A 3D Model of the Local Bubble’s Magnetic Field: Insights from Dust and Starlight Polarization

Abstract Clustered stellar feedback creates expanding voids in the magnetized interstellar medium known as superbubbles. Although theory suggests that superbubble expansion is influenced by interstellar magnetic fields, direct observational data on 3D superbubble magnetic field geometry is limited. The Sun’s location inside the Local Bubble provides a unique opportunity to infer a superbubble’s 3D magnetic field orientation, under the assumptions that: (I) the Local Bubble’s surface is the primary contributor to plane-of-the-sky polarization observations across much of the sky, and (II) the Local Bubble’s magnetic field is tangent to its dust-traced shell. In this work, we validate these assumptions and construct a model of the Local Bubble’s 3D B -field orientation from Planck 353 GHz polarization observations and a 3D-dust-derived model of the Local Bubble’s shell. We test Assumption I by examining correlations between the Local Bubble’s 3D geometry, dust polarization, and starlight polarization. We find that the Local Bubble likely dominates the polarized signal in the majority of lines of sight. We jointly test Assumptions I and II by applying our reconstruction method to a simulated superbubble, where we successfully reconstruct the 3D magnetic field orientation over the bulk of its surface. Finally, we use our 3D B -field model to infer the initial magnetic field orientation in the solar neighborhood prior to the Local Bubble’s formation, and derive an orientation parallel to the present-day Local Arm of the galaxy. These findings provide new insights into the coevolution of superbubbles and the magnetized interstellar medium.