arXiv subjects

K. Muraoka

Publications and source records attributed to K. Muraoka.

10 recordsLinked to original sources

arXiv2024

Giant molecular clouds and their Type classification in M74: Toward understanding star formation and cloud evolution

We investigated the giant molecular clouds (GMCs) in M74 (NGC 628), using data obtained from the PHANGS project. We applied the GMC Types according to the activity of star formation: Type I without star formation, Type II with H$α$ luminosity ($L_\mathrm{Hα}$) less than $10^{37.5}~\mathrm{erg~s^{-1}}$, and Type III with $L_\mathrm{Hα}$ greater than $10^{37.5}~\mathrm{erg~s^{-1}}$. A total of 432 GMCs were identified, with 59, 201, and 172 GMCs, for Type I, II, and III, respectively. The size and mass of the GMCs range from 23 to 238 pc and $10^{4.9}$ to $10^{7.1}$ M$_{\odot}$, indicating that the mass and radius increase from Type I to III. Clusters younger than 4 Myr and HII regions are concentrated within 150 pc of a GMC, indicating a tight association between these young objects and GMCs. The virial ratio decreases from Type I to Type III, indicating that Type III GMCs are the most gravitationally relaxed among the three. We interpret that the GMCs evolve from Type I to Type III, as previously observed in the LMC. Based on a steady-state assumption, the estimated evolutionary timescales of Type I, II, and III are 1, 5, and 4 Myr, respectively. We assume that the timescale of Type III is equal to the age of the associated clusters, indicating a GMC lifetime of 10 Myr or longer. Although Chevance et al. (2020, MNRAS, 493, 2872) investigated GMCs using the same PHANGS dataset of M74, they did not define a GMC, reaching an evolutionary picture with a 20 Myr duration of the non-star-forming phase, which was five times longer than 4 Myr. We compare the present results with those of Chevance et al. (2020) and argue that defining individual GMCs is essential for understanding GMC evolution.

astro-ph.GA↗

arXiv2022

Discovery of a giant molecular loop in the central region of NGC 253

NGC 253 is a starburst galaxy of SAB(s)c type with increasing interest because of its high activity at unrivaled closeness. Its energetic event is manifested as the vertical gas features in its central molecular zone, for which stellar feedback was proposed as the driving engine. In order to pursue details of the activity, we have undertaken a kinematic analysis of the ALMA archive data of $^{12}$CO($J$=3-2) emission at the highest resolution $\sim$3 pc. We revealed that one of the non-rotating gas components in the central molecular zone shows a loop-like structure of $\sim$200 pc radius. The loop-like structure is associated with a star cluster, whereas the cluster is not inside the loop-like structure and is not likely as the driver of the loop-like structure formation. Further, we find that the bar potential of NGC 253 seems to be too weak to drive the gas motion by the eccentric orbit. As an alternative, we frame a scenario that magnetic acceleration by the Parker instability is responsible for the creation of the loop-like structure. We show that the observed loop-like structure properties are similar to those in the Milky Way, and argue that recent magneto-hydrodynamics simulations lend support for the picture having the magnetic field strength of $\gtrsim$100 $μ$G. We suggest that cluster formation was triggered by the falling gas to the footpoint of the loop, which is consistent with a typical dynamical timescale of the loop $\sim$1 Myr.

astro-ph.GA↗

arXiv2020

Systematic Variations of CO J=2-1/1-0 Ratio in The Barred Spiral Galaxy M83

We present spatial variations of the CO J=2-1/1-0 line ratio in M83 using Total Power array data from ALMA. While the intensities of these two lines correlate tightly, the ratio varies over the disk, with a disk average ratio of 0.69, and shows the galactic center and a two-arm spiral pattern. It is high (>0.7) in regions of high molecular gas surface density, but ranges from low to high ratios in regions of low surface density. The ratio correlates well with the spatial distributions and intensities of FUV and IR emissions, with FUV being the best correlated. It also correlates better with the ratio of IR intensities (70/350mic), a proxy for dust temperature, than with the IR intensities. Taken together, these results suggest either a direct or indirect link between the dust heating by the interstellar radiation field and the condition of GMCs, even though no efficient mechanism is known for a thermal coupling of dust and bulk gas in GMCs.

astro-ph.GA↗

arXiv2019

Discovery of Shocked Molecular Clouds Associated with the Shell-Type Supernova Remnant RX J0046.5$-$7308 in the Small Magellanic Cloud

RX J0046.5$-$7308 is a shell-type supernova remnant (SNR) in the Small Magellanic Cloud (SMC). We carried out new $^{12}$CO($J$ = 1-0, 3-2) observations toward the SNR using Mopra and the Atacama Submillimeter Telescope Experiment. We found eight molecular clouds (A-H) along the X-ray shell of the SNR. The typical cloud size and mass are $\sim$10-15 pc and $\sim$1000-3000 $M_{\odot}$, respectively. The X-ray shell is slightly deformed and has the brightest peak in the southwestern shell where two molecular clouds A and B are located. The four molecular clouds A, B, F, and G have high intensity ratios of $^{12}$CO($J$ = 3-2) / $^{12}$CO($J$ = 1-0) $> 1.2$, which are not attributable to any identified internal infrared sources or high-mass stars. The HI cavity and its expanding motion are found toward the SNR, which are likely created by strong stellar winds from a massive progenitor. We suggest that the molecular clouds A-D, F, and G and HI clouds within the wind-blown cavity at $V_\mathrm{LSR} = 117.1$-122.5 km s$^{-1}$ are to be associated with the SNR. The X-ray spectroscopy reveals the dynamical age of $26000^{+1000}_{-2000}$ yr and the progenitor mass of $\gtrsim 30$ $M_{\odot}$, which is also consistent with the proposed scenario. We determine physical conditions of the giant molecular cloud LIRS 36A using the large velocity gradient analysis with archival datasets of the Atacama Large Millimeter/submillimeter Array; the kinematic temperature is $72^{+50}_{-37}$ K and the number density of molecular hydrogen is $1500^{+600}_{-300}$ cm$^{-3}$. The next generation of $γ$-ray observations will allow us to study the pion-decay $γ$-rays from the molecular clouds in the SMC SNR.

astro-ph.GA↗

arXiv2012

Initial Results from the Nobeyama Molecular Gas Observations of Distant Bright Galaxies

We present initial results from the CO survey toward high redshift galaxies using the Nobeyama 45m telescope. Using the new wide bandwidth spectrometer equipped with a two-beam SIS receiver, we have robust new detections of three high redshift (z=1.6-3.4) submillimeter galaxies (SXDF 1100.001, SDP9, and SDP17), one tentative detection (SDSS J160705+533558), and one non-detection (COSMOS-AzTEC1). The galaxies observed during the commissioning phase are sources with known spectroscopic redshifts from previous optical or from wide-band submm spectroscopy. The derived molecular gas mass and line widths from Gaussian fits are ~10^11 Msun and 430-530 km/s, which are consistent with previous CO observations of distant submm galaxies and quasars. The spectrometer that allows a maximum of 32 GHz instantaneous bandwidth will provide new science capabilities at the Nobeyama 45m telescope, allowing us to determine redshifts of bright submm selected galaxies without any prior redshift information.

astro-ph.CO↗

arXiv2011

Temperature Variations of the Cold Dust in the Triangulum Galaxy M33

We present wide-field 1.1 mm continuum imaging of the nearby spiral galaxy M 33, conducted with the AzTEC bolometer camera on ASTE. We show that the 1.1 mm flux traces the distribution of dust with T ~20 K. Combined with far-infrared imaging at 160um, we derive the dust temperature distribution out to a galactic radius of ~7 kpc with a spatial resolution of ~100 parsecs. Although the 1.1 mm flux is observed predominantly near star forming regions, we find a smooth radial temperature gradient declining from ~20 K to ~13 K, consistent with recent results from the Herschel satellite. Further comparison of individual regions show a strong correlation between the cold dust temperature and the Ks band brightness, but not with the ionizing flux. The observed results imply that the dominant heating source of cold dust at few hundred parsec scales are due to the non-OB stars, even when associated with star forming regions.

astro-ph.GA↗

arXiv2008

Molecular Gas Distribution in Barred and Non-Barred Galaxies along the Hubble Sequence

We present results from a survey of 12CO(J=1-0) spectra obtained for the central regions of 68 nearby galaxies at an angular resolution of 16 arcseconds using the Nobeyama Radio Observatory 45m telescope, aimed at characterizing the properties of star forming molecular gas. Combined with similar resolution observations in the literature, the compiled sample set of 166 galaxies span a wide range of galactic properties. NGC 4380, which was previously undetected in CO, was detected. This initial paper of a series will focus on the data and the gaseous properties of the samples, and particularly on the degree of central concentration of molecular gas in a range of morphological types, from early (S0/Sa) to late (Sd/Sm) galaxies with and without bars. The degree of molecular central concentration in the central kiloparsec, compared to the central several kiloparsecs of galaxies, is found to vary smoothly with Hubble type, so that early type galaxies show larger central concentration. The comparison of barred and non-barred galaxies within early and late type galaxies suggest that difference in Hubble type, representing the effect of bulges, is the more important factor in concentrating gas into the central regions than bars.

astro-ph↗

arXiv2008

Tracing star formation in galaxies with molecular line and continuum observations

We report our recent progress on extragalactic spectroscopic and continuum observations, including HCN(J=1-0), HCO$^+$(J=1-0), and CN(N=1-0) imaging surveys of local Seyfert and starburst galaxies using the Nobeyama Millimeter Array, high-J CO observations (J=3-2 observations using the Atacama Submillimeter Telescope Experiment (ASTE) and J=2-1 observations with the Submillimeter Array) of galaxies, and $λ$ 1.1 mm continuum observations of high-z violent starburst galaxies using the bolometer camera AzTEC mounted on ASTE.

astro-ph↗

arXiv2008

ASTE CO(3--2) Observations of the Southern Barred Spiral Galaxy NGC 986: a Large Gaseous Bar Filled with Dense Molecular Medium

We present CO(3-2) emission observations toward the 3'x3' (or 20x20kpc at a distance of 23Mpc) region of the southern barred spiral galaxy NGC 986 using the Atacama Submillimeter Telescope Experiment (ASTE). This effort is a part of our on-going extragalactic CO(3-2) imaging project ADIoS (ASTE Dense gas Imaging of Spiral galaxies). Our CO(3-2) image revealed the presence of a large (the major axis is 14 kpc in total length) gaseous bar filled with dense molecular medium along the dark lanes observed in optical images. This is the largest ``dense-gas rich bar'' known to date. The dense gas bar discovered in NGC 986 could be a huge reservoir of possible ``fuel'' for future starbursts in the central region, and we suggest that the star formation in the central region of NGC 986 could still be in a growing phase. We found a good spatial coincidence between the overall distributions of dense molecular gas traced by CO(3-2) and the massive star formation depicted by H$α$. The global CO(3-2) luminosity $L'_{\rm CO(3-2)}$ of NGC 986 was determined to be $(5.4 \pm 1.1) \times 10^8$ K km s$^{-1}$ pc$^2$. The CO(3-2)/CO(1-0) integrated intensity ratio was found to be 0.60 +/- 0.13 at a spatial resolution of 44'' or 5 kpc, and a CO(3-2)/CO(2-1) ratio was 0.67 +/- 0.14 at a beam size of ~25'' or ~2.8 kpc. These line ratios suggest moderate excitation conditions of CO lines ($n_{\rm H_2} \sim 10^{3-4}$ cm$^{-3}$) in the central a few kpc region of NGC 986.

astro-ph↗

arXiv2004

Nobeyama Millimeter Array Observations of GRB 030329: a Decay of Afterglow with Bumps and Molecular Gas in the Host Galaxy

Nobeyama Millimeter Array was used to observe millimeter-wave afterglow of GRB 030329 at 93 GHz and 141 GHz from 2003 April 6 to 2003 May 30. A sensitive search for CO(J=1-0) emission/absorption from the host galaxy of GRB 030329 was also carried out. Unresolved millimeter continuum emission at the position of GRB 030329 was detected until 2003 April 21. We found a steep decline of continuum flux (\propto t^{-2.0}) during this period, in accord with a previous report. Moreover, our data implies that the decay was accompanied by possible plateaus phases, or bumps, on a time scale of several days. From an integrated spectrum, produced by summing up the data from 2003 April 10 to 2003 May 30, we found a possible emission feature, which could be a redshifted CO(J=1-0) line. Its position and redshift coincide well with those of GRB 030329, though further observations are required to confirm the detection. If the emission feature is real, the observed CO flux is 1.4 +/- 0.52 Jy km/s, corresponding to a large molecular gas mass of M(H_2) > 10^9 Mo. This implies that the host galaxy, which is optically faint, is highly obscured due to a rich interstellar medium.

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