A multi-scale molecular and atomic gas view on the HII region N113 in the Large Magellanic Cloud:Evidence for high-mass star formation triggered by supersonically-colliding HI flows

Abstract: The Large Magellanic Cloud (LMC) exhibits vigorous high-mass star formation, including the HII regions 30~Dor that is the most active site of star formation in the local group. The present paper focuses on the Giant Molecular Cloud (GMC) in the HII region N113 in the central part of the LMC. Based on the $^{12}$CO($J$ =1-0) and $^{13}$CO($J$ = 1-0) data at a resolution of approximately 0.2 pc taken with ALMA+APEX, we reveal that the GMC consists of two filamentary structures each of approximately 10 pc in length, forming a V-shape pattern with a vertex angle of 90 degrees. The filamentary structures host high-mass young stellar objects in gravitationally bound dense gas. Large-scale HI 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-) HI 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 HI gas during the last 1.3 Myr triggered the GMC formation and the high-mass star formation. This lends support for the large scale collision driven by the tidal interaction is playing a role in evolution of interstellar medium in N113.

• The paper reveals that supersonic collisions between atomic HI flows compress gas, forming a GMC and triggering high-mass star formation in N113.
• The study employs multi-scale ALMA+APEX observations to resolve sub-parsec molecular filaments and gravitationally bound clumps associated with emerging massive stars.
• The findings support cloud–cloud collision models with a key 53-pc displacement and 40–56 km/s velocity offset driving the observed star formation dynamics.

Multi-Scale Gas Dynamics and Triggered High-Mass Star Formation in N113 (LMC)

Introduction

The study offers a comprehensive analysis of the interstellar medium (ISM) surrounding the H II region N113 in the central Large Magellanic Cloud (LMC), with an emphasis on unveiling the star formation processes at sub-parsec to 100-pc scales. Utilizing ALMA+APEX observations for $^{12}$CO and $^{13}$CO transitions (resolution ~0.2 pc), complemented by ATCA/Parkes H I data at 15 pc resolution, the paper examines both the fine molecular structures and the kinematics of atomic gas. Key objectives include identifying the spatial and kinematic signatures of cloud-cloud collisions (CCC) and evaluating their role in high-mass star formation within a galactic context shaped by tidal interactions.

Multi-Phase ISM and Morphological Context

The region of N113 is characterized by distinct multi-phase ISM components that trace the progression from atomic H I, through molecular gas, to ionized hydrogen associated with ongoing star formation. The large-scale three-color composite image delineates the spatial relationship between H$\alpha$ (ionized gas), 8 $\mu$m PAH emission (tracing photodissociation regions/low-density H$_2$), and H I, with embedded CO peaks at the base of an arc-like structure. Massive YSOs, WR, and OB stars are coincident with these CO intensities, while the GMC resides in the region of maximal multiphase gas overlap.

Figure 1: A holistic view of the multi-phase ISM toward N113, highlighting the spatial correlation of CO, H I, and star-forming sites.

Molecular Filamentary Structure and Kinematics

High-resolution ALMA+APEX imaging resolves the GMC into two principal $^{12}$CO/$^{13}$CO filaments ($\sim$10–20 pc in length), configured in a V-shape with a $\sim$90° vertex. These filaments exhibit widths (2–3 pc for $^{12}$CO; ~1 pc for $^{13}$0CO) and peak integrated intensities exceeding 400 K km\,s$^{13}$1, especially toward massive YSO positions. Astrodendro segmentation of $^{13}$2CO identifies ~200 gravitationally bound clumps, nearly all massive-star-forming, concentrated along the V-shaped filaments. The highest-mass cores ($^{13}$3 $^{13}$4) co-locate with spectroscopically confirmed high-mass YSOs ($^{13}$5). Localized velocity dispersion clearly correlates with protostellar activity, reaching $^{13}$6–$^{13}$7 km/s around the most massive YSOs, indicative of active or recent energy injection likely associated with ongoing massive star formation.

Figure 2: Moment-maps for $^{13}$8CO and $^{13}$9CO emission towards N113, with overlayed YSO locations and kinematic diagnostics.

Figure 3: ALMA+APEX resolved molecular distribution highlighting the V-shaped GMC and sub-filamentary gas extensions associated with YSOs.

The morphology critically resembles features predicted by MHD simulations of shock-induced cloud compression: hub-filament networks and enhanced self-gravitating clump formation within compressed layers shaped by cloud collisions.

Atomic Gas Kinematics and Evidence for Collisions

The atomic gas (H I) at 100-pc scale reveals two distinct velocity components: an L-component (low velocity; $\alpha$0 to $\alpha$1 km\,s$\alpha$2) and a D-component (disk; $\alpha$3 to $\alpha$4 km\,s$\alpha$5), separated by over 40 km/s. The L-component (interpreted as tidal debris from historic LMC/SMC interaction) spatially fits a cavity within the D-component, and the filaments of CO (tracing the current GMC) are positioned on the cavity’s edge. Position–velocity diagrams reveal “bridge” features—intermediate-velocity gas linking L and D components—consistent with simulations of supersonic CCCs.

Figure 4: H I intensity maps of both the L- and D-components, showing the large-scale context and the GMC location relative to the interacting atomic flows.

Figure 5: Position-velocity diagrams revealing bridge features that kinematically link the L- and D-components, a hallmark of CCC-driven dynamics.

Quantitative assessment via pixel-by-pixel spatial correlation (Pearson coefficient mapping) between the L- and D-components locates two minima, identifying the regions/statistics of maximal anti-correlation (“complementary distribution”). The optimal spatial displacement for maximal complementarity is $\alpha$653 pc—an offset consistent with the L-component having traversed and excavated a cavity within the denser D-component over a timescale of $\alpha$71.3 Myr at the measured velocity separation.

Figure 6: Schematic illustration of the inferred cloud-cloud collision geometry and resulting GMC/filament formation.

CCC Scenario: GMC and High-Mass Star Formation

The empirically established signatures—(i) bridge features, (ii) displaced complementary distribution, (iii) cavity in the D-component—are canonical CCC diagnostics. The inferred three-stage scenario is as follows:

1. HI cloud collision and cavity formation: The blue-shifted L-component, moving toward the observer and from the east, collides supersonically with the D-component (relative velocity $\alpha$840–56 km/s), driving a cavity and intermediate-velocity interface.
2. Compressive formation of the GMC: Shock compression at the interface triggers rapid cooling, H$\alpha$9 and CO formation on $\mu$0Myr timescales (given $\mu$1 cm$\mu$2). GMC materializes in situ at the cavity wall—specifically along the high-density, V-shaped, post-shock slab. This structural outcome matches MHD models (e.g., [Inoue+2018]).
3. Triggered high-mass star formation: Gravitationally bound clumps rapidly contract, forming a burst of massive YSOs along the compressed filaments. High-velocity dispersion and increased star formation efficiency are focused where the filament intersection (vertex) and sub-filament extensions concentrate mass and turbulence. The spatial configuration of O-star and YSO populations, their association with wider PAH (8 µm) and ionized (H$\mu$3) features, and the alignment of substructures, are all consistent with fast, turbulent, high-pressure collapse induced by the CCC.

   Figure 7: Composite, LMC-wide mapping of L-, D-, and intermediate (I-) components, placing N113 in the context of galaxy-scale tidal flows.

Implications and Future Directions

The study reinforces CCC as a primary mechanism for the assembly of GMCs and high-mass clusters in the LMC, a process modulated on galactic scales by recent ($\mu$40.2 Gyr) LMC/SMC tidal interactions. The observed kinematic and spatial congruence with MHD simulation outcomes cements the role of dynamical triggering—through collisions with $\mu$5 km/s—as a prerequisite for high-mass star birth in N113. Importantly, comparative analysis with similar LMC regions (N159, 30 Dor) highlights how collision velocity, pre-shock density, and geometry regulate GMC structure (V-shaped vs. conical filaments) and star formation morphology (distributed vs. centrally peaked cluster formation).

The results have direct implications for scaling star formation models from galactic interaction events to sub-pc structures. They suggest that enhanced cluster formation rates in interacting/irregular galaxies are structurally the result of recurrent, supersonic CCCs between massive atomic flows—an insight immediately relevant to extragalactic surveys (PHANGS, etc.) and high-z starburst/merger systems.

Conclusion

This paper provides robust, multi-scale evidence that the N113 GMC and its current burst of high-mass star formation are the products of a supersonic collision between two atomic H I flows, a process initiated and sustained by the dynamical legacy of LMC/SMC interactions. The multi-phase integration of observational data, rigorous kinematic assessment, and alignment with theoretical frameworks establishes a paradigm for triggered cluster formation by CCCs in both local and extragalactic contexts. The findings will inform future high-resolution studies of star formation efficiency and clustering in interaction-driven environments.