v 1 1 5 Ju l 1 99 8 WHAM Observations of H α 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 in the M, A, and C complexes, with most of the observations on the M I cloud. We present results including clear detections of Hα emission from all three complexes with intensities ranging from 0.06 to 0.20 R. In every observed direction where there is significant high velocity H I gas seen in 21 cm emission we have found associated ionized hydrogen emitting the Hα line. The velocities of the Hα and the 21 cm emissions are well correlated in every case except one, but the intensities are not correlated. There is some evidence that the ionized gas producing the Hα emission envelopes the 21 cm emitting neutral gas but the Hα " halo " , if present, is not large. If the Hα emission arises from the photoionization of the H I clouds, then the implied incident Lyman continuum flux F LC at the location of the clouds ranges from 1.3 to 4.2 × 10 5 photons cm −2 s −1. If, on the other hand, the ionization is due to a shock arising from the collision of the high-velocity gas with an ambient medium in the halo, then the density of the pre-shocked gas can be constrained. We have also detected the [S II] λ6716 line from the M I cloud and have evidence that the [S II] λ6716 to Hα ratio varies with location on the cloud.