Hot Interstellar Gas in the Irregular Galaxy NGC 4449 1 Dominik

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 HII regions, superbubbles, supergiant shells, and “chimney-like” radial filaments. ROSAT X-ray observations of NGC 4449 have revealed four point-like sources and a wide-spread diffuse emission. The spectral properties of the diffuse component suggest that the emission originates from hot interstellar gas. We have compared deep ground-based Hα images with the X-ray images of NGC 4449 to determine the relationship between the hot (10 K) and the warm (10 K) components of the interstellar gas. We have also used an archival Hubble Space Telescope WFPC2 image of NGC 4449 taken through the F606W filter to examine the massive stellar content of the X-ray-emission regions. We find that hot interstellar gas exists in (1) active star forming regions, including the giant H II region CM 16, (2) probable outflows from star forming regions, and (3) the supergiant shell SGS2. The X-ray data have been used to derive the rms electron density, mass, and thermal energy of the hot interior of SGS2. Finally we discuss the origin of SGS2 and implications of the detection of diffuse X-rays in irregular galaxies. Subject headings: Galaxies: Irregular — Galaxies: NGC 4449 — Galaxies: ISM — Xrays: Galaxies 1Partly based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555. 2Feodor Lynen Fellow of the Alexander von Humboldt-Foundation 3Visiting Astronomer, Calar Alto Observatory