Astronomy and Astrophysics – Astrophysics
Scientific paper
2001-10-01
ASP Conf.Ser.240:369-382,2001
Astronomy and Astrophysics
Astrophysics
14 pages, 6 figures; delayed submission due to embargo on HIPASS map. To appear in Gas & Galaxy Evolution, ASP Conf. 240, 369-
Scientific paper
Significant numbers of high-velocity HI clouds (HVCs) have now been detected in H-alpha, with a subset seen in low ionization lines (e.g. [NII]). It was originally hoped that the observed H-alpha strength would provide a distance constraint to individual clouds. This idea requires that a useful fraction (f_esc > 1%) of ionizing photons escape the Galaxy, and that the halo ionizing field is relatively smooth, as we discuss. Most HVCs which are known to be close to the Sun are H-alpha; the brightest clouds also show enhanced [NII] emission, in contrast to the Magellanic Stream where the low ionization emission lines are always weak compared to H-alpha. But an acute complication for H-alpha distances is the apparent H-alpha brightness of the Magellanic Stream along several sight lines. To account for this, we present three possible configurations for the Magellanic Stream and propose a follow-up experiment. If we normalize the distances to local HVCs, some HVCs appear to be scattered throughout the Galactic halo on scales of tens of kiloparsecs.
Bland-Hawthorn Jonathan
Putman Mary E.
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