Starburst-driven outflows in low-luminosity galaxies: ASCA and ROSAT observations of NGC 4449

NGC 4449 is a nearby (D = 5.55 Mpc for H-0 = 50 km s(-1) Mpc(-1)) high surface brightness Magellanic-type irregular galaxy with a blue luminosity about 10% of that of a fiducial L* galaxy. This object has been extensively studied in the radio, infrared, optical, and ultraviolet domains. In this paper we investigate and discuss its X-ray properties, using a data set comprised of ROSAT PSPC images and spectra, and ASCA spectra from both SIS and GIS instruments. At the spatial resolution of the ROSAT PSPC detector, the soft (E < 2 keV) X-ray emission from NGC 4449 comprises at least three pointlike X-ray sources superposed on a diffuse X-ray component. These pointlike sources, which are positionally coincident with groups of bright H II regions and/or supernova remnants, can account for about 50% of the total counts deriving from NGC 4449 in the 0.5-2.0 keV energy band. The diffuse X-ray component shows an irregular and asymmetric morphology. This component can be detected (>2.5 sigma) up to about 3' (similar to 4.8 kpc) from the center of the galaxy and appears to be embedded inside the filamentary H alpha envelope in which NGC 4449 is immersed. The overall 0.2-6 keV X-ray spectrum of NGC 4449 is very complex. Assuming an absorbing column density along the line of sight equal to the Galactic value (N-HGal = 1.4 x 10(20) cm(-2)), it is best described by a model with at least three thermal components with temperatures of kT similar to 0.25 keV (the ''very soft'' component), kT similar to 0.8 keV (the ''soft'' component), and kT similar to 3.6 keV (the ''hard'' component). The total unabsorbed X-ray luminosity of NGC 4449 is about 3.1 x 10(39) ergs s(-1) in the 0.5-2.0 keV energy band and about 1.8 x 10(39) ergs s(-1) in the 2.0-10.0 keV energy band. The very soft and the soft thermal components provide approximately 34% and 18% of the total luminosity in the 0.5-2.0 keV energy band, respectively. Their overall contribution is less than about 3% in the 2-10.0 keV energy band. We suggest that the hard component is produced by X-ray binaries and/or young supernova remnants, while the soft and very soft components are due largely to hot, diffuse gas. We have modeled the diffuse thermal X-ray emission in terms of the steady outflow of gas that has been heated to a temperature of about 10(7) K by supernovae. In order to account for the relatively low temperature, large mass, and large luminosity of this gas, Each supernova must heat an average of a few hundred M. of ambient interstellar gas (the flow must be strongly ''mass loaded''). In principle, the X-ray gas is hot enough to escape the gravitational potential well of the galaxy, and in so doing could carry away much of the newly metal-enriched gas. However, it is not clear whether this hot gas can actually ''blow out'' of the extensive H I envelope of NGC 4449.