HOT INTERSTELLAR GAS IN THE IRREGULAR GALAXY NGC-4449

NGC 4449 is an irregular galaxy with a moderately high star formation activity. The massive stars in NGC 4449 have given rise to many bright I-III regions, superbubbles, supergiant shells, and ''chimney-like'' radial filaments. ROSAT X-ray observations of NGC 4449 have revealed f our point-like sources and a wide-spread diffuse emission. The spectra l properties of the diffuse component suggest that the emission origin ates from hot interstellar gas. We have compared deep ground-based H a lpha images with the X-ray images of NGC 4449 to determine the relatio nship between the hot (10(6) K) and the warm (10(4) K) components of t he interstellar gas. We have also used an archival Hubble Space Telesc ope WFPC2 image of NGC 4449 taken through the F606W filter to examine the massive stellar content of the X-ray-emission regions. We find tha t hot interstellar gas exists in (1) active star forming regions, incl uding the giant H II region CM 16, (2) probable outflows from star for ming regions, and (3) the supergiant shell SGS2. The X-ray data have b een used to derive the rms electron density, mass, and thermal energy of the hut interior of SGS2. Finally, we discuss the origin of SGS2 an d implications of the detection of diffuse X-rays in irregular galaxie s. (C) 1997 American Astronomical Society.