Optical emission from high velocity clouds and the ionization sources in the Galactic halo

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dc.creatorWeiner, Benjamin J.
dc.creatorVogel, Stuart N.
dc.creatorWilliams, T. B.
dc.date2001-09-05
dc.date.accessioned2026-07-07T01:59:50Z
dc.date.available2026-07-07T01:59:50Z
dc.descriptionOptical emission lines have now been detected from about 20 high velocity clouds. These emission lines -- primarily H-alpha, secondarily [N II] and [S II] -- are very faint and diffuse, spread over the surfaces of the clouds. We compile emission line measurements and present a model in which the H-alpha is recombination caused by photoionizing radiation escaping the Milky Way. In such a model, we infer HVC distances of 5--30 kpc. The photoionization model fails to explain the relatively strong H-alpha emission from the Magellanic Stream, and the O VI absorption seen by FUSE in HVCs and the MS, which require a second source of ionization (likely collisional). Regardless of mechanism, the fact that HVCs are detectable in H-alpha indicates they are not far away enough to be Local Group objects. Adopting the HVC distances from the model, there appear to be two classes of HVCs: H-alpha-bright clouds with low velocity deviations from Galactic rotation, and often strong [N II], which are presumably affiliated with the Galactic disk; and H-alpha-faint clouds with high velocity deviations, which are likely to be infalling gas.
dc.description10 pages, 5 figures, uses newpasp.sty. To appear in the proceedings of "Extragalactic Gas at Low Redshift," J. Mulchaey and J. Stocke, eds
dc.identifierhttps://arxiv.org/abs/astro-ph/0109055
dc.identifierhttp://arxiv.org/abs/astro-ph/0109055
dc.identifier.urihttp://salesiana.dossiersoluciones.com/handle/123456789/3275
dc.subjectAstrophysics
dc.titleOptical emission from high velocity clouds and the ionization sources in the Galactic halo
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