A Comparison of Diffuse Ionized and Neutral Hydrogen Away from the Galactic Plane: H alpha -emitting H i Clouds

Abstract

view Abstract Citations (77) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Comparison of Diffuse Ionized and Neutral Hydrogen Away from the Galactic Plane: H alpha -emitting H i Clouds Reynolds, R. J. ; Tufte, S. L. ; Kung, D. T. ; McCullough, P. R. ; Heiles, C. Abstract 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. Publication: The Astrophysical Journal Pub Date: August 1995 DOI: 10.1086/175999 Bibcode: 1995ApJ...448..715R Keywords: ISM: CLOUDS; ISM: GENERAL; ISM: H II REGIONS; RADIO LINES: ISM full text sources ADS | data products SIMBAD (1)