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To reveal the origin of the mini-starbursts in the Milky Way, we carried out large-scale CO observations toward the RCW 106 giant molecular cloud (GMC) complex using the NANTEN2 4 m radio telescope operated by Nagoya University. We also analyzed the Mopra Southern Galactic Plane CO survey and Herschel infrared continuum archival data. The RCW 106 GMC complex contains the radial velocity components of −68 km s−1 and −50 km s−1 reported by H. Nguyen et al. (2015). Focusing on the RCW 106 East and West region with the massive star formation having the bright infrared dust emission, we found that these regions have three different velocity components with ∼10 km s−1 differences. The two out of three velocity components show morphological correspondence with the infrared cold dust emission and connect with the bridge feature on a position–velocity diagram. Therefore, two molecular clouds with ∼10 km s−1 differences are likely to be physically associated with massive star-forming regions in the GMC complex. Based on these observational results, we argue that mini-starbursts and massive star/cluster formation in the RCW 106 GMC complex are induced by supersonic cloud–cloud collisions in an agglomerate of molecular gas on the Scutum–Centaurus arm.

Young massive clusters (YMCs) are dense aggregates of young stars and are often speculated as potential precursors to globular clusters. However, the formation mechanism of massive and compact gas clumps that precede YMCs remains unknown. In this paper, we study the formation of such massive clumps via fast H i gas collisions (∼100 km s−1) as suggested by recent observations and their subsequent evolution into YMCs by using three-dimensional magnetohydrodynamics simulations involving self-gravity and detailed thermal/chemical processes. In particular, the impact of ionization feedback from stellar radiation is included in an approximate fashion where the temperature within the H ii regions is elevated to 10,000 K, while supernova feedback is not included. We examine whether the resulting massive clumps can survive this ionization feedback and evolve into YMCs. Our simulations reveal the emergence of gas clumps that not only possess substantial mass (∼105 M ⊙) but also sufficient compactness (∼5 pc). Notably, these clumps exhibit significantly higher escape velocities compared to the sound speed of the H ii region, indicating effective gravitational retention of gas against feedback-induced evaporation. Consequently, these conditions foster efficient star formation within the massive gas clumps, ultimately leading to their evolution into YMCs. We also perform simulations involving lower-velocity gas collisions, approximately 15 km s−1, typical shock velocities induced by galactic superbubbles. In contrast to the high-velocity collisions, we find that molecular cloud formation does not occur in the case of 1 cm−3 gas collision, while YMC formation is observed in the presence of denser gas of 10 cm−3. However, the formation of YMCs requires compression periods exceeding 10 Myr in these cases, indicating a potential preference for gas collisions driven by intergalactic interactions rather than galactic superbubbles for YMC formation.

The formation of super star clusters (SSCs) in galaxies remains a fundamental yet unresolved problem. Among the proposed mechanisms, cloud–cloud collisions (CCCs) have been suggested as a potential trigger, although observational validation has been limited. Here we present high-resolution ( 0.″12 , ∼14 pc) Atacama Large Millimeter/submillimeter Array observations of CO (J = 1–0) emission toward a super giant molecular cloud (SGMC) in the overlap region of the Antennae galaxies. The data resolve the SGMC into two distinct velocity components separated by ∼50 km s−1. One component exhibits a “U”-shaped structure within a large filament likely shaped by ram pressure, while the other shows hub-filament morphology. Such a morphology is naturally interpreted as a CCC scenario. The 108 GHz continuum emission detected at the apparent collision interface is dominated by free–free radiation, with an ionizing photon rate consistent with the stellar mass and age of the optically identified SSCs. Supplementary infrared imaging with JWST reveals emission spatially coincident with the inferred collision interface, further supporting the CCC scenario. These results provide compelling, multiwavelength evidence that CCCs play a key role in triggering SSC formation in merging galaxies.

The nature of molecular clouds and their statistical behavior in subsolar metallicity environments are not fully explored yet. We analyzed data from an unbiased CO (J = 2–1) survey at the spatial resolution of ∼2 pc in the northern region of the Small Magellanic Cloud with the Atacama Compact Array to characterize the CO cloud properties. A cloud-decomposition analysis identified 426 spatially/velocity-independent CO clouds and their substructures. Based on the cross-matching with known infrared catalogs by Spitzer and Herschel, more than 90% CO clouds show spatial correlations with point sources. We investigated the basic properties of the CO clouds and found that the radius–velocity linewidth (R–σ v ) relation follows the Milky Way-like power-law exponent, but the intercept is ∼1.5 times lower than that in the Milky Way. The mass functions (dN/dM) of the CO luminosity and virial mass are characterized by an exponent of ∼1.7, which is consistent with previously reported values in the Large Magellanic Cloud and in the Milky Way.

The Large Magellanic Cloud (LMC) exhibits vigorous high-mass star formation, including the H ii regions of 30 Dor, which is the most active site of star formation in the Local Group. This paper focuses on the Giant Molecular Cloud (GMC) in the H ii region N113 in the central part of the LMC. Based on the 12CO(J = 2–1) and 13CO(J = 2–1) data at a resolution of ~0.2 pc obtained by the Atacama Millimeter/submillimeter Array and the Atacama Pathfinder Experiment​​​, we reveal that the GMC consists of two filamentary structures, each ~10 pc in length, forming a V-shaped pattern with a vertex angle of 90°. The filamentary structures host high-mass young stellar objects in gravitationally bound dense gas. Large-scale H i gas data covering 100 pc reveal two distinct velocity components separated by more than 40 km s−1 that correspond to the low-velocity (L-) and disk (D-) H i components of the LMC. The L-component appears to be located in a cavity-like distribution of the D-component, and the CO filaments are positioned at the cavity’s edge. We find evidence for the L-component to fit the cavity by a 53 pc displacement, and suggest that collisional compression of the H i gas during the last 1.3 Myr triggered the GMC formation and the high-mass star formation. This lends support to the idea that the large-scale collision driven by the tidal interaction is playing a role in the evolution of the interstellar medium in N113.

We present the results of ALMA-ACA 7 m array observations in 12CO(J = 2–1), 13CO(J = 2–1), and C18O(J = 2–1) line emission toward the molecular-gas disk in the Local Group spiral galaxy M33 at an angular resolution of 7.″31 × 6.″50 (30 × 26 pc). We combined the ACA 7 m array 12CO(J = 2–1) data with the IRAM 30 m data to compensate for emission from diffuse molecular-gas components. The ACA+IRAM combined 12CO(J = 2–1) map clearly depicts the cloud-scale molecular-gas structure over the M33 disk. Based on the ACA+IRAM 12CO(J = 2–1) cube data, we cataloged 848 molecular clouds with a mass range from 103–106 M ⊙. We found that high-mass clouds (≥105 M ⊙) tend to associate with the 8 μm bright sources in the spiral arm region, while low-mass clouds (<105 M ⊙) tend to be apart from such 8 μm bright sources and to exist in the inter-arm region. We compared the cataloged clouds with GMCs observed by the IRAM 30 m telescope at 49 pc resolution (IRAM GMC), and found that a small IRAM GMC is likely to be identified as a single molecular cloud even in ACA+IRAM CO data, while a large IRAM GMC can be resolved into multiple ACA+IRAM clouds. The velocity dispersion of a large IRAM GMC is mainly dominated by the line-of-sight velocity difference between small clouds inside the GMC rather than the internal cloud velocity broadening.

The evolution of giant molecular clouds (GMCs), which are the main sites of star formation, is essential for unraveling how stars form and how galaxies evolve. We analyzed the M33 CO(J = 2–1) data with spatial resolution of 39 pc obtained by ALMA-ACA 7 m array combined with IRAM 30 m. We identified 736 GMCs and classified them into three types; Type I: associated with no Hii regions, Type II: associated with Hii regions with the Hα luminosity L(Hα) < 1037.5 erg s-1, Type III: associated with Hii regions with L(Hα) > 1037.5erg s-1. We found that mass, size, and velocity dispersion of GMCs slightly increase in the order of Type I, II, and III GMCs. Type III GMCs mainly exist in the spiral arm, while many of Type I and Type II GMCs are distributed in the inter-arm. Assuming that the star formation proceeds steadily, we roughly estimated the total GMC lifetime of 30 Myr.

Massive dense clumps in the Large Magellanic Cloud can be an important laboratory to explore the formation of populous clusters. We report multiscale ALMA observations of the N159W-North clump, which is the most CO-intense region in the galaxy. High-resolution CO isotope and 1.3 mm continuum observations with an angular resolution of ∼0.″25 (∼0.07 pc) revealed more than five protostellar sources with CO outflows within the main ridge clump. One of the thermal continuum sources, MMS-2, shows an especially massive/dense nature whose total H2 mass and peak column density are ∼104 M ⊙ and ∼1024 cm−2, respectively, and harbors massive (∼100 M ⊙) starless core candidates identified as its internal substructures. The main ridge containing this source can be categorized as one of the most massive protocluster systems in the Local Group. The CO high-resolution observations found several distinct filamentary clouds extending southward from the star-forming spots. The CO (1–0) data set with a larger field of view reveals a conical, ∼30 pc long complex extending toward the northern direction. These features indicate that a large-scale gas compression event may have produced the massive star-forming complex. Based on the striking similarity between the N159W-North complex and the other two previously reported high-mass star-forming clouds in the nearby regions, we propose a “teardrops inflow model” that explains the synchronized, extreme star formation across >50 pc, including one of the most massive protocluster clumps in the Local Group.

Young massive clusters (YMCs) are dense aggregates of young stars and are often speculated as potential precursors to globular clusters. However, the formation mechanism of massive and compact gas clumps that precede YMCs remains unknown. In this paper, we study the formation of such massive clumps via fast HI gas collisions (~100 km/s) as suggested by recent observations and their subsequent evolution into YMCs by using three-dimensional magnetohydrodynamics simulations involving self-gravity and detailed thermal/chemical processes. In particular, the impact of ionization feedback from stellar radiation is included in an approximate fashion where the temperature within the HII regions is elevated to 10,000 K, while supernova feedback is not included. We examine whether the resulting massive clumps can survive this ionization feedback and evolve into YMCs. Our simulations reveal the emergence of gas clumps that do not only possess substantial mass (~10^5 M_sun) but also sufficient compactness (~5 pc). Notably, these clumps exhibit significantly higher escape velocities compared to the sound speed of the HII region, indicating effective gravitational retention of gas against feedback-induced evaporation. Consequently, these conditions foster efficient star formation within the massive gas clumps, ultimately leading to their evolution into YMCs. We also perform simulations involving lower-velocity gas collisions, approximately 15 km/s, typical shock velocities induced by galactic superbubbles.

We derive the entire dust extinction (Av) map for the Large Magellanic Cloud (LMC) estimated from the color excess at near-infrared wavelengths. Using the percentile method we recently adopted to evaluate Av distribution along the line of sight, we derive the three-dimensional(3D)Av maps of the three massive star forming regions of N44, N79 and N11 based on the IRSF/SIRIUS point source catalog. The 3D AV maps are compared with the hydrogen column densities N(H) of three different velocity components where one is of the LMC disk velocity and the other two are of velocities lower than the disk velocity. As a result, we obtain 3D dust geometry suggesting that gas collision is on-going between the different velocity components. We also find difference in the timing of the gas collision between the massive star forming regions, which indicates that the gas collision in N44, N79 and N11 occurred later than that in 30 Doradus. In addition, difference by a factor of two in Av/N(H) is found between the velocity components for N44, while significant difference is not found for N79 and N11. From the 3D geometry and Av/N(H) in each star forming region, we suggest that the massive star formation in N44 was induced by an external trigger of tidal interaction between the LMC and the SMC, while that in N79 and N11 is likely to have been induced by internal triggers such as gas converging from the galactic spiral arm and expansion of a supershell although the possibility of tidal interaction cannot be ruled out.