arXiv subjects

Tomonari Michiyama

Publications and source records attributed to Tomonari Michiyama.

At least 37 records · Page 2Linked to original sources

arXiv2022

Multi-wavelength and Multi-CO View of The Minor Merger Driven Star Formation in the Nearby LIRG NGC 3110

We present Atacama Large Millimeter/submillimeter Array observations of multiple CO(1-0), $^{13}$CO(1-0), and C$^{18}$O(1-0) lines and 2.9 mm and 1.3 mm continuum emission toward the nearby interacting luminous infrared galaxy NGC 3110, supplemented with similar spatial resolution H$α$, 1.4GHz continuum, and $K$-band data. We estimate the typical CO-to-H$_2$ conversion factor of 1.7 $M_{\odot}$ (K km s$^{-1}$ pc$^2$)$^{-1}$ within the disk using LTE-based and dust-based H$_2$ column densities, and measure the 1-kpc scale surface densities of star formation rate ($Σ_{\rm SFR}$), super star clusters ($Σ_{\rm SSC}$), molecular gas mass, and star formation efficiency (SFE) toward the entire gas disk. These parameters show a peak at the southern part of the southern spiral arm (SFE $\sim$ 10$^{-8.2}$ yr$^{-1}$, $Σ_{\rm SFR}$ $\sim$ 10$^{-0.6}$ $M_{\odot}$ kpc$^{-2}$ yr$^{-1}$, $Σ_{\rm SSC}$ $\sim$ 6.0 kpc$^{-2}$), which is likely attributed to the on-going tidal interaction with the companion galaxy MCG-01-26-013, as well as toward the circumnuclear region. We also find that thermal free-free emission contributes to a significant fraction of the millimeter continuum emission at the southern peak position. Those measurements imply that the peak of the southern arm is an active and young star-forming region, whereas the central part of NGC 3110 is a site of long-continued star formation. We suggest that, during the early stage of the galaxy-galaxy interaction with large mass ratio that in NGC 3110, fragmentation along the main galaxy's arms is an important driver of merger-induced star formation and massive gas inflow results in dusty nuclear starbursts.

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arXiv2022

Detection of nitrogen and oxygen in a galaxy at the end of reionization

We present observations of [NII] 205 $μ$m, [OIII] 88 $μ$m and dust emission in a strongly-lensed, submillimeter galaxy (SMG) at $z=6.0$, G09.83808, with the Atacama Large Millimeter/submillimeter Array (ALMA). Both [NII] and [OIII] line emissions are detected at $>12σ$ in the 0.8$"$-resolution maps. Lens modeling indicates that the spatial distribution of the dust continuum emission is well characterized by a compact disk with an effective radius of 0.64$\pm$0.02 kpc and a high infrared surface brightness of $Σ_\mathrm{IR}=(1.8\pm0.3)\times10^{12}~L_\odot$ kpc$^{-2}$. This result supports that G09.83808 is the progenitors of compact quiescent galaxies at $z\sim4$, where the majority of its stars are expected to be formed through a strong and short burst of star formation. G09.83808 and other lensed SMGs show a decreasing trend of the [NII] line to infrared luminosity ratio with increasing continuum flux density ratio between 63 $μ$m and 158 $μ$m, as seen in local luminous infrared galaxies (LIRGs). The decreasing trend can be reproduced by photoionization models with increasing ionization parameters. Furthermore, by combining the [NII]/[OIII] luminosity ratio with far-infrared continuum flux density ratio in G09.83808, we infer that the gas phase metallicity is already $Z\approx 0.5-0.7~Z_\odot$. G09.83808 is likely one of the earliest galaxies that has been chemically enriched at the end of reionization.

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arXiv2022

Spatially-resolved relation between [CI] $^{3}P_{1}$-$^{3}P_{0}$ and $^{12}$CO (1-0) in Arp 220

We present $\sim$0."3 (114 pc) resolution maps of [CI] $^{3}P_{1}$-$^{3}P_{0}$ (hereafter [CI] (1-0)) and $^{12}$CO (1-0) obtained toward Arp 220 with the Atacama Large Millimeter/submillimeter Array. The overall distribution of the [CI] (1-0) emission is consistent with the CO (1-0). While the [CI] (1-0) and CO (1-0) luminosities of the system follow the empirical linear relation for the unresolved ULIRG sample, we find a sublinear relation between [CI] (1-0) and CO (1-0) using the spatially-resolved data. We measure the [CI] (1-0)/CO (1-0) luminosity ratio per pixel in star-forming environments of Arp 220 and investigate its dependence on the CO (3-2)/CO (1-0) ratio ($R_{\rm CO}$). On average, the [CI] (1-0)/CO (1-0) luminosity ratio is almost constant up to $R_{\rm CO} \simeq 1$ and then increases with $R_{\rm CO}$. According to the radiative transfer analysis, a high CI/CO abundance ratio is required in regions with high [CI] (1-0)/CO (1-0) luminosity ratios and $R_{\rm CO} > 1$, suggesting that the CI/CO abundance ratio varies at $\sim$100 pc scale in Arp 220. The [CI] (1-0)/CO (1-0) luminosity ratio depends on multiple factors and may not be straightforward to interpret. We also find the high-velocity components traced by [CI] (1-0) in the western nucleus, likely associated with the molecular outflow. The [CI] (1-0)/CO (1-0) luminosity ratio in the putative outflow is 0.87 $\pm$ 0.28, which is four times higher than the average ratio of Arp 220. While there is a possibility that the [CI] (1-0) and CO (1-0) emission traces different components, we suggest that the high line ratios are likely because of elevated CI/CO abundance ratios based on our radiative transfer analysis. A CI-rich and CO-poor gas phase in outflows could be caused by the irradiation of the cosmic rays, the shock heating, and the intense radiation field.

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arXiv2021

Cold Molecular Gas in Merger Remnants. II. The properties of dense molecular gas

We present the 3 mm wavelength spectra of 28 local galaxy merger remnants obtained with the Large Millimeter Telescope. Fifteen molecular lines from 13 different molecular species and isotopologues were identified, and 21 out of 28 sources were detected in one or more molecular lines. On average, the line ratios of the dense gas tracers, such as HCN (1-0) and HCO$^{+}$(1-0), to $^{13}$CO (1-0) are 3-4 times higher in ultra/luminous infrared galaxies (U/LIRGs) than in non-LIRGs in our sample. These high line ratios could be explained by the deficiency of $^{13}$CO and high dense gas fractions suggested by high HCN (1-0)/$^{12}$CO (1-0) ratios. We calculate the IR-to-HCN (1-0) luminosity ratio as a proxy of the dense gas star formation efficiency. There is no correlation between the IR/HCN ratio and the IR luminosity, while the IR/HCN ratio varies from source to source (1.1-6.5) $\times 10^{3}$ $L_{\odot}$/(K km s$^{-1}$ pc$^{2}$). Compared with the control sample, we find that the average IR/HCN ratio of the merger remnants is higher by a factor of 2-3 than those of the early/mid-stage mergers and non-merging LIRGs, and it is comparable to that of the late-stage mergers. The IR-to-$^{12}$CO (1-0) ratios show a similar trend to the IR/HCN ratios. These results suggest that star formation efficiency is enhanced by the merging process and maintained at high levels even after the final coalescence. The dynamical interactions and mergers could change the star formation mode and continue to impact the star formation properties of the gas in the post-merger phase.

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arXiv2021

An ACA Survey of [CI] $^3P_1-^3P_0$, CO $J=4-3$, and Dust Continuum in Nearby U/LIRG

We present the results of surveying [CI] $^3P_1-^3P_0$, $^{12}$CO $J=4-3$, and 630 $μ$m dust continuum emission for 36 nearby ultra/luminous infrared galaxies (U/LIRGs) using the Band 8 receiver mounted on the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter Array. We describe the survey, observations, data reduction, and results; the main results are as follows. (i) We confirmed that [CI] $^3P_1-^3P_0$ has a linear relationship with both the $^{12}$CO $J=4-3$and 630 $μ$m continuum. (ii) In NGC 6052 and NGC 7679, $^{12}$CO $J=4-3$ was detected but [CI] $^3P_1-^3P_0$ was not detected with a [CI] $^3P_1-^3P_0$/ $^{12}$CO $J=4-3$ ratio of $\lesssim0.08$. Two possible scenarios of weak [CI] $^3P_1-^3P_0$ emission are C$^0$-poor/CO-rich environments or an environment with an extremely large [CI] $^3P_1-^3P_0$ missing flux. (iii) There is no clear evidence showing that galaxy mergers, AGNs, and dust temperatures control the ratios of [CI] $^3P_1-^3P_0$/ $^{12}$CO $J=4-3$ and $L'_{\rm [CI](1-0)}/L_{\rm 630μm}$. (iv) We compare our nearby U/LIRGs with high-z galaxies, such as galaxies on the star formation main sequence (MS) at z$\sim1$ and submillimeter galaxies (SMGs) at $z=2-4$. We found that the mean value for the [CII] $^3P_1$--$^3P_0$/ $^{12}$CO $J=4-3$ ratio of U/LIRGs is similar to that of SMGs but smaller than that of galaxies on the MS.

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arXiv2020

VALES VII: Molecular and ionized gas properties in pressure balanced interstellar medium of starburst galaxies at z ~ 0.15

Context. Spatially resolved observations of the ionized and molecular gas are critical for understanding the physical processes that govern the interstellar medium (ISM) in galaxies. Aims. To study the morpho-kinematic properties of the ionized and molecular gas in three dusty starburst galaxies at $z = 0.12-0.17$ to explore the relation between molecular ISM gas phase dynamics and the star-formation activity. Methods. We analyse $\sim$kpc-scale ALMA CO(1--0) and seeing limited SINFONI Paschen-$α$ observations. We use a dynamical mass model, which accounts for beam-smearing effects, to constrain the CO-to-H$_2$ conversion factor. Results. One starburst galaxy shows irregular morphology which may indicate a major merger, while the other two systems show disc-like morpho-kinematics. The two disc-like starbursts show molecular gas velocity dispersion values comparable with that seen in local LIRG/ULIRGs, but in an ISM with molecular gas fraction and surface density values consistent to that reported for local star-forming galaxies. These molecular gas velocity dispersion values can be explained by assuming vertical pressure equilibrium. The star-formation activity is correlated with the molecular gas content suggesting depletion times of the order of $\sim 0.1-1$ Gyr. The star formation rate surface density ($Σ_{\rm SFR}$) correlates with the ISM pressure set by self-gravity ($P_{\rm grav}$) following a power law with an exponent close to 0.8. Conclusions. In dusty disc-like starburst galaxies, our data support the scenario in which the molecular gas velocity dispersion values are driven by the ISM pressure set by self-gravity, responsible to maintain the vertical pressure balance. The correlation between $Σ_{\rm SFR}$ and $P_{\rm grav}$ suggests that, in these dusty starbursts galaxies, the star formation activity arises as a consequence of the ISM pressure balance.

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arXiv2020

Hunting for wandering massive black holes

We investigate low-density accretion flows onto massive black holes (BHs) with masses of $\gtrsim 10^5~M_\odot$ orbiting around in the outskirts of their host galaxies, performing three-dimensional hydrodynamical simulations. Those wandering BHs are populated via ejection from the galactic nuclei through multi-body BH interactions and gravitational wave recoils associated with galaxy and BH coalescences. We find that when a wandering BH is fed with hot and diffuse plasma with density fluctuations, the mass accretion rate is limited at $\sim 10-20\%$ of the canonical Bondi-Hoyle-Littleton rate owing to a wide distribution of inflowing angular momentum. We further calculate radiation spectra from radiatively inefficient accretion flows onto the wandering BH using a semi-analytical two-temperature disk model and find that the predicted spectra have a peak at the millimeter band, where the Atacama Large Millimeter/submillimeter Array (ALMA) has the highest sensitivity and spatial resolution. Millimeter observations with ALMA and future facilities such as the next generation Very Large Array (ngVLA) will enable us to hunt for a population of wandering BHs and push the detectable mass limit down to $M_\bullet \simeq 2\times10^7~M_\odot$ for massive nearby ellipticals, e.g., M87, and $M_\bullet \simeq 10^5~M_\odot$ for the Milky Way. This radiation spectral model, combined with numerical simulations, will be applied to give physical interpretations of off-nuclear BHs detected in dwarf galaxies, which may constrain BH seed formation scenarios.

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arXiv2020

The 300-pc Scale ALMA View of [CI] $^3P_1$-$^3P_0$, CO $J$=1-0, and 609 $μ$m Dust Continuum in A Luminous Infrared Galaxy

We present high-quality ALMA Band 8 observations of the [CI] $^3P_1$-$^3P_0$ line and 609 $μ$m dust continuum emission toward the nearby luminous infrared galaxy (LIRG) IRAS F18293-3413, as well as matched resolution (300-pc scale) Band 3 CO $J=$1-0 data, which allow us to assess the use of the [CI] $^3P_1$-$^3P_0$ line as a total gas mass estimator. We find that the [CI] line basically traces structures detected in CO (and dust), and a mean (median) [CI]/CO luminosity ($L'_{\rm [CI]}$/$L'_{\rm CO}$) ratio of 0.17 (0.16) with a scatter of 0.04. However, a pixel-by-pixel comparison revealed that there is a radial $L'_{\rm [CI]}$/$L'_{\rm CO}$ gradient and a superlinear $L'_{\rm CO}$ vs. $L'_{\rm [CI]}$ relation (slope = 1.54 $\pm$ 0.02) at this spatial scale, which can be explained by radial excitation and/or line opacity gradients. Based on the molecular gas masses converted from the dust continuum emission, we found that the CO-to-H$_2$ and [CI]-to-H$_2$ conversion factors are relatively flat across the molecular gas disk with a median value of 3.5$^{+1.9}_{-1.3}$ and 20.7$^{+9.2}_{-4.9}$ $M_{\odot}$ (K km s$^{-1}$ pc$^2$)$^{-1}$, respectively. A non-LTE calculation yields that typical molecular gas properties seen in nearby (U)LIRGs ($n_{\rm H_2}$ = 10$^{3-4}$ cm$^{-3}$, $T_{\rm kin}$ $\sim$ 50 K, and $X_{\rm CI}$ = (0.8-2.3) $\times$ 10$^{-5}$) can naturally reproduce the derived [CI]-to-H$_2$ conversion factor. However, we caution that a careful treatment of the physical gas properties is required in order to measure H$_2$ gas mass distributions in galaxies using a single [CI] line. Otherwise, a single [CI] line is not a good molecular gas estimator in a spatially resolved manner.

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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

Discovery of a [CI]-faint, CO-bright Galaxy: ALMA Observations of the Merging Galaxy NGC 6052

We report sensitive [\ion{C}{1}]~$^3P_1$--$^3P_0$ and $^{12}$CO~$J$=4--3 observations of the nearby merging galaxy NGC 6052 using the Morita (Atacama Compact) Array of ALMA. We detect $^{12}$CO~$J$=4--3 toward the northern part of NGC 6052, but [\ion{C}{1}]~$^3P_1$--$^3P_0$ is not detected with a [\ion{C}{1}]~$^3P_1$--$^3P_0$ to $^{12}$CO~$J$=4--3 line luminosity ratio of$~\lesssim0.07$. According to models of photodissociation regions, the unusual weakness of [\ion{C}{1}]~$^3P_1$--$^3P_0$ relative to $^{12}$CO~$J$=4--3 can be explained if the interstellar medium has a hydrogen density larger than $10^5\,{\rm cm}^{-3}$, conditions that might arise naturally in the ongoing merging process in NGC 6052. Its [\ion{C}{1}]~$^3P_1$--$^3P_0$ emission is also weaker than expected given the molecular gas mass inferred from previous measurements of $^{12}$CO~$J$=1--0 and $^{12}$CO~$J$=2--1. This suggests that [\ion{C}{1}]~$^3P_1$--$^3P_0$ may not be a reliable tracer of molecular gas mass in this galaxy. NGC 6052 is a unique laboratory to investigate how the merger process impacts the molecular gas distribution.

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arXiv2020

Star formation traced by optical and millimeter hydrogen recombination lines and free-free emissions in the dusty merging galaxy NGC 3256 -- MUSE/VLT and ALMA synergy

A galaxy-galaxy merger and the subsequent triggering of starburst activity are fundamental processes linked to the morphological transformation of galaxies and the evolution of star formation across the history of the Universe. Both nuclear and disk-wide starbursts are assumed to occur during the merger process. However, quantifying both nuclear and disk-wide star formation activity is non-trivial because the nuclear starburst is dusty in the most active merging starburst galaxies. This paper presents a new approach to this problem: combining hydrogen recombination lines in optical, millimeter, and free-free emission. Using NGC~3256 as a case study, H$β$, H40$α$, and free-free emissions are investigated using the Multi Unit Spectroscopic Explorer at the Very Large Telescope of the European Southern Observatory (MUSE/VLT) and the Atacama Large Millimeter/submillimeter Array (ALMA). The H$β$ image obtained by MUSE identifies star-forming regions outside the nuclear regions, suggesting a disk-wide starburst. In contrast, the H40$α$ image obtained by ALMA identifies a nuclear starburst where optical lines are undetected due to dust extinction ($A_{\rm V}\sim25$). Combining both MUSE and ALMA observations, we conclude that the total SFR is $49\pm2~M_{\odot}$~yr$^{-1}$ and the contributions from nuclear and disk-wide starbursts are $\sim34~\%$ and $\sim66~\%$, respectively. This suggests the dominance of disk-wide star formation in NGC~3256. In addition, pixel-by-pixel analyses for disk-wide star-forming regions suggest that shock gas tracers (e.g., CH$_3$OH) are enhanced where gas depletion time ($τ_{\rm gas}$=$M_{\rm gas}/SFR$) is long. This possibly means that merger-induced shocks regulate disk-wide star formation activities.

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arXiv2020

A non-corotating gas component in an extreme starburst at z=4.3

We report the detection of a non-corotating gas component in a bright unlensed submillimeter galaxy at z=4.3, COSMOS-AzTEC-1, hosting a compact starburst. ALMA 0.17 and 0.09 arcsec resolution observations of [CII] emission clearly demonstrate that the gas kinematics is characterized by an ordered rotation. After subtracting the best-fit model of a rotating disk, we kinematically identify two residual components in the channel maps. Both observing simulations and analysis of dirty images confirm that these two subcomponents are not artificially created by noise fluctuations and beam deconvolution. One of the two has a velocity offset of 200 km/s and a physical separation of 2 kpc from the primary disk and is located along the kinematic minor axis of disk rotation. We conclude that this gas component is falling into the galaxy from a direction perpendicular to the disk rotation. The accretion of such small non-corotating gas components could stimulate violent disk instability, driving radial gas inflows into the center of galaxies and leading to formation of in-situ clumps such as identified in dust continuum and CO. We require more theoretical studies on high gas fraction mergers with mass ratio of 1:>10 to verify this process.

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arXiv2019

CNO emission of an unlensed submillimeter galaxy at z=4.3

We present the results from ALMA observations of [NII]205 micron, [CII]158 micron, and [OII]88 micron lines in an unlensed submillimeter galaxy at z=4.3, COSMOS-AzTEC-1, hosting a compact starburst core with an effective radius of 1 kpc. The [CII] and [NII] emission are spatially-resolved in 0.3 arcsec-resolution (1 kpc in radius). The kinematic properties of the [NII] emission are consistent with those of the CO(4-3) and [CII] emission, suggesting that the ionized gas feels the same gravitational potential as the associated molecular gas and photodissociation regions (PDRs). On the other hand, the spatial extent is different among the lines and dust continuum: the [CII] emitting gas is the most extended and the dust is the most compact, leading to a difference of the physical conditions in the interstellar medium. We derive the incident far-ultraviolet flux and the hydrogen gas density through PDR modeling by properly subtracting the contribution of ionized gas to the total [CII] emission. The observed [CII] emission is likely produced by dense PDRs with nH(PDR)=10^(5.5-5.75) cm^-3 and G0=10^(3.5-3.75) in the central 1 kpc region and nH(PDR)=10^(5.0-5.25) cm^-3 and G0=10^(3.25-3.5) in the central 3 kpc region. We have also successfully measured the line ratio of [OIII/[NII] in the central 3 kpc region of COSMOS-AzTEC-1 at z=4.3, which is the highest redshift where both nitrogen and oxygen lines are detected. Under the most likely physical conditions, the measured luminosity ratio of L([OIII])/L([NII])=6.4+-2.2 indicates a near solar metallicity with Zgas=0.7-1.0 Zsol, suggesting a chemically evolved system at z=4.3.

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arXiv2018

ALMA observations of HCN and HCO+ outflows in the merging galaxy NGC 3256

We report ~2" resolution Atacama Large Millimeter/submillimeter Array observations of the HCN(1-0), HCO+(1-0), CO(1-0), CO(2-1), and CO(3-2) lines towards the nearby merging double-nucleus galaxy NGC 3256. We find that the high density gas outflow traced in HCN(1-0) and HCO+(1-0) emission is co-located with the diffuse molecular outflow emanating from the southern nucleus, where a low-luminosity active galactic nucleus (AGN) is believed to be the dominant source of the far-infrared luminosity. On the other hand, the same lines were undetected in the outflow region associated with the northern nucleus, whose primary heating source is likely related to starburst activity without obvious signs of AGN. Both HCO+(1-0)/CO(1-0) line ratio (i.e. dense gas fraction) and the CO(3-2)/CO(1-0) line ratio are larger in the southern outflow (0.20$\pm$0.04 and 1.3$\pm$0.2, respectively) than in the southern nucleus (0.08$\pm$0.01, 0.7$\pm$0.1, respectively). By investigating these line ratios for each velocity component in the southern outflow, we find that the dense gas fraction increases and the CO(3-2)/CO(1-0) line ratio decreases towards the largest velocity offset. This suggests the existence of a two-phase (diffuse and clumpy) outflow. One possible scenario to produce such a two-phase outflow is an interaction between the jet and the interstellar medium, which possibly triggers shocks and/or star formation associated with the outflow.

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arXiv2018

A gravitationally unstable gas disk of a starburst galaxy 12 billion years ago

Submillimeter bright galaxies in the early Universe are vigorously forming stars at ~1000 times higher rate than the Milky Way. A large fraction of stars is formed in the central 1 kiloparsec region, that is comparable in size to massive, quiescent galaxies found at the peak of the cosmic star formation history, and eventually the core of giant elliptical galaxies in the present-day Universe. However, the physical and kinematic properties inside a compact starburst core are poorly understood because dissecting it requires angular resolution even higher than the Hubble Space Telescope can offer. Here we report 550 parsec-resolution observations of gas and dust in the brightest unlensed submillimeter galaxy at z=4.3. We map out for the first time the spatial and kinematic structure of molecular gas inside the heavily dust-obscured core. The gas distribution is clumpy while the underlying disk is rotation-supported. Exploiting the high-quality map of molecular gas mass surface density, we find a strong evidence that the starburst disk is gravitationally unstable, implying that the self-gravity of gas overcomes the differential rotation and the internal pressure by stellar radiation feedback. The observed molecular gas would be consumed by star formation in a timescale of 100 million years, that is comparable to those in merging starburst galaxies. Our results suggest that the most extreme starburst in the early Universe originates from efficient star formation due to a gravitational instability in the central 2 kpc region.

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arXiv2018

A high dust emissivity index beta for a CO-faint galaxy in a filamentary Lyman-alpha nebula at z=3.1

We present CO J=4-3 line and 3 mm dust continuum observations of a 100 kpc-scale filamentary Lyα nebula (SSA22 LAB18) at z=3.1 using the Atacama Large Millimeter/submillimeter Array (ALMA). We detected the CO J=4-3 line at a systemic z(CO)=3.093 {\pm} 0.001 at 11 σ from one of the ALMA continuum sources associated with the Lyα filament. We estimated the CO J=4-3 luminosity of L'CO(4-3)=(2.3\pm0.2)x10^9 K km s^{-1} pc^2 for this CO source, which is one order of magnitude smaller than those of typical z>1 dusty star-forming galaxies (DSFGs) of similar far-infrared luminosity L(IR)~10^{12} Lsun. We derived a molecular gas mass of Mgas=(4.4^{+0.9}_{-0.6})x10^9 Msun and a star-formation rate of SFR=270\pm160 Msun yr^{-1}. We also estimated a gas depletion time of τ(dep)=17\pm10 Myr, being shorter than those of typical DSFGs. It is suggested that this source is in a transition phase from DSFG to a gas-poor, early-type galaxy. From ALMA to Herschel multi-band dust continuum observations, we measured a dust emissivity index β=2.3\pm0.2, which is similar to those of local gas-poor, early-type galaxies. Such a high β can be reproduced by specific chemical compositions for interstellar dust at the submillimeter wavelengths from recent laboratory experiments. ALMA CO and multi-band dust continuum observations can constrain the evolutionary stage of high-redshift galaxies through τ(dep) and β, and thus we can investigate dust chemical compositions even in the early Universe.

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arXiv2018

Spatially-resolved Dense Molecular Gas Excitation in the Nearby LIRG VV 114

We present high-resolution observations (0".2-1".5) of multiple dense gas tracers, HCN and HCO$^+$ ($J$ = 1-0, 3-2, and 4-3), HNC ($J$ = 1-0), and CS ($J$ = 7-6) lines, toward the nearby luminous infrared galaxy VV 114 with the Atacama Large Millimeter/submillimeter Array. All lines are robustly detected at the central gaseous filamentary structure including the eastern nucleus and the Overlap region, the collision interface of the progenitors. We found that there is no correlation between star formation efficiency and dense gas fraction, indicating that the amount of dense gas does not simply control star formation in VV 114. We predict the presence of more turbulent and diffuse molecular gas clouds around the Overlap region compared to those at the nuclear region assuming a turbulence-regulated star formation model. The intracloud turbulence at the Overlap region might be excited by galaxy-merger-induced shocks, which also explains the enhancement of gas-phase CH$_3$OH abundance previously found there. We also present spatially resolved spectral line energy distributions of HCN and HCO$^+$ for the first time, and derive excitation parameters by assuming optically-thin and local thermodynamic equilibrium (LTE) conditions. The LTE model revealed that warmer, HCO$^+$-poorer molecular gas medium is dominated around the eastern nucleus, harboring an AGN. The HCN abundance is remarkably flat ($\sim$3.5 $\times$ 10$^{-9}$) independently of the various environments within the filament of VV 114 (i.e., AGN, star formation, and shock).

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arXiv2016

Clumpy and Extended Starbursts in the Brightest Unlensed Submillimeter Galaxies

The central structure in three of the brightest unlensed z=3-4 submillimeter galaxies are investigated through 0.015" - 0.05" (120 -- 360~pc) 860 micron continuum images obtained using the Atacama Large Millimeter/submillimeter Array (ALMA). The distribution in the central kpc in AzTEC1 and AzTEC8 are extremely complex, and they are composed of multiple ~200 pc clumps. AzTEC4 consists of two sources that are separated by ~1.5 kpc, indicating a mid-stage merger. The peak star formation rate densities in the central clumps are ~300 - 3000 Msun/yr/kpc^2, suggesting regions with extreme star formation near the Eddington Limit. By comparing the flux obtained by ALMA and Submillimeter Array (SMA), we find that 68-90% of the emission is extended (> 1 kpc) in AzTEC 4 and 8. For AzTEC1, we identify at least 11 additional compact (~200 pc) clumps in the extended 3 - 4 kpc region. Overall, the data presented here suggest that the luminosity surface densities observed at < 150 pc scales are roughly similar to that observed in local ULIRGs, as in the eastern nucleus of Arp 220. Between 10 to 30% of the 860 micron continuum is concentrated in clumpy structures in the central kpc while the remaining flux is distributed over > 1 kpc regions, some of which could also be clumpy. These sources can be explained by a rapid inflow of gas such as a merger of gas-rich galaxies, surrounded by extended and clumpy starbursts. However, the cold mode accretion model is not ruled out.

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