New Insights in the Bubble Wall of NGC 3324: Intertwined Substructures and a Bipolar Morphology Uncovered by JWST

Abstract

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 μ m 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 μ m polycyclic aromatic hydrocarbon (PAH) emission, the 4.05 μ m ionized emission, and the 4.693 μ m H _2 emission. However, the PAH-emitting structures are depicted between the other two. The H _2 emission reveals numerous intertwined substructures that are not prominently traced in the 3.3 μ m PAH emission. The separation between two substructures in the H _2 emission is ∼1.″1 or 2420 au. The intertwined substructures are traced in the spatial areas associated with the neutral to H _2 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 μ m 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.

Keywords

• Interstellar dust extinction
• H II regions
• Interstellar medium
• Interstellar clouds
• Star formation
• Pre-main sequence stars