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Title:
A Comparison of Diffuse Ionized and Neutral Hydrogen Away from the Galactic Plane: H alpha -emitting H i Clouds
Authors:
Reynolds, R. J.; Tufte, S. L.; Kung, D. T.; McCullough, P. R.; Heiles, C.
Publication:
Astrophysical Journal v.448, p.715 (ApJ Homepage)
Publication Date:
08/1995
Origin:
APJ; KNUDSEN
Astronomy Keywords:
ISM: CLOUDS, ISM: GENERAL, ISM: H II REGIONS, RADIO LINES: ISM
DOI:
10.1086/175999
Bibliographic Code:
1995ApJ...448..715R

Abstract

We compare velocity-resolved maps of the interstellar Halpha emission at 1° angular resolution within a 10° × 12° region of the sky, and 3' resolution within a 1° × 1° region, with corresponding maps of 21 cm emission. The results indicate that at least 30% of the Halpha background and 10%-30% of the 21 cm emission are spatially and kinematically associated with "clouds" containing both neutral and ionized hydrogen. These Halpha-emitting H I clouds, identified as prominent Halpha and 21 cm emission enhancements of several degrees extent on narrow (12 km s-1) velocity interval maps, have H I column densities that range from about 2 × 1019 cm-2 to 2 × 1020 cm-2 and emission measures from 2 cm-6 pc to 10 cm-6 pc. Their radial velocities suggest distances |z| from the Galactic midplane that range from |z| ≲ 100 pc to |z| ≈ 1 kpc, with a |z|-distribution that is much thicker than that of the non-Halpha-emitting H I. The high |z| Halpha-emitting H I clouds identified on these maps are approximately 40% ionized, have densities n ≈ 0.2-0.3 cm-3, and are associated with large filament and loop structures. The relationship between the Ho and the H+ within these clouds is not clear; however, the available data suggest that the neutral and ionized components of each cloud are spatially separated. If the clouds are photoionized, the Halpha surface brightnesses imply that at |z| ≈ 1 kpc the strength of the incident Lyman continuum flux 4piJ ≈ 2 × 106 photons cm-2 s-1.

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