X-Ray Emission from an Expanding Supergiant Shell in IC 2574

Abstract

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 pc in size and is expanding at »25 km s 21 . The H i data suggest 1000 # 500 an age of » yr; the energy input must have been of order ergs. Massive star-forming 6 53 1.4 # 10 (2.6 5 1) # 10 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 cm show that these star-forming regions are the main l 5 6 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 38