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 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 ~3300 pc, implying a shell diameter of ~290 pc with a vertical extension of ~400 pc away from the Galactic plane. The Scutum shell contains 6.2 × 10⁵ 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^°, culminating in a large cloud of neutral hydrogen, 3.74 × 10⁴ M_☉, located ~630 pc from the plane. ROSAT data show X-ray emission that 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 find 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 ~5 × 10³⁶ ergs s^-1. The Wisconsin H-Alpha Mapper (WHAM) survey shows the presence of Hα 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 μm wavebands reveal the presence of dust correlated with the neutral hydrogen. Infrared surface brightness indicates an excess in the 100 μm 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 177989 (l = 1789, b = -1188) and HD 175754 (l = 1640, b = -992) reveal the presence of very 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 flows. The Hα 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" phenomena whereby hot material produced within the Scutum Supershell has blown through the top of the shell and been pushed to high latitude.