Contrasting geometries of kiloparsec-scale stellar flows associated with giant Galactic cavities

Abstract

Giant bubbles and cavities are widespread in star-forming galactic discs, yet the stellar dynamics associated with such structures remain poorly constrained. Here we combine Gaia Data Release 3 with three-dimensional extinction information to identify 3,502 massive-star candidates corresponding to initial masses $M_{\rm ini}\gtrsim8\,M_{\odot}$ within 5 kpc of the Sun. A search of the stellar-density field alone, independent of interstellar-medium morphology and stellar kinematics, reveals two prominent overdensities, R1 and R2, with characteristic scales of approximately 1.3 kpc. Both are subsequently found to be associated with previously identified giant interstellar cavities in projected position and line-of-sight location. After subtracting locally fitted background velocity fields, stars with full three-dimensional velocities reveal coherent divergent in-plane motions across both structures. R1 extends well beyond a known massive-star association and exhibits strongly anisotropic expansion, with one principal strain rate robustly positive and the other consistent with zero. R2 instead exhibits positive strain rates along both principal directions, indicating broadly two-dimensional internal expansion. These results show that density-selected, cavity-associated stellar structures of comparable scale can host coherent kiloparsec-scale flows with contrasting internal flow geometries and need not share a common stellar kinematic organisation.