arXiv subjects

K. Torii

Publications and source records attributed to K. Torii.

At least 19 recordsLinked to original sources

arXiv2020

CHIMPS2: Survey description and $^{12}$CO emission in the Galactic Centre

The latest generation of Galactic-plane surveys is enhancing our ability to study the effects of galactic environment upon the process of star formation. We present the first data from CO Heterodyne Inner Milky Way Plane Survey 2 (CHIMPS2). CHIMPS2 is a survey that will observe the Inner Galaxy, the Central Molecular Zone (CMZ), and a section of the Outer Galaxy in $^{12}$CO, $^{13}$CO, and C$^{18}$O $(J = 3\rightarrow2)$ emission with the Heterodyne Array Receiver Program on the James Clerk Maxwell Telescope (JCMT). The first CHIMPS2 data presented here are a first look towards the CMZ in $^{12}$CO J = 3$\rightarrow$2 and cover $-3^{\circ}\leq\,\ell\,\leq\,5^{\circ}$ and $\mid$b$\mid \leq 0.5^{\circ}$ with angular resolution of 15 arcsec, velocity resolution of 1 km s$^{-1}$, and rms $\Delta T_A ^\ast =$ 0.58 K at these resolutions. Such high-resolution observations of the CMZ will be a valuable data set for future studies, whilst complementing the existing Galactic Plane surveys, such as SEDIGISM, the Herschel infrared Galactic Plane Survey, and ATLASGAL. In this paper, we discuss the survey plan, the current observations and data, as well as presenting position-position maps of the region. The position-velocity maps detect foreground spiral arms in both absorption and emission.

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arXiv2018

FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45-m Telescope (FUGIN) IV: Galactic Shock Wave and Molecular Bow Shock in the 4-kpc Arm of the Galaxy

The FUGIN CO survey with the Nobeyama 45-m Telescope revealed the 3D structure of a galactic shock wave in the tangential direction of the 4-kpc molecular arm. The shock front is located at G30.5+00.0+95 km/s on the up-stream (lower longitude) side of the star-forming complex W43 (G30.8-0.03), and composes a molecular bow shock (MBS) concave to W43, exhibiting an arc-shaped molecular ridge perpendicular to the galactic plane with width $\sim 0^\circ.1$ (10 pc) and vertical length $\sim 1^\circ \ (100\ {\rm pc})$. The MBS is coincident with the radio continuum bow of thermal origin, indicating association of ionized gas and similarity to a cometary bright-rimmed cloud. The up-stream edge of the bow is sharp with a growth width of $\sim 0.5$ pc indicative of shock front property. The velocity width is $\sim 10$ km/s, and the center velocity decreases by $\sim 15$ \kms from bottom to top of the bow. The total mass of molecular gas in MBS is estimated to be $\sim 1.2\times 10^6 <_\odot$ and ionized gas $\sim 2\times 10^4 M_\odot$. The vertical disk thickness increases step like at the MBS by $\sim 2$ times from lower to upper longitude, which indicates hydraulic-jump in the gaseous disk. We argue that the MBS was formed by the galactic shock compression of an accelerated flow in the spiral-arm potential encountering the W43 molecular complex. A bow-shock theory can well reproduce the bow morphology. We argue that molecular bows are common in galactic shock waves not only in the Galaxy but also in galaxies, where MBS are associated with giant cometary HII regions. We also analyzed the HI data in the same region to obtain a map of HI optical depth and molecular fraction. We found a firm evidence of HI-to-H$_{2}$ transition in the galactic shock as revealed by a sharp molecular front at the MBS front.

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arXiv2018

Possible Evidence for Cosmic-Ray Acceleration in the Type Ia SNR RCW 86: Spatial Correlation between TeV Gamma rays and Interstellar Atomic Protons

We present a detailed morphological study of TeV gamma rays, synchrotron radiation, and interstellar gas in the young Type Ia supernova remnant (SNR) RCW 86. We find that the interstellar atomic gas shows good spatial correlation with the gamma rays, indicating that the TeV gamma rays from RCW 86 are likely to be dominantly of hadronic origin. In contrast, the spatial correlation between the interstellar molecular cloud and the TeV gamma rays is poor in the southeastern shell of the SNR. We argue that this poor correlation can be attributed to the low-energy cosmic rays (~ 1 TeV) not penetrating into the dense molecular cloud due to an enhancement of the turbulent magnetic field around the dense cloud of ~ 10-100 $\mu$G. We also find that the southwestern shell, which is bright in both synchrotron X-ray and radio continuum radiation, shows a significant gamma-ray excess compared with the interstellar proton column density, suggesting that leptonic gamma rays via inverse Compton scattering possibly contributes along with hadronic gamma rays. The total cosmic-ray energies of the young TeV gamma-ray SNRs$-$RX J1713.7$-$3946, Vela Jr, HESS J1731$-$347, and RCW 86$-$are roughly similar, which indicates that cosmic rays can be accelerated in both the core-collapse and Type Ia supernovae. The total energy of cosmic rays derived using the gas density, ~ 10$^{48}$-10$^{49}$ erg, gives a safe lower limit due mainly to the low filling factor of interstellar gas within the shell.

astro-ph.HE↗

arXiv2017

Discovery of molecular and atomic clouds associated with the gamma-ray supernova remnant Kesteven 79

We carried out $^{12}$CO($J$ = 1-0) observations of the Galactic gamma-ray supernova remnant (SNR) Kesteven 79 using the Nobeyama Radio Observatory 45 m radio telescope, which has an angular resolution of $\sim20$ arcsec. We identified molecular and atomic gas interacting with Kesteven 79 whose radial velocity is $\sim80$ km s$^{-1}$. The interacting molecular and atomic gases show good spatial correspondence with the X-ray and radio shells, which have an expanding motion with an expanding velocity of $\sim4$ km s$^{-1}$. The molecular gas associated with the radio and X-ray peaks also exhibits a high-intensity ratio of CO 3-2/1-0 $>$ 0.8, suggesting a kinematic temperature of $\sim24$ K, owing to heating by the supernova shock. We determined the kinematic distance to the SNR to be $\sim5.5$ kpc and the radius of the SNR to be $\sim8$ pc. The average interstellar proton density inside of the SNR is $\sim360$ cm$^{-3}$, of which atomic protons comprise only $\sim10$ $\%$. Assuming a hadronic origin for the gamma-ray emission, the total cosmic-ray proton energy above 1 GeV is estimated to be $\sim5 \times 10^{48}$ erg.

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arXiv2017

A detailed study of the interstellar protons toward the TeV $\gamma$-ray SNR RX J0852.0$-$4622 (G266.2$-$1.2, Vela Jr.); a third case of the $\gamma$-rays and ISM spatial correspondence

We present a new analysis of the interstellar protons toward the TeV $\gamma$-ray SNR RX J0852.0$-$4622 (G266.2$-$1.2, Vela Jr.). We used the NANTEN2 $^{12}$CO($J$ = 1-0) and ATCA & Parkes HI datasets in order to derive the molecular and atomic gas associated with the TeV $\gamma$-ray shell of the SNR. We find that atomic gas over a velocity range from $V_\mathrm{LSR}$ = $-4$ km s$^{-1}$ to 50 km s$^{-1}$ or 60 km s$^{-1}$ is associated with the entire SNR, while molecular gas is associated with a limited portion of the SNR. The large velocity dispersion of the HI is ascribed to the expanding motion of a few HI shells overlapping toward the SNR but is not due to the Galactic rotation. The total masses of the associated HI and molecular gases are estimated to be $\sim2.5 \times 10^4 $ $M_{\odot}$ and $\sim10^3$ $M_{\odot}$, respectively. A comparison with the H.E.S.S. TeV $\gamma$-rays indicates that the interstellar protons have an average density around 100 cm$^{-3}$ and shows a good spatial correspondence with the TeV $\gamma$-rays. The total cosmic ray proton energy is estimated to be $\sim10^{48}$ erg for the hadronic $\gamma$-ray production, which may still be an underestimate by a factor of a few due to a small filling factor of the SNR volume by the interstellar protons. This result presents a third case, after RX J1713.7$-$3946 and HESS J1731$-$347, of the good spatial correspondence between the TeV $\gamma$-rays and the interstellar protons, lending further support for a hadronic component in the $\gamma$-rays from young TeV $\gamma$-ray SNRs.

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arXiv2017

Properties of Hi-GAL clumps in the inner Galaxy]{The Hi-GAL compact source catalogue. I. The physical properties of the clumps in the inner Galaxy ($-71.0^{\circ}< \ell < 67.0^{\circ}$)

Hi-GAL is a large-scale survey of the Galactic plane, performed with Herschel in five infrared continuum bands between 70 and 500 $\mu$m. We present a band-merged catalogue of spatially matched sources and their properties derived from fits to the spectral energy distributions (SEDs) and heliocentric distances, based on the photometric catalogs presented in Molinari et al. (2016a), covering the portion of Galactic plane $-71.0^{\circ}< \ell < 67.0^{\circ}$. The band-merged catalogue contains 100922 sources with a regular SED, 24584 of which show a 70 $\mu$m counterpart and are thus considered proto-stellar, while the remainder are considered starless. Thanks to this huge number of sources, we are able to carry out a preliminary analysis of early stages of star formation, identifying the conditions that characterise different evolutionary phases on a statistically significant basis. We calculate surface densities to investigate the gravitational stability of clumps and their potential to form massive stars. We also explore evolutionary status metrics such as the dust temperature, luminosity and bolometric temperature, finding that these are higher in proto-stellar sources compared to pre-stellar ones. The surface density of sources follows an increasing trend as they evolve from pre-stellar to proto-stellar, but then it is found to decrease again in the majority of the most evolved clumps. Finally, we study the physical parameters of sources with respect to Galactic longitude and the association with spiral arms, finding only minor or no differences between the average evolutionary status of sources in the fourth and first Galactic quadrants, or between "on-arm" and "inter-arm" positions.

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arXiv2016

Interstellar Gas and X-rays toward the Young Supernova Remnant RCW 86; Pursuit of the Origin of the Thermal and Non-Thermal X-ray

We have analyzed the atomic and molecular gas using the 21 cm HI and 2.6/1.3 mm CO emissions toward the young supernova remnant (SNR) RCW 86 in order to identify the interstellar medium with which the shock waves of the SNR interact. We have found an HI intensity depression in the velocity range between $-46$ and $-28$ km s$^{-1}$ toward the SNR, suggesting a cavity in the interstellar medium. The HI cavity coincides with the thermal and non-thermal emitting X-ray shell. The thermal X-rays are coincident with the edge of the HI distribution, which indicates a strong density gradient, while the non-thermal X-rays are found toward the less dense, inner part of the HI cavity. The most significant non-thermal X-rays are seen toward the southwestern part of the shell where the HI gas traces the dense and cold component. We also identified CO clouds which are likely interacting with the SNR shock waves in the same velocity range as the HI, although the CO clouds are distributed only in a limited part of the SNR shell. The most massive cloud is located in the southeastern part of the shell, showing detailed correspondence with the thermal X-rays. These CO clouds show an enhanced CO $J$ = 2-1/1-0 intensity ratio, suggesting heating/compression by the shock front. We interpret that the shock-cloud interaction enhances non-thermal X-rays in the southwest and the thermal X-rays are emitted by the shock-heated gas of density 10-100 cm$^{-3}$. Moreover, we can clearly see an HI envelope around the CO cloud, suggesting that the progenitor had a weaker wind than the massive progenitor of the core-collapse SNR RX J1713.7$-$3949. It seems likely that the progenitor of RCW 86 was a system consisting of a white dwarf and a low-mass star with low-velocity accretion winds.

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arXiv2016

The two molecular clouds in RCW 38; evidence for formation of the youngest super star cluster in the Milky Way triggered by cloud-cloud collision

We present distributions of two molecular clouds having velocities of 2 km s$^{-1}$ and 14 km s$^{-1}$ toward RCW 38, the youngest super star cluster in the Milky Way, in the $^{12}$CO ($J=$1--0 and 3--2) and $^{13}$CO ($J=$1--0) transitions. The two clouds are likely physically associated with the cluster as verified by the high intensity ratio of the $J$=3--2 emission to the $J$=1--0 emission, the bridging feature connecting the two clouds in velocity and their morphological correspondence with the infrared dust emission. The total mass of the clouds and the cluster is too small to gravitationally bind the velocity difference. We frame a hypothesis that the two clouds are colliding with each other by chance to trigger formation of the $\sim$20 candidate O stars which are localized within $\sim$0.3 pc of the cluster center in the 2 km s$^{-1}$ cloud. We suggest that the collision is currently continuing toward part of the 2 km s$^{-1}$ cloud where the bridging feature is localized. This is the third super star cluster alongside of Westerlund2 and NGC3603 where cloud-cloud collision triggered the cluster formation. RCW38 is the most remarkable and youngest cluster, holding a possible sign of on-going O star formation, and is the most promising site where we may be able to witness the moment of O-star formation.

astro-ph.SR↗

arXiv2015

Isolating signatures of major cloud-cloud collisions II: The lifetimes of broad bridge features

We investigate the longevity of broad bridge features in position-velocity diagrams that appear as a result of cloud-cloud collisions. Broad bridges will have a finite lifetime due to the action of feedback, conversion of gas into stars and the timescale of the collision. We make a series of analytic arguments with which to estimate these lifetimes. Our simple analytic arguments suggest that for collisions between clouds larger than R~10 pc the lifetime of the broad bridge is more likely to be determined by the lifetime of the collision rather than the radiative or wind feedback disruption timescale. However for smaller clouds feedback becomes much more effective. This is because the radiative feedback timescale scales with the ionising flux Nly as R^{7/4}Nly^{-1/4} so a reduction in cloud size requires a relatively large decrease in ionising photons to maintain a given timescale. We find that our analytic arguments are consistent with new synthetic observations of numerical simulations of cloud-cloud collisions (including star formation and radiative feedback). We also argue that if the number of observable broad bridges remains ~ constant, then the disruption timescale must be roughly equivalent to the collision rate. If this is the case our analytic arguments also provide collision rate estimates, which we find are readily consistent with previous theoretical models at the scales they consider (clouds larger than about 10 pc) but are much higher for smaller clouds.

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arXiv2015

Enhancement of the 6.4 keV line in the inner Galactic ridge: Proton-induced fluorescence?

A common idea for the origin of the Galactic diffuse X-ray emission, particularly that of the iron lines from neutral and highly ionized atoms, is a superposition of many cataclysmic variables and coronally active binaries. In this scenario, the flux should symmetrically distribute between the east and west on the plane with respect to Sagittarius A$^*$ because the stellar mass distribution determined by infrared observations is nearly symmetric. This symmetry is confirmed for the highly ionized iron line as well as the continuum emission. However, a clear excess of the neutral iron line in the near east of the Galactic center compared to the near-west side is found. The flux distribution of the excess emission well correlates with molecular column density. The X-ray spectrum of the excess emission is described by a power-law continuum plus a 6.4 keV line with the large equivalent width of $\sim1.3$ keV, which is hardly explained by the low-energy electron bombardment scenario. The longitudinal and latitudinal distribution of the excess emission disfavors the X-ray irradiation, neither by Sagittarius A$^*$ nor by nearby X-ray binaries. Then the low-energy proton bombardment is the most probable origin although the high energy density $\sim 80$ eV cm$^{-3}$ in 0.1-1000 MeV is required and there is no conventional proton source in the vicinity.

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arXiv2015

Cloud-cloud collision as a trigger of the high-mass star formation; a molecular line study in RCW120

RCW120 is a Galactic HII region having a beautiful ring shape bright in infrared. Our new CO J=1-0 and J=3-2 observations performed with the NANTEN2, Mopra, and ASTE telescopes have revealed that two molecular clouds with a velocity separation of 20km/s are both physically associated with RCW120. The cloud at -8km/s apparently traces the infrared ring, while the other cloud at -28km/s is distributed just outside the opening of the infrared ring, interacting with the HII region as supported by high kinetic temperature of the molecular gas and by the complementary distribution with the ionized gas. A spherically expanding shell driven by the HII region is usually discussed as the origin of the observed ring structure in RCW120. Our observations, however, indicate no evidence of the expanding motion in the velocity space, being inconsistent with the expanding shell model. We here postulate an alternative that, by applying the model introduced by Habe & Ohta (1992), the exciting O star in RCW120 was formed by a collision between the present two clouds at a colliding velocity ~30km/s. In the model, the observed infrared ring can be interpreted as the cavity created in the larger cloud by the collision, whose inner surface is illuminated by the strong UV radiation after the birth of the O star. We discuss that the present cloud-cloud collision scenario explains the observed signatures of RCW120, i.e., its ring morphology, coexistence of the two clouds and their large velocity separation, and absence of the expanding motion.

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arXiv2015

Isolating signatures of major cloud-cloud collisions using position-velocity diagrams

Collisions between giant molecular clouds are a potential mechanism for triggering the formation of massive stars, or even super star clusters. The trouble is identifying this process observationally and distinguishing it from other mechanisms. We produce synthetic position-velocity diagrams from models of: cloud-cloud collisions, non-interacting clouds along the line of sight, clouds with internal radiative feedback and a more complex cloud evolving in a galactic disc, to try and identify unique signatures of collision. We find that a broad bridge feature connecting two intensity peaks, spatially correlated but separated in velocity, is a signature of a high velocity cloud-cloud collision. We show that the broad bridge feature is resilient to the effects of radiative feedback, at least to around 2.5Myr after the formation of the first massive (ionising) star. However for a head on 10km/s collision we find that this will only be observable from 20-30 per cent of viewing angles. Such broad-bridge features have been identified towards M20, a very young region of massive star formation that was concluded to be a site of cloud-cloud collision by Torii et al (2011), and also towards star formation in the outer Milky Way by Izumi et al (2014).

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arXiv2014

Optically thick HI dominant in the local interstellar medium; an alternative interpretation to "dark gas"

Dark gas in the interstellar medium (ISM) is believed to not be detectable either in CO or Hi radio emission, but it is detected by other means including gamma-rays, dust emission and extinction traced outside the Galactic plane at |b|>5 degree. In these analyses, the 21-cm HI emission is usually assumed to be completely optically thin. We have reanalyzed the HI emission from the whole sky at |b|>15 degree by considering temperature stratification in the ISM inferred from the Planck/IRAS analysis of the dust properties. The results indicate that the HI emission is saturated with an optical depth ranging from 0.5 to 3 for 85% of the local HI gas. This optically thick HI is characterized by spin temperature in the range 10K-60K, significantly lower than previously postulated in the literature, whereas such low temperature is consistent with emission/absorption measurements of the cool HI toward radio continuum sources. The distribution and the column density of the HI are consistent with those of the dark gas sug- gested by gamma-rays, and it is possible that the dark gas in the Galaxy is dominated by optically thick cold HI gas. This result implies that the average density of HI is 2-2.5 times higher than that derived on the optically-thin assumption in the local ISM.

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arXiv2014

A detailed study of non-thermal X-ray properties and interstellar gas toward the γ-ray supernova remnant RX J1713.7-3946

We have carried out a spectral analysis of the Suzaku X-ray data in the 0.4-12 keV range toward the shell-type very-high-energy γ-ray supernova remnant RX J1713.7-3946. The aims of this analysis are to estimate detailed X-rays spectral properties at a high angular resolution up to 2 arcmin, and to compare them with the interstellar gas. The X-ray spectrum is non-thermal and used to calculate absorbing column density, photon index, and absorption-corrected X-ray flux. The photon index varies significantly from 2.1 to 2.9. It is shown that the X-ray intensity is well correlated with the photon index, especially in the west region, with a correlation coefficient of 0.81. The X-ray intensity tends to increase with the averaged interstellar gas density while the dispersion is relatively large. The hardest spectra having the photon index less than 2.4 are found outside of the central 10 arcmin of the SNR, from the north to the southeast (~430 arcmin^2) and from the southwest to the northwest (~150 arcmin^2). The former region shows low interstellar gas density, while the latter high interstellar gas density. We present discussion for possible scenarios which explain the distribution of the photon index and its relationship with the interstellar gas.

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arXiv2014

Origin of the 6.4-keV line of the Galactic Ridge X-ray Emission

We report the first results from high-statistics observation of the 6.4-keV line in the region of $l= +1.5^\circ$ to $+3.5^\circ$ (hereafter referred to as GC East), with the goal to uncover the origin of the Galactic ridge X-ray emission (GRXE). By comparing this data with that from the previous observations in the region $l=-1.5^\circ$ to $-3.5^\circ$ (hereafter referred to as GC West), we discovered that the 6.4-keV line is asymmetrically distributed with respect to the Galactic center, whereas the 6.7-keV line is symmetrically distributed. The distribution of the 6.4-keV line follows that of $^{13}$CO and its flux is proportional to the column density of the molecular gas. This correlation agrees with that seen between the 6.4-keV line and the cold interstellar medium (ISM) (H$_{\rm I}$ $+$ H$_2$) in the region $|l|>4^\circ$. This result suggests that the 6.4-keV emission is diffuse fluorescence from the cold ISM not only in GC East and West but also in the entire Galactic plane. This observational result suggests that the surface brightness of the 6.4-keV line is proportional to the column density of the cold ISM in the entire Galactic plane. For the ionizing particles, we consider X-rays and low energy cosmic-ray protons and electrons .

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arXiv2014

Interstellar protons in the TeV gamma-ray SNR HESS J1731-347:Possible evidence for the coexistence of hadronic and leptonic gamma-rays

HESS J1731-347 (G353.6-0.7) is one of the TeV gamma-ray SNRs which shows the shell-like morphology. We have made a new analysis of the interstellar protons toward the SNR by using both the 12CO(J=1-0) and HI datasets. The results indicate that the TeV gamma-ray shell shows significant spatial correlation with the interstellar protons at a velocity range from -90 km/s to -75 km/s, and the distance corresponding to the velocity range is ~5.2 kpc, a factor of 2 larger than the previous figure 3 kpc. The total mass of the interstellar protons is estimated to be 6.4x10^4 M_sun, 25 % of which is atomic gas. We have identified the cold HI gas observed as self-absorption which shows significant correspondence with the northeastern gamma-ray peak. While the good correspondence between the interstellar protons and TeV gamma-rays in the north of the SNR lends support to the hadronic scenario for the TeV gamma-rays, the southern part of the shell shows a break in the correspondence; in particular, the southwestern rim of the SNR shell shows a significant decrease of the interstellar protons by a factor of 2. We argue that this discrepancy can be explained as due to leptonic gamma-rays, because this region well coincides with the bright shell which emit non-thermal radio continuum emission and non-thermal X-rays, suggesting that the gamma-rays of HESS J1713-347 consist of both the hadronic and leptonic components. The leptonic contribution then corresponds to ~20 % of the total gamma-rays. The total energy of cosmic-ray protons is estimated to be 10^49 erg for the gamma-ray energy range of 1 GeV - 100 TeV by assuming that 80 % of the total gamma-ray is of the hadronic origin.

astro-ph.HE↗

arXiv2014

Mapping the Milky Way bulge at high resolution: the 3D dust extinction, CO, and X factor maps

Three dimensional interstellar extinction maps provide a powerful tool for stellar population analysis. We use data from the VISTA Variables in the Via Lactea survey together with the Besançon stellar population synthesis model of the Galaxy to determine interstellar extinction as a function of distance in the Galactic bulge covering $ -10 < l < 10$ and $-10 < b <5$. We adopted a recently developed method to calculate the colour excess. First we constructed the H-Ks vs. Ks and J-Ks vs. Ks colour-magnitude diagrams based on the VVV catalogues that matched 2MASS. Then, based on the temperature-colour relation for M giants and the distance-colour relations, we derived the extinction as a function of distance. The observed colours were shifted to match the intrinsic colours in the Besançon model as a function of distance iteratively. This created an extinction map with three dimensions: two spatial and one distance dimension along each line of sight towards the bulge. We present a 3D extinction map that covers the whole VVV area with a resolution of 6' x 6', using distance bins of 0.5 kpc. The high resolution and depth of the photometry allows us to derive extinction maps for a range of distances up to 10 kpc and up to 30 magnitudes of extinction in $A_{V}$. Integrated maps show the same dust features and consistent values as other 2D maps. We discuss the spatial distribution of dust features in the line of sight, which suggests that there is much material in front of the Galactic bar, specifically between 5-7 kpc. We compare our dust extinction map with high-resolution $\rm ^{12}CO$ maps towards the Galactic bulge, where we find a good correlation between $\rm ^{12}CO$ and $\rm A_{V}$. We determine the X factor by combining the CO map and our dust extinction map. Our derived average value is consistent with the canonical value of the Milky Way.

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arXiv2014

Detailed molecular observations toward the Double Helix Nebula

The Double Helix Nebula (DHN), located 100 pc above Sgr A* in the Galactic center (GC), is a unique structure whose morphology suggests it is a magnetic feature (Morris, Uchida & Do 2006). Recent molecular observations toward the DHN by Enokiya et al. (2014) revealed two candidate molecular counterparts of the DHN at radial velocities of -35 km/s and 0 km/s and discussed the model in which the DHN has its origin at the circumnuclear disk in the GC. In this paper, new CO observations toward the DHN using the CSO and Mopra telescopes are presented. The higher-resolution observations of ~1 pc scale reveal the detailed distributions and kinematics of the two CO counterparts (the 0 km/s and -35 km/s features) and provide new information on their physical conditions. As a result, we find that the 0 km/s feature with a mass of 3.3x10^4 Msun coincides with the infrared emission of the DHN, indicating clear association with the DHN. The association of the -35 km/s feature, with a mass of 0.8x10^4 Msun, is less clear compared with the 0 km/s feature, but the complementary distribution between the molecular gas and the DHN and velocity variation along the DHN support its association with the DHN. The two molecular features are highly excited, as shown by the relatively high CO J=2-1/J=1-0 intensity ratios of ~1.0, and have kinetic temperatures of ~30 K, consistent with the typical molecular clouds in the GC.

astro-ph.SR↗