An analytical model for the magnetic field in the thick shell of Galactic bubbles with uniform initial conditions

TL;DR

This paper develops an analytical model for the magnetic field in the thick shells of Galactic bubbles, accounting for various bubble shapes and using observational data to estimate their magnetic contributions.

Contribution

It introduces a novel analytical framework for modeling magnetic fields in non-spherical Galactic bubble shells based on initial conditions and radial displacements.

Findings

Minimal contribution to Faraday rotation measures.

Significant contribution to synchrotron emission at high Galactic latitudes.

Highlights the importance of Local Bubble in Galactic magnetic field studies.

Abstract

Bubbles and super-bubbles are ubiquitous in the interstellar medium and influence their local magnetic field. Starting from the assumption that bubbles result from violent explosions that sweep matter away in a thick shell, we derive the analytical equations for the divergence-free, regular magnetic field in the shell. The explosion velocity field is assumed to be radial but not necessarily spherical, making it possible to model various-shaped bubbles. Assuming an explosion center, the magnetic field at the present time is fully determined by the initial uniform magnetic field, the present-time geometry of the bubble shell, and a radial vector field that encodes the explosion-induced displacement of matter, from its original location to its present-time location. We present the main characteristics of our magnetic-field model using a simple linear model for the radial displacements. Next, we use our analytical prescription, informed by a three-dimensional dust density map, to estimate the expected contribution of the shell of the Local Bubble, the super-bubbles in which the Sun resides, to the integrated Faraday rotation measures and synchrotron emission and compare these to full-sky observational data. We find that, while the contribution to the former is minimal, the contribution to the latter is very significant at Galactic latitudes $|b|>45^\circ$. Our results underline the need to take the Local Bubble into account in large-scale Galactic magnetic field studies.