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Original article can be found at: http://adsabs.harvard.edu/abs/ Copyright American Astronomical Society. DOI: 10.1086/172780 [Full text of this article is not available in the UHRA]The B array of the VLA has observed a sample of seven H II galaxies at 325 and 1489 MHz; the radio continuum spectra thus obtained exhibit a wide variety of spectral shapes, but four show a significant flattening toward lower frequencies. A detailed discussion is conducted of this LF flattening, invoking several radio emission and absorption mechanisms and energy losses of relativistic electrons. Such model-dependent parameters as the spectral index of injected electrons, the radio-emission spectral index, the thermal electron density, and the age of the relativistic electrons, are derived

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 Ϫ1 . The H i data suggest 1000 # 500 an age of ∼ yr; the energy input must have been of order ergs. Massive star-forming 6 531.4 # 10 (2.6 ‫ע‬ 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 ϭ 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 L ϭ (1.6 ‫ע‬ 0.5) # 10 X ergs s Ϫ1 . The X-ray data are compatible with emission coming from a Raymond-Smith plasma at a temperature of about and a density of cm Ϫ3 . The energy content of the coronal gas corresponds log (T [K]) ϭ 6.8 n ∼ 0.03 e to ergs, or broadly in agreement with the energy input derived on the basis of the H i observations. 53(4 ‫ע‬ 2) # 10

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