AFar Ultraviolet Spectroscopic ExplorerSurvey of Interstellar OviAbsorption in the Small Magellanic Cloud

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Abstract
We present the results of a Far Ultraviolet Spectroscopic Explorer (FUSE) survey of O VI λ1031.93 and λ1037.62 absorption toward 18 OB stars in the Small Magellanic Cloud (SMC). The FUSE data are of very high quality, allowing a detailed study of the coronal temperature gas in the SMC. We find that O VI is ubiquitous in the SMC, with a detection along every sight line. The average value of the O VI column density in the SMC is logN(O VI) = 14.53. This value is 1.7 times higher than the average value for the Milky Way halo (perpendicular to the Galactic plane) of log N⊥(O VI) = 14.29 found by FUSE, even though the SMC has much lower metallicity than the Galaxy. The column density in the SMC is higher along sight lines that lie close to star-forming regions, in particular NGC 346 in the northern part of the SMC, and to a lesser degree the southwestern complex of H II regions. This correlation with star formation suggests that local processes have an important effect on the distribution of coronal gas in the SMC. If the sight lines within NGC 346 are excluded, the mean column density for the SMC is log N(O VI) = 14.45, only 1.4 times higher than the Milky Way average. The standard deviation of the column densities for sight lines outside of NGC 346 is ±27%, somewhat lower than the deviation seen in the Milky Way halo. The lowest O VI column densities, log N(O VI)~14.3, occur in the central region and in the southeastern wing of the galaxy. Even these low column densities are as high as the Milky Way average, establishing the presence of a substantial, extended component of coronal gas in the SMC. The O VI absorption is always shifted to higher velocities than the main component of lower ionization gas traced by Fe II absorption. The O VI line widths are broader than expected for pure thermal broadening at 3 × 105 K, the temperature at which the O VI peaks in abundance, so large nonthermal motions or multiple hot gas components are likely present. We discuss several mechanisms that may be able to explain the observed properties of the hot gas, including supershells, a galactic fountain, and the infall of gas previously stripped from the SMC by tidal interactions with the Milky Way and the Large Magellanic Cloud. If a galactic fountain produces the hot gas, the mass flux per unit surface area is /Ω ~ 2 × 10-2 M☉ yr-1 kpc-2.