X-ray emission from an expanding supergiant shell in IC 2574
We present a multiwavelength study of a supergiant shell within the violent interstellar medium of the nearby dwarf galaxy IC 2574, which is a member of the M81 group of galaxies. Neutral hydrogen (H i) observations obtained with the Very Large Array (VLA) reveal a prominent expanding supergiant H i shell in the northeast quadrant of IC 2574 which is thought to be produced by the combined effects of stellar winds and supernova explosions. It measures roughly 1000#500 pc in size and is expanding at »25 km s21. The H i data suggest an age of »1.4#106 yr; the energy input must have been of order (2.651)#1053 ergs. Massive star-forming regions, as traced by Ha emission, are situated predominantly on the rim of this H i shell. This supports the view that the accumulated H i on the rim has reached densities that are high enough for secondary star formation to commence. VLA radio continuum observations at l 5 6 cm show that these star-forming regions are the main sources of radio continuum emission in this galaxy. This emission is mainly thermal in origin. Soft X-ray emission from within the H i hole is detected by a pointed ROSAT PSPC observation. The emission is resolved, coinciding in size and orientation with the H i shell. These spatial properties suggest that the emission is generated by an X-ray–emitting plasma located within the H i shell, although a contribution from X-ray binaries cannot be completely ruled out. The X-ray luminosity within the 0.11–2.4 keV energy range is L 5 (1.650.5)#1038 X ergs s21. The X-ray data are compatible with emission coming from a Raymond-Smith plasma at a temperature of about log (T [K]) 5 6.8 and a density of n » 0.03 cm23. The energy content of the coronal gas corresponds e to (452)#1053 ergs, or broadly in agreement with the energy input derived on the basis of the H i observations.