Remarkable Symmetries in the Milky Way Disc's Magnetic Field

Abstract

AbstractWe apply a new, expanded compilation of extragalactic source Faraday rotation measures (RM) to investigate the broad underlying magnetic structure of the Galactic disk at latitudes ∣b∣ ≲15° over all longitudes l, where our total number of RMs is comparable to those in the combined Canadian Galactic Plane Survey (CGPS) at ∣b∣ < 4° and the Southern Galactic Plane (SGPS) ∣b∣<1.5°. We report newly revealed, remarkably coherent patterns of RM at ∣b∣≲15° from l∼270° to ∼90° and RM(l) features of unprecedented clarity that replicate in l with opposite sign on opposite sides of the Galactic center. They confirm a highly patterned bisymmetric field structure toward the inner disc, an axisymmetic pattern toward the outer disc, and a very close coupling between the CGPS/SGPS RMs at ∣b∣≲3° (‘mid-plane’) and our new RMs up to ∣b∣∼15° (‘near-plane’). Our analysis also shows the vertical height of the coherent component of the disc field above the Galactic disc's mid-plane—to be ∼1.5 kpc out to ∼6 kpc from the Sun. This identifies the approximate height of a transition layer to the halo field structure. We find no RM sign change across the plane within ∣b∣∼15° in any longitude range. The prevailing disc field pattern and its striking degree of large-scale ordering confirm that our side of the Milky Way has a very organized underlying magnetic structure, for which the inward spiral pitch angle is 5.5°±1° at all ∣b∣ up to ∼12° in the inner semicircle of Galactic longitudes. It decreases to ∼0° toward the anticentre.