Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...448..715r&link_type=abstract
Astrophysical Journal v.448, p.715
Astronomy and Astrophysics
Astronomy
61
Ism: Clouds, Ism: General, Ism: H Ii Regions, Radio Lines: Ism
Scientific paper
We compare velocity-resolved maps of the interstellar Hα 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 Hα background and 10%-30% of the 21 cm emission are spatially and kinematically associated with "clouds" containing both neutral and ionized hydrogen. These Hα-emitting H I clouds, identified as prominent Hα 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-Hα-emitting H I. The high |z| Hα-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 Hα surface brightnesses imply that at |z| ≍ 1 kpc the strength of the incident Lyman continuum flux 4πJ ≍ 2 × 106 photons cm-2 s-1.
Heiles Carl
Kung David T.
McCullough Peter R.
Reynolds Ron J.
Tufte Stephen Louis
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