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The Galactic Halo Ionising Field

Published online by Cambridge University Press:  05 March 2013

J. Bland-Hawthorn  and
P. R. Maloney
J. Bland-Hawthorn
Affiliation:
Anglo-Australian Observatory, PO Box 296, Epping, NSW 2121, Australia; [email protected]
P. R. Maloney
Affiliation:
CASA, University of Colorado, Boulder, CO 80309-0389, USA
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Abstract

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There has been much debate in recent decades as to what fraction of ionising photons from star-forming regions in the Galactic disk escape into the halo. The recent detection of the Magellanic Stream in optical line emission at the CTIO 4 m and the AAT 3·9 m telescopes may now provide the strongest evidence that at least some of the radiation escapes the disk completely. We present a simple model to demonstrate that, while the distance to the Magellanic Stream is uncertain, the observed emission measures (εm ≈ 0·5 – 1 cm−6 pc) are most plausibly explained by photoionisation due to hot, young stars. This model requires that the mean Lyman-limit opacity perpendicular to the disk is τLL ≈ 3, and the covering fraction of the resolved clouds is close to unity. Alternative sources (e.g. shock, halo, LMC or metagalactic radiation) contribute negligible ionising flux.

Keywords

interstellar mediumintergalactic mediumindividual object: Magellanic StreamGalaxy: corona, halointerferometry
Type
Research Article
Information
Publications of the Astronomical Society of Australia , Volume 14 , Issue 1 , 1997 , pp. 59 - 63
Copyright
Copyright © Astronomical Society of Australia 1997

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