Observational evidence of supershell blowout in GS 018-04+44: The Scutum Supershell

Emission in the H I 21 cm line has been mapped for a region of the Galaxy t hat includes two known supershells, GS 018 -04+44 and GS 034-06+65. We focu s on the GS 018 -04+44, hereafter referred to as the Scutum Supershell, whi ch is an elongated shell about 5 degrees in diameter extending to -7 degree s below the Galactic plane. The Scutum shell lies at a kinematic distance o f similar to 3300 pc, implying a shell diameter of similar to 290 pc with a vertical extension of similar to 400 pc away from the Galactic plane. The Scutum shell contains 6.2 x 10(5) M. swept into the walls. We observe that the top of the shell is missing, and a substantial column of H I rises from the shell walls to b = -11 degrees, culminating in a large cloud of neutra l hydrogen, 3.74 x 10(4) M., located similar to 630 pc from the plane. ROSA T data show X-ray emission that closely anticorrelates with the 21 cm emiss ion. This emission probably originates from hot gas within the Scutum Super shell. After approximately correcting for the foreground absorption, we fin d that the 1.5 keV X-rays peak at the base of the shell, the 0.75 keV emiss ion peaks in the interior and at the top of the shell, and the 0.25 keV emi ssion extends to high latitudes above the shell. The X-ray luminosity is ro ughly similar to 5 x 10(36) ergs s(-1). The Wisconsin II-Alpha Mapper (WHAM ) survey shows the presence of Hot emission that exhibits a morphology simi lar to that of the H I. Spectra indicate the presence of ionized hydrogen a t velocities similar to the H I, placing ionized material at the same kinem atic distance as the neutral material. IRAS images in the 60 and 100 mu m w avebands reveal the presence of dust correlated with the neutral hydrogen. Infrared surface brightness indicates an excess in the 100 mu m emission, w hich could indicate a molecular hydrogen component with a column density of 2.4 x 10(21) cm(-2) in the densest regions of the high-latitude cloud of n eutral hydrogen. IUE ultraviolet high dispersion spectra of HD 177989 (l = 17 degrees 89, b = -11 degrees 88) and HD 175754 (l = 16 degrees 40, b = -9 degrees 92) reveal the presence of very strong absorption by highly ionize d gas at a velocity that associates the absorption with the ejecta of the S cutum Supershell. In the case of HD 177989, the high ion column density rat ios suggest an origin in a turbulent mixing layer where hot and cool gases mix in the presence of shear flows. The Ha and X-ray emission suggest that a multitude of energetic phenomena exist in this region, providing the nece ssary ionizing radiation. Indeed, there are multiple supernova remnants, H II regions, and hot stars, which could all contribute sizeable amounts of e nergy and ionizing radiation. The combination of these data sets indicates observational evidence of a "blowout" phenomena whereby hot material produc ed within the Scutum Supershell has blown through the top of the shell and been pushed to high latitude.