The Galactic halo magnetic field revisited

TL;DR

This paper revisits the Galactic halo magnetic field model, incorporating a revised thermal gas scale-height, leading to more physically plausible magnetic field strengths and cosmic-ray distributions that still match observational data.

Contribution

It updates the 3D Galactic magnetic field model by adjusting the thermal gas scale-height, resulting in more realistic halo magnetic field parameters while maintaining consistency with all-sky observational data.

Findings

Halo magnetic field strength reduced to 2 microG or less

Removal of the unrealistic cosmic-ray electron density cutoff

Models still reproduce all-sky polarization and rotation measure data

Abstract

Recently, Sun et al. (2008) published new Galactic 3D-models of magnetic fields in the disk and halo of the Milky Way and the distribution of cosmic-ray electron density by taking into account the thermal electron density model NE2001 by Cordes & Lazio (2002, 2003). The models successfully reproduce observed continuum and polarization all-sky maps and the distribution of rotation measures of extragalactic sources across the sky. However, the model parameters obtained for the Galactic halo, although reproducing the observations, seem physically unreasonable: the magnetic field needs to be significantly stronger in the Galactic halo than in the plane and the cosmic-ray distribution must be truncated at about 1 kpc to avoid excessive synchrotron emission from the halo. The reason for these unrealistic parameters was the low scale-height of the warm thermal gas of about 1 kpc adapted in the NE2001 model. However, this scale-height seemed well settled by numerous investigations. Recently, the scale-height of the warm gas in the Galaxy was revised by Gaensler et al. (2008) to about 1.8 kpc, by showing that the 1 kpc scale-height results from a systematic bias in the analysis of pulsar data. This implies a higher thermal electron density in the Galactic halo, which in turn reduces the halo magnetic field strength to account for the observed rotation measures of extragalactic sources. We slightly modified the NE2001 model for the new scale-height and revised the Sun et al. (2008) model parameters accordingly: the strength of the regular halo magnetic field is now 2 microG or lower, and the physically unrealistic cutoff in z for the cosmic-ray electron density is removed. The simulations based on the revised 3D-models reproduce all-sky observations as before.