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The Vertical Structure of Warm Ionised Gas in the Milky Way

Published online by Cambridge University Press:  05 March 2013

B. M. Gaensler ,
G. J. Madsen ,
S. Chatterjee  and
S. A. Mao
B. M. Gaensler*
Affiliation:
Institute of Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia
G. J. Madsen
Affiliation:
Institute of Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia Department of Astronomy, University of Wisconsin, Madison, WI 53706, USA
S. Chatterjee
Affiliation:
Institute of Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia
S. A. Mao
Affiliation:
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
*
DCorresponding author. Email: bgaensler@usyd.edu.au
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Abstract

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We present a new joint analysis of pulsar dispersion measures and diffuse Hα emission in the Milky Way, which we use to derive the density, pressure and filling factor of the thick disk component of the warm ionised medium (WIM) as a function of height above the Galactic disk. By excluding sightlines at low Galactic latitude that are contaminated by Hii regions and spiral arms, we find that the exponential scale-height of free electrons in the diffuse WIM is 1830–250+120 pc, a factor of two larger than has been derived in previous studies. The corresponding inconsistent scale heights for dispersion measure and emission measure imply that the vertical profiles of mass and pressure in the WIM are decoupled, and that the filling factor of WIM clouds is a geometric response to the competing environmental influences of thermal and non-thermal processes. Extrapolating the properties of the thick-disk WIM to mid-plane, we infer a volume-averaged electron density 0.014 ± 0.001 cm−3, produced by clouds of typical electron density 0.34 ± 0.06 cm−3 with a volume filling factor 0.04 ± 0.01. As one moves off the plane, the filling factor increases to a maximum of ∼30% at a height of ≈1–1.5 kpc, before then declining to accommodate the increasing presence of hot, coronal gas. Since models for the WIM with a ≈1 kpc scale-height have been widely used to estimate distances to radio pulsars, our revised parameters suggest that the distances to many high-latitude pulsars have been substantially underestimated.

Keywords

galaxies: ISMGalaxy: halo, structureglobular clusters: generalISM: structurepulsars: general
Type
Research Article
Information
Publications of the Astronomical Society of Australia , Volume 25 , Issue 4 , 2008 , pp. 184 - 200
Copyright
Copyright © Astronomical Society of Australia 2008

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