Cloud-cloud Collisions Research Articles

Multiple collisions in N59 bubble: sequential cloud–cloud collisions

ABSTRACT We report that the gas components in the N59 bubble suffered from sequential multiple cloud–cloud collision (CCC) processes. The molecular gas in the N59 bubble can be decomposed into four velocity components, namely Cloud A [95, 108] km s$^{-1}$, Cloud B [86, 95] km s$^{-1}$, Cloud C [79, 86] km s$^{-1}$, and Cloud D [65, 79] km s$^{-1}$. Four CCC processes occurred among these four velocity components, i.e. Cloud A versus Cloud B, Cloud A versus Cloud C, Cloud C versus Cloud D, and Cloud A versus Cloud D. Using the near- and mid-infrared photometric point source catalogues, we identified 514 young stellar object (YSO) candidates clustered in 13 YSO groups, and most of them ($\sim 60~{{\ \rm per\ cent}}$) were located at the colliding interfaces, indicating that they were mainly triggered by these four CCC processes. We also found that these four collisions occurred in a time sequential order: the earliest and most violent collision occurred between Cloud A and Cloud D about 2 Myr ago, then Cloud B collided with Cloud A about 1 Myr ago, and finally, Cloud C collided with Clouds A and D simultaneously about 0.4 Myr ago.

Cloud–Cloud Collision: Formation of Hub-filament Systems and Associated Gas Kinematics. Mass-collecting Cone—A New Signature of Cloud–Cloud Collision

Massive star-forming regions (MSFRs) are commonly associated with hub-filament systems (HFSs) and sites of cloud–cloud collision (CCC). Recent observational studies of some MSFRs suggest a possible connection between CCC and the formation of HFSs. To understand this connection, we analyzed the magnetohydrodynamic simulation data from Inoue et al. This simulation involves the collision of a spherical turbulent molecular cloud with a plane-parallel sea of dense molecular gas at a relative velocity of about 10 km s−1. Following the collision, the turbulent and nonuniform cloud undergoes shock compression, rapidly developing filamentary structures within the compressed layer. We found that CCC can lead to the formation of HFSs, which is the combined effect of turbulence, shock compression, magnetic field, and gravity. The collision between the cloud components shapes the filaments into a cone and drives inward flows among them. These inward flows merge at the vertex of the cone, rapidly accumulating high-density gas, which can lead to the formation of massive star(s). The cone acts as a mass-collecting machine, involving a nongravitational early process of filament formation, followed by gravitational gas attraction to finalize the HFS. The gas distribution in the position–velocity (PV) and position–position spaces highlights the challenges in detecting two cloud components and confirming their complementary distribution if the colliding clouds have a large size difference. However, such CCC events can be confirmed by the PV diagrams presenting gas flow toward the vertex of the cone, which hosts gravitationally collapsing high-density objects, and by the magnetic field morphology curved toward the direction of the collision.

High-resolution observations of 12CO and 13CO J = 3 2 towards the NGC 6334 extended filament. I. Emission morphology and velocity structure

NGC 6334 is a giant molecular cloud (GMC) complex that exhibits elongated filamentary structure and harbours numerous OB-stars, H II regions, and star-forming clumps. To study the emission morphology and velocity structure of the gas in the extended NGC 6334 region using high-resolution molecular line data, we made observations of the 12CO and 13CO $J=3 2$ lines with the LAsMA instrument at the APEX telescope . The LAsMA data provided a spatial resolution of 20" (sim 0.16 pc) and sensitivity of 0.4 K at a spectral resolution of 0.25 km $. Our observations revealed that gas in the extended NGC 6334 region exhibits connected velocity coherent structure over sim 80 pc parallel to the Galactic plane. The NGC 6334 complex has its main velocity component at $ -3.9$ km $ with two connected velocity structures at velocities $ -9.2$ km $ (the `bridge' features) and $ km $ (the northern filament, NGC 6334-NF). We observed local velocity fluctuations at smaller spatial scales along the filament that are likely tracing local density enhancement and infall, while the broader V-shaped velocity fluctuations observed towards the NGC 6334 central ridge and G352.1 region located in the eastern filament EF1 indicate globally collapsing gas onto the filament. We investigated the 13CO emission and velocity structure around 42 WISE H II regions located in the extended NGC 6334 region and found that most H II regions show signs of molecular gas dispersal from the centre (36 of 42) and intensity enhancement at their outer radii (34 of 42). Furthermore most H II regions (26 of 42) are associated with least one ATLASGAL clump within or just outside of their radii, the formation of which may have been triggered by H II bubble expansion. Typically towards larger H II regions we found visually clear signatures of bubble shells emanating from the filamentary structure. Overall the NGC 6334 filamentary complex exhibits sequential star formation from west to east. Located in the west, the GM-24 region exhibits bubbles within bubbles and is at a relatively evolved stage of star formation. The NGC 6334 central ridge is undergoing global gas infall and exhibits two gas bridge features possibly connected to the cloud-cloud collision scenario of the NGC 6334-NF and the NGC 6334 main gas component. The relatively quiescent eastern filament (EF1 - G352.1) is a hub-filament in formation, which shows the kinematic signature of global gas infall onto the filament. Our observations highlight the important role of H II regions in shaping the molecular gas emission and velocity structure as well as the overall evolution of the molecular filaments in the NGC 6334 complex.

Shock waves in Interstellar Cloud-Cloud and Wind-Cloud Collisions

Abstract The interstellar medium (ISM) is a key ingredient of galaxies and their evolution, consisting of multiphase, turbulent dust and gas. Some of the star-forming regions in our Galaxy originate from cloud-cloud and wind-cloud collisions, which generate shock waves that change the physical and chemical properties of the gas. We utilise our own python-based shock-finding algorithm to study the properties and distribution of shocks in interstellar collisions. Such interactions are studied via 3D numerical simulations with different initial conditions: Cloud-cloud collisions (CCc): We identify four stages of evolution: pre-collision, compression, pass-through, and dissipation. We also vary the size of one of the colliding clouds. Larger clouds facilitate cloud erosion and the formation of more and stronger shocks at early stages. Shock distributions are also time-dependent, as strong shocks are only produced during the early stages. As the collisions evolve, turbulent kinetic energy is rapidly dissipated, so most perturbations become subsonic waves at late times. Wind-cloud collisions (WCc): we identify four stages: compression, stripping, expansion, and break-up. We study the evolution of several diagnostics in these clouds: energies (thermal and kinetic), temperature, displacement of the centre of mass, and mass-weighted averages of the cloud density and acceleration. We show, that the geometry of the cloud impact the diagnostic parameters, for example, smoothing the edges of the cloud leads to enhanced mass losses and dispersion, but has little impact on the shock distribution.

Deciphering the hidden structures of HH 216 and Pillar IV in M16: results from JWST and HST

ABSTRACT To probe the star formation process, we present an observational investigation of the Pillar IV and an ionized knot HH 216 in the Eagle Nebula (M16). Pillar IV is known to host a Class I protostar that drives a bipolar outflow. The outflow has produced the bow shock, HH 216, which is associated with the red-shifted outflow lobe. The JWST near- and mid-infrared images (resolution ∼0${_{.}^{\prime\prime}}$07–0${_{.}^{\prime\prime}}$7) reveal the protostar as a single, isolated object (below 1000 au). The outer boundary of Pillar IV is depicted with the 3.3 $\mu$m Polycyclic aromatic hydrocarbon (PAH) emission. HH 216 is traced with the 4.05 $\mu$m Brα and the radio continuum emission; however, it is undetected with 4.693 $\mu$m H2 emission. HH 216 seems to be associated with both thermal and non-thermal radio emissions. High-resolution images reveal entangled ionized structures (below 3000 au) of HH 216, which appear to be located towards termination shocks. New knots in 4.693 $\mu$m H2 emission are detected and are mainly found on Pillar IV’s northern side. This particular result supports the previously proposed episodic accretion in the powering source of HH 216. One part of the ionized jet (extent ∼0.16 pc) is discovered on the southern side of the driving source. Using the 12CO(J = 1–0), 12CO(J = 3–2), and 13CO(J = 1–0) emission, observational signposts of cloud-cloud collision (or interacting clouds) towards Pillar IV are investigated. Overall, our results suggest that the interaction of molecular cloud components around 23 and 26 km s−1 might have influenced star formation activity in Pillar IV.

Cloud–cloud collision in S235: triggered the formation of high-mass stars and young star clusters

ABSTRACT We present the analysis of cloud–cloud collision (CCC) process in the Galactic molecular complex S235. Our new CO observations performed with the PMO-13.7 m telescope reveal two molecular clouds, namely the S235-Main and the S235-ABC, with ∼4 km s−1 velocity separation. The bridge feature, the possible colliding interface and the complementary distribution of the two clouds are significant observational signatures of cloud–cloud collision in S235. The most direct evidence of cloud–cloud collision process in S235 is that the S235-Main (in a distance of 1547$^{+44}_{-43}$ pc) and S235-ABC (1567$^{+33}_{-39}$ pc) meet at almost the same position (within 1σ error range) at a supersonic relative speed. We identified ten 13CO clumps from PMO-13.7 m observations, 22 dust cores from the archival SCUBA-2 data, and 550 YSOs from NIR–MIR data. 63 per cent of total YSOs are clustering in seven MST groups (M1−M7). The tight association between the YSO groups (M1 $\&$ M7) and the bridge feature suggests that the CCC process triggers star formation there. The collisional impact subregion (the South) shows 3 ∼ 5 times higher CFE and SFE (average value of 12.3 and 10.6 per cent, respectively) than the non-collisional impact subregion (2.4 and 2.6 per cent, respectively), suggesting that the CCC process may have enhanced the CFE and SFE of the clouds compared to those without collision influence.

AFGL 5180 and AFGL 6366S: sites of hub–filament systems at the opposite edges of a filamentary cloud

ABSTRACT We present a multiscale and multiwavelength study to unveil massive star formation (MSF) processes around sites AFGL 5180 and AFGL 6366S, both hosting a Class ii 6.7 GHz methanol maser emission. The radio continuum map at 8.46 GHz reveals a small cluster of radio sources towards AFGL 5180. Signatures of the early stages of MSF in our target sites are spatially seen at the opposite edges of a filamentary cloud (length ∼5 pc), which is observed in the submillimetre dust continuum maps. Using the near-infrared photometric data, the spatial distribution of young stellar objects is found towards the entire filament, primarily clustered at its edges. The getsf utility on the Herschel far-infrared images reveals a hub–filament system (HFS) towards each target site. The analysis of the molecular line data, which benefits from large area coverage (∼1° × 1°), detects two cloud components with a connection in both position and velocity space. This supports the scenario of a cloud–cloud collision (CCC) that occurred ∼1 Myr ago. The filamentary cloud, connecting AFGL 5180 and AFGL 6366S, seems spatially close to an H ii region Sh 2−247 excited by a massive O9.5 star. Based on the knowledge of various pressures exerted by the massive star on its surroundings, the impact of its energetic feedback on the filamentary cloud is found to be insignificant. Overall, our observational outcomes favour the possibility of the CCC scenario driving MSF and the formation of HFSs towards the target sites.

Massive core/star formation triggered by cloud–cloud collision – II. High-speed collisions of magnetized clouds

ABSTRACT We present a study of the effects of magnetic fields on the formation of massive, self-gravitationally bound cores (MBCs) in high-speed cloud–cloud collisions (CCCs). Extending our previous work, we perform magnetohydrodynamic simulations of the high-speed (20–40 km s−1) collisions between two magnetized (initial 4 $\mu\!$ G), turbulent clouds of different sizes in the range of 7–20 pc. We show that a magnetic field hinders the growth of cores, particularly in the case of short-duration collisions where cores are not seen to reach a highly bound state. In such cases, a shocked region created by the collision rapidly expands into the ambient medium owing to the enhanced magnetic pressure, resulting in the destruction of the highly unbound cores and suppression of gas accretion. This negative effect on the formation of MBCs has not been reported in previous hydrodynamic simulations of CCCs. Together with our previous work, we conclude that magnetic fields provide two competing effects on the MBC formation in CCCs; while they promote the mass accumulation onto cores during a collision, they operate to destroy cores or hinder the core growth after the collision. The duration of such collisions determines which effect prevails, resulting in a maximum collision speed for the MBC formation within given colliding clouds. Our results agree with the observed trend among CCC samples in the column density range probed in the simulations presented here; CCCs with higher relative velocities require higher column densities for the formation of massive stars.

Cloud-cloud collision and star formation in G323.18+0.15

We studied the cloud-cloud collision candidate G323.18+0.15 based on signatures of induced filaments, clumps, and star formation. We used archival molecular spectrum line data from the SEDIGISM 13CO (J = 2−1) survey, from the Mopra southern Galactic plane CO survey, and infrared to radio data from the GLIMPSE, MIPS, Hi-GAL, and SGPS surveys. Our new result shows that the G323.18+0.15 complex is 3.55 kpc away from us and consists of three cloud components, G323.18a, G323.18b, and G323.18c. G323.18b shows a perfect U-shape structure, which can be fully complemented by G323.18a, suggesting a collision between G323.18a and the combined G323.18bc filamentary structure. One dense compressed layer (filament) is formed at the bottom of G323.18b, where we detect a greatly increased velocity dispersion. The bridge with an intermediate velocity in a position-velocity diagram appears between G323.18a and G323.18b, which corresponds to the compressed layer. G323.18a plus G323.18b as a whole are probably not gravitationally bound. This indicates that high-mass star formation in the compressed layer may have been caused by an accidental event. The column density in the compressed layer of about 1.36 × 1022 cm−2 and most of the dense clumps and high-mass stars are located there. The average surface density of class I and class II young stellar objects (YSOs) inside the G323.18+0.15 complex is much higher than the density in the surroundings. The timescale of the collision between G323.18a and G323.18b is 1.59 Myr. This is longer than the typical lifetime of class I YSOs and is comparable to the lifetime of class II YSOs.

Methanol masers in NGC 253 with ALCHEMI

Context. Methanol masers of Class I (collisionally pumped) and Class II (radiatively pumped) have been studied in great detail in our Galaxy in a variety of astrophysical environments such as shocks and star-forming regions and are they are helpful to analyze the properties of the dense interstellar medium. However, the study of methanol masers in external galaxies is still in its infancy. Aims. Our main goal is to search for methanol masers in the central molecular zone (CMZ; inner 500 pc) of the nearby starburst galaxy NGC 253. Methods. Covering a frequency range between 84 and 373 GHz (λ = 3.6–0.8 mm) at high angular (1.″6 ∼ 27 pc) and spectral (∼8–9 km s−1) resolution with ALCHEMI (ALMA Comprehensive High-resolution Extragalactic Molecular Inventory), we have probed different regions across the CMZ of NGC 253. In order to look for methanol maser candidates, we employed the rotation diagram method and a set of radiative transfer models. Results. We detect for the first time masers above 84 GHz in NGC 253, covering an ample portion of the J−1 → (J − 1)0 − E line series (at 84, 132, 229, and 278 GHz) and the J0 → (J − 1)1 − A series (at 95, 146, and 198 GHz). This confirms the presence of the Class I maser line at 84 GHz, which was already reported, but now being detected in more than one location. For the J−1 → (J− 1)0 − E line series, we observe a lack of Class I maser candidates in the central star-forming disk. Conclusions. The physical conditions for maser excitation in the J−1 → (J − 1)0 − E line series can be weak shocks and cloud-cloud collisions as suggested by shock tracers (SiO and HNCO) in bi-symmetric shock regions located in the outskirts of the CMZ. On the other hand, the presence of photodissociation regions due to a high star-formation rate would be needed to explain the lack of Class I masers in the very central regions.

Unraveling the Observational Signatures of Cloud–Cloud Collision and Hub-filament Systems in W31

To understand the formation process of massive stars, we present a multiscale and multiwavelength study of the W31 complex hosting two extended H ii regions (i.e., G10.30-0.15 (hereafter, W31-N) and G10.15-0.34 (hereafter, W31-S)) powered by a cluster of O-type stars. Several Class i protostars and a total of 49 ATLASGAL 870 μm dust clumps (at d = 3.55 kpc) are found toward the H ii regions where some of the clumps are associated with the molecular outflow activity. These results confirm the existence of a single physical system hosting the early phases of star formation. The Herschel 250 μm continuum map shows the presence of a hub-filament system (HFS) toward both W31-N and W31-S. The central hubs harbor H ii regions and they are depicted with extended structures (with T d ∼ 25–32 K) in the Herschel temperature map. In the direction of W31-S, an analysis of the NANTEN2 12CO(J = 1−0) and SEDIGISM 13CO(J = 2−1) line data supports the presence of two cloud components around 8 and 16 km s−1, and their connection in velocity space. A spatial complementary distribution between the two cloud components is also investigated toward W31-S, where the signposts of star formation, including massive O-type stars, are concentrated. These findings favor the applicability of cloud–cloud collision (CCC) around ∼2 Myr ago in W31-S. Overall, our observational findings support the theoretical scenario of CCC in W31, which explains the formation of massive stars and the existence of HFSs.

A Multiwavelength Study of the Sgr B Region: Contiguous Cloud–Cloud Collisions Triggering Widespread Star Formation Events?

The Sgr B region, including Sgr B1 and Sgr B2, is one of the most active star-forming regions in the Galaxy. Hasegawa et al. originally proposed that Sgr B2 was formed by a cloud–cloud collision (CCC) between two clouds with velocities of ∼45 km s−1 and ∼75 km s−1. However, some recent observational studies conflict with this scenario. We have reanalyzed this region, by using recent, fully sampled, dense-gas data and by employing a recently developed CCC identification methodology, with which we have successfully identified more than 50 CCCs and compared them at various wavelengths. We found two velocity components that are widely spread across this region and that show clear signatures of a CCC, each with a mass of ∼106 M ⊙. Based on these observational results, we suggest an alternative scenario, in which contiguous collisions between two velocity features with a relative velocity of ∼20 km s−1 created both Sgr B1 and Sgr B2. The physical parameters, such as the column density and the relative velocity of the colliding clouds, satisfy a relation that has been found to apply to the most massive Galactic CCCs, meaning that the triggering of high-mass star formation in the Galaxy and starbursts in external galaxies can be understood as being due to the same physical CCC process.

Cloud-cloud collision as origin of the G31.41+0.31 massive protocluster

The G31.41+0.31 (G31) hot molecular core (HMC) is a high-mass protocluster showing accelerated infall and rotational spin-up that is well studied at high-angular resolution. To complement the accurate view of the small scale in G31, we traced the kinematics of the large-scale material by carrying out N2H+ (1–0) observations with the Institute de Radioastronomie Millimétrique 30m telescope of an area of ∼6 × 6 arcmin2 around the HMC. The N2H+ observations have revealed a large-scale (5 pc) hub-filament system (HFS) composed of at least four filamentary arms and a NNE–SSW velocity gradient (∼0.4 km s−1 pc−1) between the northern and southern filaments. The linewidth increases toward the hub at the center of the HFS reaching values of 2.5–3 km s−1 in the central 1 pc. The origin of the large-scale velocity gradient is likely a cloud-cloud collision. In this scenario, the filaments in G31 would have formed by compression resulting from the collision, and the rotation of the HMC observed at scales of 1000 au would have been induced by shear caused by the cloud-cloud collision at scales of a few parsecs. We conclude that G31 represents a HFS in a compressed layer with an orthogonal orientation to the plane of the sky, and it represents a benchmark for the filaments-to-clusters paradigm of star formation.

Spatially resolved star-formation relations of dense molecular gas in NGC 1068

Context. According to the current understanding of star formation (SF), the regulation of this phenomenon in galaxy disks reflects a complex balance between processes that operate in molecular gas on local cloud scales as well as on global disk scales. Aims. We analyse the influence of the dynamical environment on the SF relations of the dense molecular gas in the starburst (SB) ring of the Seyfert 2 galaxy NGC 1068. Methods. We used ALMA to image the emission of the 1–0 transitions of HCN and HCO+, which trace dense molecular gas in the r ∼ 1.3 kpc SB ring of NGC 1068, with a resolution of 56 pc. We also used ancillary data of CO(1–0) as well as CO(3–2) and its underlying continuum emission at the resolutions of ∼100 pc and ∼40 pc, respectively. These observations allow us to probe a wide range of molecular gas densities (nH2 ∼ 103 − 5 cm−3). The star-formation rate (SFR) in the SB ring of NGC 1068 is derived from Paα line emission imaged by HST/NICMOS. We analyse how different formulations of SF relations change depending on the adopted aperture sizes and on the choice of molecular gas tracer. Results. The scatter in the Kennicutt–Schmidt relation, linking the SFR density (ΣSFR) with the (dense) molecular gas surface density (Σdense), is about a factor of two to three lower for the HCN and HCO+ lines compared to that derived from CO(1–0) for a common aperture. Correlations lose statistical significance below a critical spatial scale of ≈300−400 pc for all gas tracers. The SF efficiency of the dense molecular gas, defined as SFEdense ≡ ΣSFR/Σdense, shows a scattered distribution as a function of the HCN luminosity (L′(HCN)) around a mean value of ≃0.01 Myr−1. An alternative prescription for SF relations, which includes the dependence of SFEdense on the combination of Σdense and the velocity dispersion (σ), resolves the degeneracy associated with the SFEdense − L′(HCN) plot. The SFEdense values show a positive trend with the boundedness of the gas, measured by the parameter b ≡ Σdense/σ2. We identify two branches in the SFEdense − b plot that correspond to two dynamical environments within the SB ring; they are defined by their proximity to the region where the spiral structure is connected to the stellar bar. This region corresponds to the crossing of two overlapping density wave resonances, where an increased rate of cloud-cloud collisions would favour an enhanced compression of molecular gas. Conclusions. These results suggest that galactic dynamics plays a major role in the efficiency of the conversion of gas into stars. Our work adds supporting evidence that density-threshold SF models, which argue that the SFEdense should be roughly constant, fail to account for spatially resolved SF relations of dense gas in the SB ring of NGC 1068.

Shocks in the surroundings of the NGC 1333 IRAS 4 system

The IRAS 4A system is part of the southern filament of the Perseus NGC 1333 molecular cloud. As most of the star forming regions, NGC 1333 is known to be heavily shaped by external triggers, such as shock fronts around OB stars or supernovae remnants, cloud-cloud collisions, and instabilities. Recently, it has been hypothesized that the entire southwest region of NGC 1333, encompassing the filament where IRAS 4A lie, is due to a colliding “turbulent” cell, a clash that triggered the birth of the protostars on the filaments. However, no specific signatures of a clash have been reported so far, leaving unanswered how and where the energy of this clash, if real, is dispersed. To answer this question, we analyzed new high spatial resolution (~600 au) observations of CH3OH and SiO, known shock tracers, obtained in the context of the Large Program IRAM/NOEMA SOLIS searching for specific signature of the clash event. We detected three parallel elongated structures, called fingers, with narrow line profiles (~1.5 km s−1), peaked at the systemic velocity of the cloud, tracing gas with high density (5-20×105 cm−3) and high temperature (80-160 K). They are chemically different with the northern finger traced by both SiO and CH3OH and the other two only by SiO. Among various possibilities, a train of three consecutive shocks, due to an expanding bubble coming behind NGC 1333 and from the southwest, can reproduce the observations. Finally, we propose a solution for the two-decades long debate on the nature and the origin of the widespread narrow SiO emission observed in the south part of NGC 1333, namely unresolved trains of shocks.

The Young Stellar Population, Distance, and Cloud–Cloud Collision Induced Star Formation Scenario of the Trifid Nebula

Abstract The Trifid Nebula is a young, nearby star-forming region where star formation is proposed to have been triggered by cloud–cloud collision (CCC), based on observations of molecular clouds. It offers a unique opportunity to test whether the CCC hypothesis is supported by the spatial distribution and star formation chronology of young stars. We present the first study of the optically visible pre-main sequence (PMS) population of the region using riHα imaging and Gaia astrometry. Combined with an analysis of young stellar objects (YSOs) using infrared imaging, we capture the spatial distribution and star formation chronology of the young stellar population. From the analysis, 15 Flat/Class I YSOs, 46 Class II YSOs, and 41 accreting PMS stars are identified (diskless/non-accreting sources are not included in the analysis). The distance based on Gaia parallaxes is ∼1250 pc, significantly closer than previously reported. The Class II YSOs and PMS stars (∼1.5 Myr old) are spread toward the edge of the molecular clouds. They are slightly younger than the estimated crossing time of ∼2.7 Myr and closer to the estimated dynamical age ∼0.85 Myr. Younger Class I YSOs are more concentrated spatially. There exists a cavity devoid of young stars where the two clouds overlap. This evidence suggests that the current generation of stars formed after the collision of two clouds ∼1 Myr ago, and this result can be corroborated using future spectroscopic studies.

Massive star formation in the Carina nebula complex and Gum 31. II. A cloud–cloud collision in Gum 31

Abstract We present the results of analyses of the 12CO (J = 1–0), 13CO (J = 1–0), and 12CO (J = 2–1) emission data toward Gum 31. Three molecular clouds separated in velocity were detected at −25, −20, and −10 km s−1. The velocity structure of the molecular clouds in Gum 31 cannot be interpreted as an expanding motion. Two of them, the −25 km s−1 cloud and the −20 km s−1 cloud, are likely to be associated with Gum 31, because their 12CO (J = 2–1)$/$12CO (J = 1–0) intensity ratios are high. We found that these two clouds show the observational signatures of cloud–cloud collisions (CCCs): a complementary spatial distribution and a V-shaped structure (bridge features) in the position–velocity diagram. In addition, their morphology and velocity structures are very similar to the numerical simulations conducted by the previous studies. We propose a scenario that the −25 km s−1 cloud and the −20 km s−1 cloud collided and triggered the formation of the massive star system HD 92206 in Gum 31. This scenario can explain the offset of the stars from the center and the morphology of Gum 31 simultaneously. The timescale of the collision was estimated to be ∼1 Myr by using the ratio between the path length of the collision and the assumed velocity separation. This is consistent with that of the CCCs in the Carina Nebula Complex in our previous study.

A kinematic analysis of the CO clouds toward a reflection nebula NGC 2023 observed using the Nobeyama 45 m telescope: Further evidence for a cloud–cloud collision in the Orion region

Abstract We performed fully sampled J = 1–0 line observations of 12CO and 13CO toward the reflection nebula NGC 2023 using the Nobeyama 45 m telescope. It covered the entire NGC 2023 region with angular and velocity resolutions of 19″ corresponding to 0.04 pc and 0.33 km s−1, respectively. We analyzed the data focusing on gas dynamics and revealed two velocity components, each with a mass of $\sim\! 3000 \mbox{$\, {M}_\odot $}$, at radial velocities of 10 and 12 km s−1. These components show spatially complementary distributions and are connected in velocity, both of which are the typical signatures of cloud–cloud collision (CCC). Although previous studies favored a scheme of triggering by the H ii region of IC 434, our results show that the effect of the H ii region is limited only to the surface of the molecular cloud, and does not contribute to the entire gas compression and the overall star formation. Comparing the present results with those for neighboring NGC 2024, we find that their active star formation can be explained by a single CCC event. We suggest that the 10 and 12 km s−1 clouds collided ∼0.4 Myr ago, which is common to the regions, but triggered the B1.5V-type star HD 37903 as well as ∼20 low-mass stars in NGC 2023, and ∼20 OB stars in NGC 2024. The column density in NGC 2023 is two to three times lower than that of NGC 2024, which may result in the different star formation activity suggested by the trend of ∼50 other CCC-triggered star-forming regions in the literature.

Submillimeter imaging of the Galactic Center starburst Sgr B2

Context. Star-forming galaxies emit bright molecular and atomic lines in the submillimeter and far-infrared (FIR) domains. However, it is not always clear which gas heating mechanisms dominate and which feedback processes drive their excitation. Aims. The Sgr B2 complex is an excellent template to spatially resolve the main OB-type star-forming cores from the extended cloud environment and to study the properties of the warm molecular gas in conditions likely prevailing in distant extragalactic nuclei. Methods. We present 168 arcmin2 spectral images of Sgr B2 taken with Herschel/SPIRE-FTS in the complete ~450−1545 GHz band. We detect ubiquitous emission from mid-J CO (up to J = 12−11), H2O 21,1−20,2, [C I] 492, 809 GHz, and [N II] 205 μm lines. We also present velocity-resolved maps of the SiO (2−1), N2H+, HCN, and HCO+ (1−0) emission obtained with the IRAM 30 m telescope. Results. The cloud environment (~1000 pc2 around the main cores) dominates the emitted FIR (~80%), H2O 752 GHz (~60%) mid-J CO (~91%), [C I] (~93%), and [N II] 205 μm (~95%) luminosity. The region shows very extended [N II] 205 μm emission (spatially correlated with the 24 and 70 μm dust emission) that traces an extended component of diffuse ionized gas of low ionization parameter (U ≃ 10−3) and low LFIR / MH2 ≃ 4−11 L⊙M⊙−1 ratios (scaling as ∝Tdust6). The observed FIR luminosities imply a flux of nonionizing photons equivalent to G0 ≈ 103. All these diagnostics suggest that the complex is clumpy and this allows UV photons from young massive stars to escape from their natal molecular cores. The extended [C I] emission arises from a pervasive component of neutral gas with nH ≃ 103 cm−3. The high ionization rates in the region, produced by enhanced cosmic-ray (CR) fluxes, drive the gas heating in this component to Tk ≃ 40−60 K. The mid-J CO emission arises from a similarly extended but more pressurized gas component (Pth / k ≃ 107 K cm−3): spatially unresolved clumps, thin sheets, or filaments of UV-illuminated compressed gas (nH ≃ 106 cm−3). Specific regions of enhanced SiO emission and high CO-to-FIR intensity ratios (ICO / IFIR ≳ 10−3) show mid-J CO emission compatible with C-type shock models. A major difference compared to more quiescent star-forming clouds in the disk of our Galaxy is the extended nature of the SiO and N2H+ emission in Sgr B2. This can be explained by the presence of cloud-scale shocks, induced by cloud-cloud collisions and stellar feedback, and the much higher CR ionization rate (>10−15 s−1) leading to overabundant H3+ and N2H+. Conclusions. Sgr B2 hosts a more extreme environment than star-forming regions in the disk of the Galaxy. As a usual template for extragalactic comparisons, Sgr B2 shows more similarities to nearby ultra luminous infrared galaxies such as Arp 220, including a “deficit” in the [C I] / FIR and [N II] / FIR intensity ratios, than to pure starburst galaxies such as M 82. However, it is the extended cloud environment, rather than the cores, that serves as a useful template when telescopes do not resolve such extended regions in galaxies.

Herschel and Odin observations of H2O, CO, CH, CH+, and [N II] in the barred spiral galaxy NGC 1365

Context. The Odin satellite is now into its twentieth year of operation, much surpassing its design life of two years. One of its major astronomical pursuits was the search for and study of water vapor in diverse regions of the Solar System and the Milky Way galaxy. The Herschel space observatory was needed to detect water vapor in external galaxies. Aims. Our goal is to study the distribution and excitation of water vapor and other molecules in the barred spiral galaxy NGC 1365. Methods. Herschel has observed the central region of NGC 1365 in two positions, and both its SPIRE and PACS observations are available in the Herschel Science Archive. Herschel PACS images have been produced of the 70 and 160 μm infrared emission from the whole galaxy, and also of the cold dust distribution as obtained from the ratio of the 160 to 70 μm images. The Herschel SPIRE observations have been used to produce simultaneously observed maps of the 557 GHz o-H2O, 752 GHz p-H2O, 691 GHz CO(6−5), 1037 GHz CO(9−8), 537 GHz CH, 835 GHz CH+, and the 1461 GHz [N II] lines (efficiently probing the warm ionized medium) in the inner bar and circumnuclear torus region; – however, these observations have no effective velocity resolution. For this reason Odin has recently observed the 557 GHz ortho-H2O ground state line in the central region with high (5 km s−1) spectral resolution. Results. The emission and absorption of H2O at 557 GHz, with a velocity resolution of 5 km s−1, has been marginally detected in NGC 1365 with Odin. The water vapor is predominantly located in a shocked 15″ (1.3 kpc) region near some central compact radio sources and hot-spot H II regions, close to the northeast component of the molecular torus surrounding the nucleus. An analysis of the H2O line intensities and velocities indicates that a shock-region is located here. This is corroborated by a statistical image deconvolution of our SEST CO(3−2) observations, yielding 5″ resolution, and a study of our Very Large Array H I absorption observations, as well as comparisons with published interferometric CO observations. Additionally, an enticing 20″ H I ridge is found to extend south-southeast from the nucleus, coinciding in position with the southern edge of an O III outflow cone, emanating from the nucleus. The molecular chemistry of the shocked central region of NGC 1365 is analyzed with special emphasis on the CO, H2O and CH, CH+ results. Conclusions. The dominating activity near the northeast (NE) torus component may have been triggered by the rapid bar-driven inflow into the circumnuclear torus causing cloud-cloud collisions and shocks, leading to the formation of stellar superclusters and, hence, also to more efficient PDR chemistry, which, here, may also benefit from cosmic ray focusing caused by the observed aligned magnetic field. The very high activity near the NE torus component may reflect the fact that the eastern bar-driven gas inflow into the NE region is much more massive than the corresponding western gas inflow into the southwest region. The H2O and CH+ emissions peak in the NE torus region, but the CO and CH emissions are more evenly distributed across the whole circumnuclear torus. The higher energy CO spectral line energy distribution (SLED) is nicely modeled by a low velocity (10 km s−1) shock, which may as well explain the required CH excitation and its high abundance in denser gas. The higher velocity (40 km s−1) shock required to model the H2O SLED in the NE torus region, paired with the intense UV radiation from the observed massive young stellar superclusters, may also explain the high abundance of CH+ in this region. The nuclear H I ridge may have been created by the action of outflow-driving X-ray photons colliding with ice-covered dust grains. A precessing nuclear engine, as is suggested by the tilted massive inner gas torus, may be necessary to explain the various nuclear outflows encountered.