Cloud-Cloud Collisions Induce Filament-Mediated Super Star Cluster Formation in the Antennae Overlap Region: Evidence from ALMA and JWST

TL;DR

This study investigates whether cloud–cloud collisions trigger super star cluster formation in the Antennae overlap by leveraging 0.12'' ALMA CO(1--0) observations of SGMC1-ALMA-3 and complementary JWST imaging. The data reveal two spatially and kinematically distinct molecular components with CCC-like morphologies, including a U-shaped filament, hub–filament structure, and a PV-bridge, all co-located with SSC candidates and free–free emission at the collision interface. The 108 GHz continuum is attributed to free–free radiation from recently formed massive stars, with an ionizing photon rate $Q(H^0)\approx1.7\times10^{52}\ \mathrm{s^{-1}}$ implying a SSC mass of $M_{\rm star}\approx5.7\times10^{5}\ M_\odot$, consistent with HST-identified clusters. JWST NIRCam/MIRI data corroborate the collision interface and younger stellar populations, supporting a scenario where merger-driven gas flows create localized CCCs that trigger SSC formation, offering a concrete observational link between galaxy interactions, cloud collisions, and the birth of massive clusters.

Abstract

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 ($0.12^{\prime\prime}$, $\sim14\,\mathrm{pc}$) ALMA 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 $\sim50\,\mathrm{km\,s^{-1}}$. 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, multi-wavelength evidence that CCCs play a key role in triggering SSC formation in merging galaxies.

Figures (6)

• Figure 1: (outer panel) CO ($J=1$--$0$) spectrum extracted at the ALMA 108 GHz peak pixel of SGMC1-ALMA-3. The background shading highlights three distinct velocity intervals: blue (1330--1390 km s$^{-1}$), green (1395--1470 km s$^{-1}$), and red (1475--1550 km s$^{-1}$). (inner panels) CO ($J=1$--$0$) integrated intensity maps of SGMC1-ALMA-3, shown for three velocity components: 1330–1390 km s$^{-1}$ (blue, left), 1395–1470 km s$^{-1}$ (green, center), and 1475–1550 km s$^{-1}$ (red, right). The contour levels are set at 5 logarithmically spaced intervals between 100 and 1000 K km s$^{-1}$ to highlight both faint and bright emission features.
• Figure 2: Channel-map sequence of the CO (1--0) emission cube toward SGMC1--ALMA-3. Each panel shows a $2\farcs\times2\farcs$ field of view, derived from the cube binned by five channels along the spectral axis, with the velocity (km s$^{-1}$) indicated at the top. Contours indicate emission at 3, 9, 15, 21, 27, and 33 times the rms noise level, where the rms noise per channel is estimated to be $2.1\,{\rm K}/\sqrt{5}$ after five-channel binning. Colored borders delineate three characteristic velocity intervals (blue, green, and red) corresponding to the distinct gas components identified in Figure \ref{['fig:spec']}.
• Figure 3: Zoomed-in view of the SGMC1–ALMA-3 region, showing the CO ($J=1$--$0$) integrated intensity (moment 0) map at 1395–1470 km s$^{-1}$ with various overlays. Top-left: Red contours trace the CO ($J=1$--$0$) emission integrated over 1475–1550 km s$^{-1}$ (contour levels: six logarithmically spaced levels between 300 and 1000 K km s$^{-1}$). Top-right: Locations of optically identified SSC candidates from Whitmore2010, marked with ‘×’ symbols and annotated with ID. Bottom-left: Cyan contours indicate JWST/NIRCam F335M emission (contour levels: 0.5–1.2 times the 99.9th percentile of the image in 6 steps), overlaid with the same red CO contours as in the top-left panel. Bottom-right: Orange contours show the ALMA 108 GHz continuum emission (contour levels: five logarithmically spaced levels between $1\times10^{-5}$ and $5\times10^{-4}$ Jy beam$^{-1}$).
• Figure 4: Left: Same background image and red contours as in Figure \ref{['fig:green']}, but with a wider field of view. The white line indicates the cut used to extract the PV diagram, and tick marks along the line denote offset positions at $-2$, $-1$, $0$, $+1$, and $+2$ arcsec. Right: PV diagram extracted along the white line shown in the left panel, using a slit width of 1 pixel. The horizontal dashed green and red lines indicate the velocity range, corresponding to Figure \ref{['fig:spec']}.
• Figure 5: Multiwavelength comparison of the SGMC1–ALMA-3 region in the Antennae overlap. Each panel covers a $2\arcsec\times2\arcsec$ field centered on the peak of the ALMA 108 GHz continuum emission. Magenta crosses mark the positions of young massive star clusters identified in HST catalogs (Table \ref{['tab:SSCs']}). (Top left) JWST NIRCam F335M map shown in grayscale with cyan contours. A 50 pc scale bar is shown for reference. (Top right) HST optical RGB composite image (constructed from archival data) overlaid with the same NIRCam contours. The HST and JWST images were manually aligned using stars, but a small systematic offset may still exist. (Bottom left) ALMA CO (1--0) moment 1 map with NIRCam contours, showing the kinematic distribution of the molecular gas relative to the NIRCam F335M emission. (Bottom right) CO (1--0) moment 2 (velocity-dispersion) map with NIRCam contours, emphasizing regions of enhanced velocity dispersion at the interface between gas components.