100 pc). The C I/H$_2$ abundance in the central 1 kpc is $\sim3-7\times10^{-5}$, consistent with the values in luminous infrared galaxies. The intensity ratios of [C I]/CO(1-0) and [C I]/CO(3-2), respectively, decrease and increase with radius in the central 1 kpc, whereas [C I]/CO(2-1) is uniform within statistical errors. The result can be explained by excitation and optical depth effects, since the effective critical density of CO (2-1) is comparable to that of [C I]. The distribution of [C I] is similar to that of $^{13}$CO (2-1), and the ratios of [C I] to $^{13}$CO (2-1) and C$^{18}$O (2-1) are uniform within $\sim30\%$ in the central $<400$ pc starburst disk. The results suggest that [C I] (${^3\mathrm{P}}_1\rightarrow{^3\mathrm{P}}_0$) luminosity can be used as a CO-equivalent tracer of molecular gas mass, although caution is needed when applied in resolved starburst nuclei (e.g., circumnuclear disk), where the [C I]/CO(1-0) luminosity ratio is enhanced due to high excitation and atomic carbon abundance. The [C I]/CO(1-0) intensity ratio toward the base of the starburst-driven outflow is $\lesssim0.15$, and the upper limits of the mass and kinetic energy of the atomic carbon outflow are $\sim1\times10^4~M_\odot$ and $\sim3\times10^{51}$ erg, respectively.

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arXiv2019

CO Multi-line Imaging of Nearby Galaxies (COMING) IV. Overview of the Project

Observations of the molecular gas in galaxies are vital to understanding the evolution and star-forming histories of galaxies. However, galaxies with molecular gas maps of their whole discs having 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 flux in $^{12}$CO, $^{13}$CO, and C$^{18}$O $J=1-0$ lines. The total molecular gas mass was derived using the standard CO-to-H$_2$ conversion factor and found to be positively correlated with the total stellar mass derived from the WISE $3.4 μ$m 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, and 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.

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arXiv2019

CO Multi-line Imaging of Nearby Galaxies (COMING). III. Dynamical effect on molecular gas density and star formation in the barred spiral galaxy NGC 4303

We present the results of $^{12}$CO($J$=1-0) and $^{13}$CO($J$=1-0) simultaneous mappings toward the nearby barred spiral galaxy NGC 4303 as a 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 $n(\rm{H}_2)$ in the eight different regions within the galactic disk with CO data combined with archival far-ultraviolet and 24 $μ$m 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. The integrated intensity ratios of $^{12}$CO to $^{13}$CO ($R_{12/13}$) range from 10 to 17 as the results of stacking. Fixing a kinetic temperature of molecular gas, $n(\rm{H}_2)$ was derived from $R_{12/13}$ via non-local thermodynamic equilibrium (non-LTE) analysis. The density $n(\rm{H}_2)$ in the bar is lower by 31-37% than that in the arms and there is a rather tight positive correlation between SFEs and $n(\rm{H}_2)$, with a correlation coefficient of $\sim 0.8$. Furthermore, we found a dependence of $n(\rm{H}_2)$ on the velocity dispersion of inter-molecular clouds ($ΔV/ \sin i$). Specifically, $n(\rm{H}_2)$ increases as $ΔV/ \sin i$ increases when $ΔV/ \sin i < 100$ km s$^{-1}$. On the other hand, $n(\rm{H}_2)$ decreases as $ΔV/ \sin i$ increases when $ΔV/ \sin i > 100$ 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.

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arXiv2019

CO Multi-line Imaging of Nearby Galaxies (COMING): VI. Radial variations in star formation efficiency

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). The radial variations in SFE for individual galaxies are typically a factor of 2 -- 3, which suggests that SFE is nearly constant along galactocentric radius. We found the averaged SFE in 80 galaxies of $(1.69 \pm 1.1) \times 10^{-9}$ yr$^{-1}$, which is consistent with Leroy et al. 2008 if we consider the contribution of helium to the molecular gas mass evaluation and the difference in the assumed initial mass function between two studies. We compared SFE among different morphological (i.e., SA, SAB, and SB) types, and found that SFE within the inner radii ($r/r_{25} < 0.3$, where $r_{25}$ is $B$-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-H$_2$ conversion factor through the metallicity. For two representative SB galaxies in our sample, NGC 3367 and NGC 7479, the ellipse of $r/r_{25}$ = 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 spiral arms. However, we found an opposite trend in NGC 4303; SFE is lower in the bar than in spiral arms, which is consistent with earlier studies (e.g., Momose et al. 2010). These results suggest diversity of star formation activities in the bar.

astro-ph.GA↗

arXiv2019

CO Multi-line Imaging of Nearby Galaxies (COMING). VII. Fourier decomposition of molecular gas velocity fields and bar pattern speed

The $^{12}$CO $(J=1\rightarrow0)$ 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 $a_\mathrm{b}$ at 1 kpc resolution, with a maximum at a radius of $R/a_\mathrm{b}\sim0.3$. 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 $R_\mathrm{CR}$ and bar pattern speed $Ω_\mathrm{b}$ in seven SAB and SB systems. The ratio of the corotation to bar radius is found to be in a range of $\mathcal{R}\equiv R_\mathrm{CR}/a_\mathrm{b}\sim0.8\mathrm{-}1.6$, suggesting that intermediate (SBb-SBc), luminous barred spiral galaxies host fast and slow rotator bars. Tentative negative correlations are found for $Ω_\mathrm{b}$ vs. $a_\mathrm{b}$ and $Ω_\mathrm{b}$ vs. total stellar mass $M_\ast$, 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.

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arXiv2018

Multi-line Imaging of the Starburst Galaxy NGC 1808 with ALMA

Revealing the properties of molecular gas in starburst galaxies is key to understand the process of star formation across cosmic history. In this paper, we present our recent high-resolution ($\sim1''$ or 50 pc) ALMA observations of the nearby, barred starburst galaxy NGC 1808, and highlight some of the main findings: (1) the discovery of a double peak revealed in the distributions of CO (3-2) and CS (2-1) in the circumnuclear disk indicating the presence of a molecular gas torus with a radius of $r\sim30$ pc; inside the torus, we found a compact source of dust continuum at 0.87 mm and CO, coincident with the location of the low-luminosity AGN revealed by X-ray observations; (2) the excitation of CO gas, traced by the line intensity ratio of CO (3-2) to CO (1-0) is elevated to $\sim1$ in the star-forming disk; the ratio is between 0.3 and 1 in the 500 pc ring; (3) the ratio of HCN (1-0) to HCO$^{+}$ (1-0) in the central 1 kpc exhibits a radial gradient: while the ratio decreases from $\sim1.5$ in the center to $\sim0.9$ in the 500 pc disk, it exhibits a peak of $\sim1.2$ in a ring-like structure at a galactocentric radius of $r\sim300$ pc. The detection of SiO (2-1) in the circumnuclear disk suggests the presence of shocks, likely generated by intense nuclear star formation (supernova explosions and winds from massive stars), as well as inflows and outflows of turbulent gas in the central 100 pc.

astro-ph.GA↗

arXiv2018

Dense molecular gas in the starburst nucleus of NGC 1808

Dense molecular gas tracers in the central 1 kpc region of the superwind galaxy NGC 1808 have been imaged by ALMA at a resolution of 1" (~50 pc). Integrated intensities and line intensity ratios of HCN (1-0), H$^{13}$CN (1-0), HCO$^+$ (1-0), H$^{13}$CO$^+$ (1-0), HOC$^+$ (1-0), HCO$^+$ (4-3), CS (2-1), C$_2$H (1-0), and previously detected CO (1-0) and CO (3-2) are presented. SiO (2-1) and HNCO (4-3) are detected toward the circumnuclear disk (CND), indicating the presence of shocked dense gas. There is evidence that an enhanced intensity ratio of HCN(1-0)/HCO$^+$(1-0) reflects star formation activity, possibly in terms of shock heating and electron excitation in the CND and a star-forming ring at radius ~300 pc. A non-LTE analysis indicates that the molecular gas traced by HCN, H$^{13}$CN, HCO$^+$, and H$^{13}$CO$^+$ in the CND is dense ($n_{\mathrm{H}_2}$~$10^5$ cm$^{-3}$) and warm (20 K$<T_\mathrm{k}$<100 K). The calculations yield a low average gas density of $n_{\mathrm{H}_2}$~$10^2\mathrm{-}10^3$ cm$^{-3}$ for a temperature of $T_\mathrm{k}\geq30$ K in the nuclear outflow. Dense gas tracers HCN (1-0), HCO$^+$ (1-0), CS (2-1), and C$_2$H (1-0) are detected for the first time in the superwind of NGC 1808, confirming the presence of a velocity gradient in the outflow direction.

astro-ph.GA↗

arXiv2018

ALMA multiline observations toward the central region of NGC 613

We report ALMA observations of molecular gas and continuum emission in the 90 and 350 GHz bands toward a nearby Seyfert galaxy NGC 613. Radio continuum emissions were detected at 95 and 350 GHz from both the circum-nuclear disk (CND) ($r\leq90$ pc) and a star-forming ring (250 pc $\leq r\leq 340$ pc), and the 95 GHz continuum was observed to extend from the center at a position angle of $20^{\circ} \pm 8^{\circ}$. The archival 4.9 GHz data and our 95 GHz data show spectral indices of $α\leq -0.6$ and $-0.2$ along the jets and in the star-forming ring; these can be produced by synchrotron emission and free-free emission, respectively. In addition, we detected the emission of CO(3-2), HCN(1-0), HCN(4-3), HCO$^+$(1-0), HCO$^+$(4-3), CS(2-1), and CS(7-6) in both the CND and ring. The rotational temperatures and column densities of molecules derived from $J=1-0$ and $4-3$ lines of HCN and HCO$^+$ and $J=2-1$ and $7-6$ of CS in the CND and ring were derived. Furthermore, a non-LTE model revealed that the kinetic temperature of $T_{\rm k}=350-550$ K in the CND is higher than $T_{\rm k}=80-300$ K in the ring, utilizing the intensity ratios of HCN, HCO$^+$, and CS. The star-formation efficiency in the CND is almost an order of magnitude lower than those at the spots in the star-forming ring, while the dominant activity of the central region is the star formation rather than active galactic nuclei. We determined that the large velocity dispersion of CO extending toward the north side of the CND and decomposing into blueshifted and redshifted features is probably explained by the effect of the radio jets. These results strongly suggest that the jets heat the gas in the CND, in which the feedback prevents star formation.

astro-ph.GA↗