WHAM Observations of Hα Emission from High‐Velocity Clouds in the M, A, and C Complexes

The first observations of the recently completed Wisconsin H-Alpha Mapper(WHAM) facility include a study of emission lines from high velocity clouds inthe M, A, and C complexes, with most of the observations on the M I cloud. Wepresent results including clear detections of H-alpha emission from all threecomplexes with intensities ranging from 0.06 R to 0.20 R. In every observeddirection where there is significant high velocity H I gas seen in the 21 cmline we have found associated ionized hydrogen emitting the H-alpha line. Thevelocities of the H-alpha and 21 cm emission are well correlated in every caseexcept one, but the intensities are not correlated. There is some evidence thatthe ionized gas producing the H-alpha emission envelopes the 21 cm emittingneutral gas but the H-alpha "halo", if present, is not large. If the H-alphaemission arises from the photoionization of the H I clouds, then the impliedLyman continuum flux F_{LC} at the location of the clouds ranges from 1.3 to4.2 x 10^5 photons cm^{-2} s^{-1}. If, on the other hand, the ionization is dueto a shock arising from the collision of the high-velocity gas with an ambientmedium in the halo, then the density of the pre-shocked gas can be constrained.We have also detected the [S II] 6716 angstrom line from the M I cloud and haveevidence that the [S II] to H-alpha ratio varies with location on the cloud.Comment: 32 pages, 18 figures, to appear in ApJ (Sept. 10, 1998