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.
The neutral and ionized gas generally trace each other quite well in the complex, but the detailed structure is not identical. In particular, the Ha emission peaks in brightness at slightly higher Galactic longitudes than corresponding 21 cm features. The ionized gas has a peak Ha intensity of 0.5 R, corresponding to an emission measure of 1.1 cm Ϫ6 pc. Structures in the complex are traced by WHAM down to about 0.1 R (0.2 cm Ϫ6 pc). Typical line widths of the Ha emission are ∼30 km s Ϫ1 , limiting temperatures in the ionized gas to less than 20,000 K. If radiation is the primary ionizing mechanism, the Lyman continuum flux required to sustain the most strongly emitting ionized regions is photons cm Ϫ2 s Ϫ1 . There appears to be no local, stellar source 6 1.2 # 10 capable of maintaining the ionization of the gas; however, the required ionizing flux is consistent with current models of the escape of Lyman continuum radiation from OB stars in the disk and of ionizing radiation produced by cooling supernova remnants.
Emission in the H I 21 cm line has been mapped for a region of the Galaxy that includes two known supershells, GS 018 [04] 44 and GS 034[06]65. We focus on the GS 018 [04]44, hereafter referred to as the Scutum Supershell, which is an elongated shell about 5¡ in diameter extending to [7¡ below the Galactic plane. The Scutum shell lies at a kinematic distance of D3300 pc, implying a shell diameter of D290 pc with a vertical extension of D400 pc away from the Galactic plane. The Scutum shell contains 6.2 ] 105 swept into the walls. We observe that the top of the shell is missing, and a substan-M _ tial column of H I rises from the shell walls to b \ [11¡, culminating in a large cloud of neutral hydrogen, 3.74 ] 104 located D630 pc from the plane. ROSAT data show X-ray emission that M _ , closely anticorrelates with the 21 cm emission. This emission probably originates from hot gas within the Scutum Supershell. After approximately correcting for the foreground absorption, we Ðnd that the 1.5 keV X-rays peak at the base of the shell, the 0.75 keV emission peaks in the interior and at the top of the shell, and the 0.25 keV emission extends to high latitudes above the shell. The X-ray luminosity is roughly D5 ] 1036 ergs s~1. The Wisconsin H-Alpha Mapper (WHAM) survey shows the presence of Ha emission that exhibits a morphology similar to that of the H I. Spectra indicate the presence of ionized hydrogen at velocities similar to the H I, placing ionized material at the same kinematic distance as the neutral material. IRAS images in the 60 and 100 km wavebands reveal the presence of dust correlated with the neutral hydrogen. Infrared surface brightness indicates an excess in the 100 km emission, which could indicate a molecular hydrogen component with a column density of 2.4 ] 1021 cm~2 in the densest regions of the high-latitude cloud of neutral hydrogen. IUE ultraviolet high dispersion spectra of HD 177989and HD 175754 reveal the presence of very92) strong absorption by highly ionized gas at a velocity that associates the absorption with the ejecta of the Scutum Supershell. In the case of HD 177989, the high ion column density ratios suggest an origin in a turbulent mixing layer where hot and cool gases mix in the presence of shear Ñows. The Ha and X-ray emission suggest that a multitude of energetic phenomena exist in this region, providing the necessary ionizing radiation. Indeed, there are multiple supernova remnants, H II regions, and hot stars, which could all contribute sizeable amounts of energy and ionizing radiation. The combination of these data sets indicates observational evidence of a "" blowout ÏÏ phenomena whereby hot material produced within the Scutum Supershell has blown through the top of the shell and been pushed to high latitude.
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