2023ApJ...958...51D

We report the discovery of intertwined/entangled substructures toward the bubble wall of NGC 3324 below a physical scale of 4500 au, which is the sharp edge/ionization front/elongated structure traced at the interface between the H II region and the molecular cloud. The sharp edge appears wavy in the Spitzer 3.6-8.0 μm images (resolution ∼2″). Star formation signatures have mostly been traced on one side of the ionization front, which lies on the molecular cloud's boundary. The James Webb Space Telescope's (JWST) near- and mid-infrared images (resolution ∼0.″07-0.″7) are employed to resolve the sharp edge, which has a curvature facing the exciting O-type stars. The elongated structures are associated with the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission, the 4.05 μm ionized emission, and the 4.693 μm H₂ emission. However, the PAH-emitting structures are depicted between the other two. The H₂ emission reveals numerous intertwined substructures that are not prominently traced in the 3.3 μm PAH emission. The separation between two substructures in the H₂ emission is ∼1.″1 or 2420 au. The intertwined substructures are traced in the spatial areas associated with the neutral to H₂ transition zone, suggesting the origin of these structures by “thin-shell” instability. Furthermore, an arc-like feature traced in the Spitzer 3.6-8.0 μm images is investigated as a bipolar H II region (extent ∼0.35 pc) at T_d ∼25-28 K using the JWST images. A massive-star candidate VPHAS-OB1 #03518 seems to be responsible for the bipolar H II region.