Remarkable symmetries in the Milky Way disk's magnetic field

Abstract

Using a new, expanded compilation of extragalactic source Faraday rotation measures (RM) we investigate the broad underlying magnetic structure of the Galactic disk at latitudes $|b|$ $\lesssim 15^{\circ }$ over all longitudes $l$, where our total number of RM's in this low-latitude range of the Galactic sky is comparable to those in the combined Canadian Galactic Plane Survey(CGPS) at $|b|<4^{\circ }$ and the Southern Galactic Plane (SGPS) $|b|<1.5^{\circ }$ survey. We report newly revealed, remarkably coherent patterns of RM at $|b|$ $\lesssim 15^{\circ }$ from $l\sim 270^{\circ }$ to $\sim 90^{\circ }$ 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 RM's at $|b|\lesssim 3^{\circ }$ ("mid-plane") and our new RM's up to $|b|\sim 15^{\circ }$ ("near-plane"). Our analysis also shows the approximate $z$-height -- the vertical height of the coherent component of the disc field above the Galactic disc's mid-plane -- to be $\sim 1.5$kpc out to $\sim 6$ kpc from the Sun. This identifies the approximate height of the transition layer to the halo field structure. We find no RM sign change across the plane within $|b|\sim 15^{\circ }$ in any longitude range. The prevailing {\it 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 $5.5^{\circ }\pm 1^{\circ }$ at all $|b|$ up to $\sim 12^{\circ }$ in the inner semicircle of Galactic longitudes. It decreases to $\sim 0^{\circ }$ toward the anticentre.

7 pages, 5 figures, Version 3. Accepted 2011 for publication in Publications of the Astronomical Society of Australia(PASA)

References in corpus (5)

• The Vertical Structure of Warm Ionised Gas in the Milky Way
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• Modeling the Magnetic Field in the Galactic Disk using New Rotation Measure Observations from the Very Large Array
• A Radio and Optical Polarization Study of the Magnetic Field in the Small Magellanic Cloud
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