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Faint Optical Line Emission from the Diffuse Interstellar Medium: Observations and Implications

Published online by Cambridge University Press:  08 February 2017

R. J. Reynolds
R. J. Reynolds*
Affiliation:
The University of Wisconsin, Madison Department of Physics 1150 University Avenue Madison, WI 53706 USA

Abstract

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Diffuse galactic Hα emission appears to cover the entire sky with an intensity that ranges from 3–12 R near the galactic equator to 0.25–0.8 R near the galactic poles. Observations of this H-recombination line and the forbidden lines, [S II] 6716 Å, [N II] 6583 Å, and [O III] 5007 Å, indicate that the emission originates from a low-density, 2–3 kpc thick layer of warm (~104 K), ionized interstellar gas that has an emission-line spectrum significantly different from that of the traditional, more localized H II regions. Along a line perpendicular to the galactic disk, the mean emission measure of this layer is 4.5 cm−6 pc, and the column density of the H+ is 2 × 1020 cm−2. The origin of this diffuse ionization is not yet clear; however, its existence requires the equivalent of about 14% of the total ionizing photon flux from O stars or nearly all of the power injected into the ISM by supernova. This optically emitting gas also may be a nonnegligible source of diffuse emission in the far ultraviolet (FUV) and infrared (IR).

Type
III. Diffuse Galactic Radiation from Dust and Gas: Observations and Models
Information
Symposium - International Astronomical Union , Volume 139: The Galactic and Extragalactic Background Radiation , 1990 , pp. 157 - 169
Copyright
Copyright © Kluwer 1990 

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