A radio-polarisation and rotation measure study of the Gum Nebula and its environment

Abstract

The Gum Nebula is 36 degree wide shell-like emission nebula at a distance of only 450 pc. It has been hypothesised to be an old supernova remnant, fossil HII 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-alpha data from the Southern H-Alpha Sky Survey Atlas. We model the upper part of the nebula as a spherical shell of ionised 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-halpha data to constrain average electron density in the shell $n_e$. Assuming a latitudinal background gradient in RM we find $n_e=1.3_{-0.4}^{+0.4}{\mathrm{cm}}^{-3}$, angular radius $\ddot{I}{•}_{\mathrm{outer}}=22.7_{-0.1}^{+0.1}\mathrm{deg}$, shell thickness $dr=18.5_{-1.4}^{+1.5}\mathrm{pc}$, ambient magnetic field strength $B_0=3.9_{-2.2}^{+4.9}\hat{I}¼\mathrm{G}$ 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 $+7^{\circ }\lesssim \wp \lesssim +44^{\circ }$, which represents a significant deviation from the median field orientation on kiloparsec scales (~-7.2${}^{\circ }$). The moderate compression factor $X=6.0{}_{-2.5}^{+5.1}$ at the edge of the H-alpha shell implies that the 'old supernova remnant' origin is unlikely. Our results support a model of the nebula as a HII region around a wind-blown bubble. Analysis of depolarisation 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.

33 pages, 16 figures. Accepted by The Astrophysical Journal