A Radio-Polarisation and Rotation Measure Study of the Gum Nebula and Its Environment

The Gum Nebula is 36°-wide shell-like emission nebula at a distance of only ∼450 pc. It has been hypothesized to be an old supernova remnant, fossil H ii region, wind-blown bubble, or combination of multiple objects. Here we investigate the magneto-ionic properties of the nebula using data from recent surveys: radio-continuum data from the NRAO VLA and S-band Parkes All Sky Surveys, and H α data from the Southern H-Alpha Sky Survey Atlas. We model the upper part of the nebula as a spherical shell of ionized gas expanding into the ambient medium. We perform a maximum-likelihood Markov chain Monte Carlo fit to the NVSS rotation measure data, using the H α data to constrain average electron density in the shell n_e. Assuming a latitudinal background gradient in rotation measure, we find {{n}_e}=1.3_-0.4^+0.4 c{{m}^-3}, angular radius {{φ }_outer}=22\buildrel{\circ}\over{.} 7_-0.1^+0.1, shell thickness dr=18.5_-1.4^+1.5 pc, ambient magnetic field strength {{B}₀}=3.9_-2.2^+4.9 μ G, and warm gas filling factor f=0.3_-0.1^+0.3. We constrain the local, small-scale (∼260 pc) pitch-angle of the ordered Galactic magnetic field to +7{}^\circ ≲ \wp ≲ +44{}^\circ , which represents a significant deviation from the median field orientation on kiloparsec scales (∼-7.°2). The moderate compression factor X=6.0_-2.5^+5.1 at the edge of the H α shell implies that the “old supernova remnant” origin is unlikely. Our results support a model of the nebula as a H ii region around a wind-blown bubble. Analysis of depolarization in 2.3 GHz S-PASS data is consistent with this hypothesis and our best-fitting values agree well with previous studies of interstellar bubbles.