Unbiased Galactic Plane Imaging Survey Research Articles

Sub-kpc scale gas density histogram of the galactic molecular gas: a new statistical method to characterise galactic-scale gas structures

Abstract To understand physical properties of the interstellar medium (ISM) on various scales, we investigate it at parsec resolution on the kiloparsec scale. Here, we report on the sub-kpc scale Gas Density Histogram (GDH) of the Milky Way. The GDH is a density probability distribution function (PDF) of the gas volume density. Using this method, we are free from an identification of individual molecular clouds and their spatial structures. We use survey data of 12CO and 13CO (J=1–0) emission in the Galactic plane (l = 10○–50○) obtained as a part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45m telescope (FUGIN). We make a GDH for every channel map of 2○ × 2○ area including the blank sky component, and without setting cloud boundaries. This is a different approach from previous works for molecular clouds. The GDH fits well to a single or double log-normal distribution, which we name the low-density log-normal (L-LN) and high-density log-normal (H-LN) components, respectively. The multi-log-normal components suggest that the L-LN and H-LN components originate from two different stages of structure formation in the ISM. Moreover, we find that both the volume ratios of H-LN components to total (fH) and the width of the L-LN along the gas density axis (σL) show coherent structure in the Galactic-plane longitude-velocity diagram. It is possible that these GDH parameters are related to strong galactic shocks and other weak shocks in the Milky Way.

Sub kpc-scale gas density histogram: a new statistical method to characterize galactic-scale gas structures

AbstractTo understand the physical properties of the interstellar medium (ISM) in various scales, we should investigate it with pc-scale resolution over kpc scale coverage. Here, we report the sub-kpc scale Gas Density Histogram (GDH) of the Milky Way. GDH is a histogram of averaged density and corresponds to the probability density distribution (PDF) of gas volume density. We use galactic plain survey data (l =10∘− 50∘) at 12CO and 13CO (J = 1 − 0) obtained as a part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45m telescope (FUGIN). With this method and data, we are free from spatial structure and molecular cloud identification. GDH can be well fitted with single or double log-normal distribution; which we call as the low-density log-normal (L-LN) and high-density log-normal (H-LN) components. We found both the H-LN fraction (fH) and L-LN width (σL) along the gas density axis show a coherent structure on the longitude-velocity diagram. It suggests that there is a relationship between the ISM property and kpc scale structure in the Milky Way.

FUGIN hot core survey. I. Survey method and initial results for l = 10°–20°

Abstract We have developed a method to make a spectral-line-based survey of hot cores, which represent an important stage of high-mass star formation, and applied the method to the data of the FUGIN (FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope) survey. First, we select hot core candidates by searching the FUGIN data for the weak hot core tracer lines (HNCO and CH3CN) by stacking, and then we conduct follow-up pointed observations on these candidates in C34S, SO, OCS, HC3N, HNCO, CH3CN, and CH3OH J = 2–1 and J = 8–7 lines to confirm and characterize them. We applied this method to the l = 10°–20° portion of the FUGIN data and identified 22 “HotCores” (compact sources with more than two significant detections of the hot core tracer lines, i.e., SO, OCS, HC3N, HNCO, CH3CN, or CH3OH J = 8–7 lines) and 14 “DenseClumps” (sources with more than two significant detection of C34S, CH3OH J = 2–1, or the hot core tracer lines). The identified HotCores are found to be associated with signposts of high-mass star formation such as ATLASGAL clumps, WISE H ii regions, and Class II methanol masers. Many of the FUGIN HotCores are identified with the Herschel Hi-GAL clumps with a median mass of 6.8 × 102 M⊙ and a median bolometric luminosity of 7.4 × 103 L⊙. Five of the seven HotCores with stronger CH3CN lines exhibit elevated gas temperatures of 50–100 K. These observations suggest that FUGIN HotCores are closely related to the formation of stars with medium to high mass. For those associated with ATLASGAL clumps, their bolometric luminosity to clump mass ratios are consistent with the star formation stages centered at the hot core phase. The catalog of FUGIN HotCores provides a useful starting point for further statistical studies and detailed observations of high-mass star forming regions.

FOREST unbiased Galactic plane imaging survey with the Nobeyama 45 m telescope (FUGIN). VII. Molecular fraction of H i clouds

ABSTRACT We analyze molecular-gas formation in neutral atomic hydrogen (H i) clouds using the latest CO data, obtained from the FOREST (four-beam receiver system on the 45 m telescope) unbiased Galactic plane imaging survey with the Nobeyama 45 m telescope, and using H i data taken from the Very Large Array Galactic plane survey. We applied a dendrogram algorithm to the H i data cube to identify H i clouds, and we calculated the H i mass and molecular-gas mass by summing the CO line intensity within each H i cloud. On the basis of the results, we created a catalog of 5737 identified H i clouds with local standard of rest (LSR) velocity of VLSR ≤ −20 km s−1 in galactic longitude and latitude ranges of 20° ≤ l ≤ 50° and −1° ≤ b ≤ 1°, respectively. We found that most of the H i clouds are distributed within a Galactocentric distance of 16 kpc, and most of them are in the cold neutral medium phase. In addition, we determined that the high-mass end of the H i mass function is fitted well with a power-law function with an index of 2.3. Although two sequences of self-gravitating and diffuse clouds are expected to appear in the M tot–$M_{\,{\rm H}_2}$ diagram according to previous works based on a plane-parallel model, the observational data show only a single sequence with large scattering within these two sequences. This implies that most of the clouds are mixtures of these two types of clouds. Moreover, we suggest the following scenario of molecular-gas formation: an H i-dominant cloud evolved with increasing H2 mass along a path of $M_{\,{\rm H}_2} \propto M_{\,\rm tot}^2$ by collecting diffuse gas before reaching and moving along the curves of the two sequences.

FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45 m telescope (FUGIN). V. Dense gas mass fraction of molecular gas in the Galactic plane

Abstract Recent observations of the nearby Galactic molecular clouds indicate that the dense gas in molecular clouds has quasi-universal properties on star formation, and observational studies of extra-galaxies have shown a galactic-scale correlation between the star formation rate (SFR) and the surface density of molecular gas. To reach a comprehensive understanding of both properties, it is important to quantify the fractional mass of dense gas in molecular clouds, fDG. In particular, for the Milky Way (MW) there are no previous studies resolving fDG disk over a scale of several kpc. In this study, fDG was measured over 5 kpc in the first quadrant of the MW, based on the CO J = 1–0 data in l = 10°–50° obtained as part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) project. The total molecular mass was measured using 12CO, and the dense gas mass was estimated using C18O. The fractional masses, including fDG, in the region within ±30% of the distances to the tangential points of the Galactic rotation (e.g., the Galactic Bar, Far-3 kpc Arm, Norma Arm, Scutum Arm, Sagittarius Arm, and inter-arm regions) were measured. As a result, an averaged fDG of $2.9^{+2.6}_{-2.6}$% was obtained for the entirety of the target region. This low value suggests that dense gas formation is the primary factor in inefficient star formation in galaxies. It was also found that fDG shows large variations depending on the structures in the MW disk. In the Galactic arms, fDG was estimated to be ∼4%–5%, while in the bar and inter-arm regions it was as small as ∼0.1%–0.4%. These results indicate that the formation/destruction processes of the dense gas and their timescales are different for different regions in the MW, leading to differences in Star formation efficiencies.

FUGIN: Molecular Gas in Spitzer Bubble N4—Possible Evidence for a Cloud–Cloud Collision as a Trigger of Massive Star Formations

Abstract Herein, we present 12CO (J = 1–0) and 13CO (J = 1–0) emission-line observations via the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) toward Spitzer bubble N4. We observed clouds at three discrete velocities: 16, 19, and 25 km s−1. Their masses were 0.1 × 104 M ⊙, 0.3 × 104 M ⊙, and 1.4 × 104 M ⊙, respectively. The distribution of the 25 km s−1 cloud likely traces the ring-like structure observed at mid-infrared wavelength. The 16 and 19 km s−1 clouds have not been recognized in previous observations of molecular lines. We could not find clear expanding motion of the molecular gas in N4. On the contrary, we found a bridge feature and a complementary distribution, which are discussed as observational signatures of a cloud–cloud collision (CCC), between the 16 and 25 km s−1 clouds. We proposed a possible scenario wherein the formation of a massive star in N4 was triggered by a collision between the two clouds. The timescale of collision is estimated to be 0.2–0.3 Myr, which is comparable to the estimated dynamical age of the H ii region of ∼0.4 Myr. In N4W, a star-forming clump located west of N4, we observed molecular outflows from young stellar objects and the observational signature of a CCC. Thus, we also proposed a possible scenario in which massive- or intermediate-mass star formation was triggered via a CCC in N4W.

Study of the 13CO/C18O abundance ratio towards the filamentary infrared dark cloud IRDC 34.43 + 0.24

Abstract Nowadays, there are several observational studies about the 13CO/C18O abundance ratio ( $X^{13/18}$ ) towards nearby molecular clouds. These works give observational support to the C18O selective photodissociation due to the interaction between the far ultraviolet (FUV) radiation and the molecular gas. It is necessary to increase the sample of molecular clouds located at different distances and affected in different ways by nearby or embedded H ii regions and OB associations to study the selective photodissociation. Using 12CO, 13CO, and C18O J = 1–0 data obtained from the FOREST unbiased Galactic plane imaging survey performed with the Nobeyama 45-m telescope, we analyse the filamentary infrared dark cloud IRDC $34.43+0.24$ located at the distance of about 3.9 kpc. This infrared dark cloud (IRDC) is related to several H ii regions and young stellar objects. Assuming local thermodynamic equilibrium, we obtain: $0.8 \times 10^{16} <$ N(13CO) $<4 \times 10^{17}$ cm–2 (average value $= 4.2 \times 10^{16}$ cm–2), $0.6 \times 10^{15} <$ N(C18O) $<4.4 \times 10^{16}$ cm–2 (average value $= 5.0 \times 10^{15}$ cm–2), and 3 $<$ $X^{13/18}$ $<$ 30 (average $= 8$ ) across the whole IRDC. Larger values of $X^{13/18}$ were found towards portions of the cloud related to the H ii regions associated with the N61 and N62 bubbles and with the photodissociation regions, precisely the regions in which FUV photons are strongly interacting with the molecular gas. Our result represents an observational support to the C18O selectively photodissociation phenomenon occurring in a quite distant filamentary IRDC. Additionally, based on IR data from the Hi-GAL survey, the FUV radiation field was estimated in Habing units, and the dust temperature (T $_{dust}$ ) and H2 column density (N(H2)) distribution were studied. Using the average of N(H2), values in close agreement with the ‘canonical’ abundance ratios [H2]/[13CO] and [H2]/[C18O] were derived. However, the obtained ranges in the abundance ratios show that if an accurate analysis of the molecular gas is required, the use of the ‘canonical’ values may introduce some bias. Thus, it is important to consider how the gas is irradiated by the FUV photons across the molecular cloud. The analysis of $X^{13/18}$ is a good tool to perform that. Effects of beam dilution and clumpiness were studied.

CO observations of the molecular gas in the Galactic H ii region Sh2-48: Evidence for cloud–cloud collision as a trigger of high-mass star formation

Abstract Sh2-48 is a Galactic H ii region, 3.8 kpc distant, with an O9.5-type star identified at its center. As a part of the FOREST Unbiased Galactic plane Imaging survey using the Nobeyama 45 m telescope (FUGIN) project, we obtained a CO J = 1–0 data set for a large area of Sh2-48 at a spatial resolution of 21″ (∼0.4 pc), and used it to find a molecular cloud with a total molecular mass of ∼3.8 × 104 M⊙ associated with Sh2-48. The molecular cloud has a systematic velocity shift in a velocity range of ∼42–47 km s−1. On the lower-velocity side the CO emission spatially corresponds to the bright 8 μm filament at the western rim of Sh2-48; however, the CO emission with higher velocities separates into the eastern and western sides of the 8 μm filament. This velocity variation forms a V-shaped feature in the east–west direction on the position–velocity diagram. We found that these lower- and higher-velocity components are, unlike the infrared and radio-continuum data, physically associated with Sh2-48. To interpret the observed V-shaped velocity distribution, we assess a cloud–cloud collision scenario, and found, from a comparison between observations and simulations, that the velocity distribution is an expected outcome of a collision between a cylindrical cloud corresponding to the lower-velocity component and a spherical cloud, and that the two separate higher-velocity components are interpretable as collision-broken remnants of the spherical cloud. Based on the consistency between an estimated formation timescale of the H ii region, ∼1.3 Myr, and a timescale of the collision, we conclude that the high-mass star formation in Sh2-48 was triggered by the collision.

“KAGONMA” NH3 mapping observations of molecular clouds with Nobeyama 45m telescope

AbstractIn the collapsing phase of a molecular cloud, the molecular gas temperature is a key to understand the evolutionary process from a dense molecular cloud to stars. In order to know this, mapping observations in NH3 lines are required. Therefore, we made them based on the FUGIN (FOREST Unbiased Galactic plane Imaging survey with Nobeyama 45m telescope). The 6 maps were observed in NH3 (J,K) = (1,1), (2,2), (3,3) and H2O maser lines and obtained temperature maps; some show temperature gradient in a cloud. Additionally 72 cores were observed. These candidates were called as KAGONMA or KAG objects as abbreviation of KAgoshima Galactic Object survey with Nobeyama 45-M telescope in Ammonia lines. We show the results of two regions in W33 and discuss their astrophysical properties.

Large-scale CO J = 1–0 observations of the giant molecular cloud associated with the infrared ring N35 with the Nobeyama 45 m telescope

Abstract We report an observational study of the giant molecular cloud (GMC) associated with the Galactic infrared ring-like structure N35 and two nearby H ii regions G024.392+00.072 (H ii region A) and G024.510−00.060 (H ii region B), using the new CO J = 1–0 data obtained as a part of the FOREST Unbiased Galactic Plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) project at a spatial resolution of 21″. Our CO data reveals that the GMC, with a total molecular mass of 2.1 × 106 M⊙, has two velocity components of over ∼10–15 km s−1. The majority of molecular gas in the GMC is included in the lower-velocity component (LVC) at ∼110–114 km s−1, while the higher-velocity components (HVCs) at ∼118–126 km s−1 consist of three smaller molecular clouds which are located near the three H ii regions. The LVC and HVCs show spatially complementary distributions along the line-of-sight, despite large velocity separations of ∼5–15 km s−1, and are connected in velocity by the CO emission with intermediate intensities. By comparing the observations with simulations, we discuss a scenario where collisions of the three HVCs with the LVC at velocities of ∼10–15 km s−1 can provide an interpretation of these two observational signatures. The intermediate-velocity features between the LVC and HVCs can be understood as broad bridge features, which indicate the turbulent motion of the gas at the collision interfaces, while the spatially complementary distributions represent the cavities created in the LVC by the HVCs through the collisions. Our model indicates that the three H ii regions were formed after the onset of the collisions, and it is therefore suggested that the high-mass star formation in the GMC was triggered by the collisions.

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN). III. Possible evidence for formation of NGC 6618 cluster in M 17 by cloud–cloud collision

Abstract We present 12CO (J = 1–0), 13CO (J = 1–0), and C18O (J = 1–0) images of the M 17 giant molecular clouds obtained as part of the FUGIN (FOREST Ultra-wide Galactic Plane Survey In Nobeyama) project. The observations cover the entire area of the M 17 SW and M 17 N clouds at the highest angular resolution (∼19″) to date, which corresponds to ∼0.18 pc at the distance of 2.0 kpc. We find that the region consists of four different velocity components: a very low velocity (VLV) clump, a low velocity component (LVC), a main velocity component (MVC), and a high velocity component (HVC). The LVC and the HVC have cavities. Ultraviolet photons radiated from NGC 6618 cluster penetrate into the N cloud up to ∼5 pc through the cavities and interact with molecular gas. This interaction is correlated with the distribution of young stellar objects in the N cloud. The LVC and the HVC are distributed complementarily after the HVC is displaced by 0.8 pc toward the east-southeast direction, suggesting that collision of the LVC and the HVC created the cavities in both clouds. The collision velocity and timescale are estimated to be 9.9 km s−1 and 1.1 × 105 yr, respectively. The high collision velocity can provide a mass accretion rate of up to 10$^{-3}{\,\,}M_{\odot }\:$yr−1, and the high column density (4 × 1023 cm−2) might result in massive cluster formation. The scenario of cloud–cloud collision likely explains well the stellar population and the formation history of the NGC 6618 cluster proposed by Hoffmeister et al. (2008, ApJ, 686, 310).

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN): Molecular clouds toward W 33; possible evidence for a cloud–cloud collision triggering O star formation

Abstract We observed molecular clouds in the W 33 high-mass star-forming region associated with compact and extended H ii regions using the NANTEN2 telescope as well as the Nobeyama 45 m telescope in the J = 1–0 transitions of 12CO, 13CO, and C18O as part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) legacy survey. We detected three velocity components at 35 km s−1, 45 km s−1, and 58 km s−1. The 35 km s−1 and 58 km s−1 clouds are likely to be physically associated with W 33 because of the enhanced 12CO J = 3–2 to J = 1–0 intensity ratio as $R_{\rm 3\mbox{-}2/1\mbox{-}0} > 1.0$ due to the ultraviolet irradiation by OB stars, and morphological correspondence between the distributions of molecular gas and the infrared and radio continuum emissions excited by high-mass stars. The two clouds show complementary distributions around W 33. The velocity separation is too large to be gravitationally bound, and yet not explained by expanding motion by stellar feedback. Therefore, we discuss whether a cloud–cloud collision scenario likely explains the high-mass star formation in W 33.

FOREST unbiased Galactic plane imaging survey with the Nobeyama 45 m telescope (FUGIN). I. Project overview and initial results

Abstract The FUGIN project is one of legacy projects using a new multi-beam FOREST (four-beam receiver system on the 45 m telescope). This project aims to simultaneously investigate the distribution, kinematics, and physical properties of both diffuse and dense molecular gases in the Galaxy by observing 12CO, 13CO, and C18O J = 1–0 lines simultaneously. Mapping regions are parts of the first quadrant (10° ≤ l ≤ 50°, |b| ≤ 1°) and the third quadrant (198° ≤ l ≤ 236°, |b| ≤ 1°) of the Galaxy, where spiral arms, bar structure, and the molecular gas ring are included. This survey achieves the highest angular resolution to date (∼20″) for the Galactic plane survey in the CO J = 1–0 lines, which makes it possible to find dense clumps located farther away than the previous surveys. FUGIN will provide us an invaluable dataset for investigating the physics of the Galactic interstellar medium (ISM), particularly the evolution of interstellar gas covering galactic-scale structures to the internal structures of giant molecular clouds, such as small filaments/clumps/cores. We present an overview of the FUGIN project, the observation plan and initial results. These results reveal wide-field and detailed structures of molecular clouds, such as entangled filaments that have not been obvious in previous surveys, and large-scale kinematics of molecular gas, such as spiral arms.