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The Warm Ionised Medium in Spiral Galaxies: A View from Above

Published online by Cambridge University Press:  16 May 2016

René A. M. Walterbos
René A. M. Walterbos*
Affiliation:
Astronomy Department, New Mexico State University, MSC 4500, Box 30001, Las Cruces, NM 88003, USA; rwalterb@nmsu.edu
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Abstract

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The warm ionised medium (WIM), also referred to as diffuse ionised gas (DIG), contains most of the mass of interstellar medium in ionised form, contributing as much as 30% of the total atomic gas mass in the solar neighborhood. The advent of CCDs has enabled unprecedented study of this medium in external galaxies, probing a variety of environments. In particular, we can derive the morphology of the WIM, its distribution across disks, and the correlation with other Population I material. Spectroscopy of the WIM makes it possible to test various ionisation models. I will review here our current understanding of the properties of the WIM in spiral galaxies. A perhaps unexpected result is that the Hα emission from the WIM contributes about 40% of the total observed Hα luminosity from spirals. This places severe constraints on possible sources of ionisation, since only photoionisation by OB stars meets this requirement. Spectroscopic measurements of forbidden line strengths appear in reasonable agreement with photoionisation models. It is not yet clear if the Lyman continuum photons that ionise the WIM are mostly from OB stars located inside traditional HII regions, or from field OB stars.

Keywords

galaxies: Local GroupspiralISM: HII regionsbubblesultraviolet: stars
Type
Research Article
Information
Publications of the Astronomical Society of Australia , Volume 15 , Issue 1 , 1998 , pp. 99 - 102
Copyright
Copyright © Astronomical Society of Australia 1998

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