Kinematical and Physical properties of a 700 pc large bubble in NGC 6946

The galaxy NGC 6946 contains a gas-star complex of 700 pc in diameter which appears populated by tens of young stellar clusters, and a Super Star Cluster (SSC) as massive as 10$^6$ M$\odot$. The ionized gas, as drawn by the H$\alpha$ emission, delineates an almost circular shape which we show here to be in expansion. Previous studies have analyzed the stellar component of the complex, as well as the structure of the atomic and ionized gas; these analyses were restricted to the blueshifted component along the whole extent of the bubble or to a smaller inner region where both sides of an expanding bubble were seen. In this work we present a complete spectroscopic study of this object for two position angles crossing each other close to the young massive SSC. We have obtained new data with a spectral resolution six times better than previous spectroscopic studies, taken under atmospheric conditions better than those previously reported, allowing us to detect the approaching and receding walls of one the largest bubbles in external galaxies ever studied in detail. The kinematical analysis shows a large expanding bubble, whose walls appear to be highly structured with superposed smaller shells, likely originated as the result of star forming events occurring at the edges of the larger scale shell, a la Huygens. We also study some diagnostic diagrams of the ionized gas and conclude that most of the observed ionization is originated by photons from hot stars, but with clear evidence that some of the gas is shock ionized. This peculiar complex is an excellent laboratory for the analysis of the interaction and feedback between the gas where the stars were formed and the young and massive generation of new born stars.

💡 Research Summary

The authors present a comprehensive spectroscopic investigation of a remarkable 700 pc‑diameter gas‑star complex in the spiral galaxy NGC 6946. This region hosts dozens of young stellar clusters and a super‑star cluster (SSC) with a mass of roughly 10⁶ M⊙. In H α imaging the ionized gas outlines an almost circular shell, but previous kinematic studies were limited to the blueshifted side of the structure or to a small inner zone where both approaching and receding walls could be seen. By obtaining long‑slit spectra along two position angles that intersect near the SSC, with a spectral resolution six times finer than earlier work and under superior atmospheric conditions, the authors succeed in detecting both the approaching and receding walls of the bubble across its full extent.

The velocity profiles reveal a classic expanding shell with a maximum expansion speed of ≈30 km s⁻¹. The shell walls are not smooth; instead they are riddled with smaller, concentric arcs and fragmented shells. The authors interpret these sub‑structures as secondary shells generated by successive star‑forming events that have been triggered at the rim of the primary bubble—a process reminiscent of Huygens’ principle, where wave fronts propagate outward and spawn new disturbances. This hierarchical shell morphology suggests that the bubble has been inflated not by a single explosive episode but by a series of stellar winds and supernova explosions associated with the SSC and surrounding clusters.

Diagnostic line‑ratio diagrams (BPT diagrams using