The Exceptionally Soft X-Ray Spectrum of the Low-Mass Starburst Galaxy NGC 1705

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Abstract
NGC 1705 is one of the optically brightest and best studied dwarf galaxies. It appears to be in the late stage of a major starburst and contains a young super-star cluster. Type II supernovae are therefore likely to have been a major event in the recent evolution of this galaxy and are likely to have produced a superbubble whose effects on the low-density ambient interstellar medium can be ideally studied. ROSAT PSPC observations of this galaxy reveal two striking blobs of X-ray emission embedded in Hα loops that can be interpreted as both sides of the upper plumes of the same superbubble. These sources are a surprise. They are much softer than those observed from other starburst dwarf galaxies and are so soft that they should have been blocked if the observed Galactic H I column density were uniformly distributed across NGC 1705 or if the sources were embedded in the H I disk of NGC 1705. In addition, the total X-ray luminosity in the ROSAT energy band of 1.2×1038 ergs s-1 is low in comparison with similar objects. We discuss possible models for the two X-ray peaks in NGC 1705 and find that the sources most likely originate from the relatively cool gas of one single superbubble in NGC 1705. The implications of the exceptional softness of these sources are addressed in terms of the intrinsic properties of NGC 1705 and the nature of the foreground Galactic absorption.
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