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

Abstract

We present a multi--wavelength study of a supergiant shell within the violent interstellar medium of the nearby dwarf galaxy IC 2574, a member of the M81 group of galaxies. Neutral hydrogen (HI) observations obtained with the Very Large Array (VLA) reveal a prominent expanding supergiant HI shell which is thought to be produced by the combined effects of stellar winds and supernova explosions. It measures roughly 1000 x 500 pc in size and is expanding at about 25 km/s. The HI data suggest an age of about 1.4 x 10^6 yrs; the energy input must have been of order (2.6\pm 1) x 10^53 ergs. Massive star forming regions, as traced by H$\alpha$ emission, are situated predominantly on the rim of this HI shell. VLA radio continuum observations at 6 cm show that these star-forming regions are the main sources of radio continuum emission in this galaxy. Soft X-ray emission from within the HI hole is detected by a pointed ROSAT PSPC observation. The emission is resolved, coinciding in size and orientation with the HI shell. These spatial properties suggest that the emission is generated by an X-ray emitting plasma located within the HI shell although a contribution from X-ray binaries cannot be completely ruled out. The X-ray data are compatible with emission coming from a Raymond & Smith plasma at a temperature of about log(T[K]) = 6.8 and a density of 0.03 cm^-3.