STARBURST-DRIVEN OUTFLOWS IN LOW-LUMINOSITY GALAXIES - ASCA AND ROSATOBSERVATIONS 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 lumin osity about 10% of that of a fiducial L galaxy. This object has been extensively studied in the radio, infrared, optical, and ultraviolet d omains. 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 N GC 4449 comprises at least three pointlike X-ray sources superposed on a diffuse X-ray component. These pointlike sources, which are positio nally coincident with groups of bright H II regions and/or supernova r emnants, can account for about 50% of the total counts deriving from N GC 4449 in the 0.5-2.0 keV energy band. The diffuse X-ray component sh ows an irregular and asymmetric morphology. This component can be dete cted (>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 alph a 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 den sity 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 unabs orbed 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 component s 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 remnan ts, while the soft and very soft components are due largely to hot, di ffuse 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 l ow temperature, large mass, and large luminosity of this gas, Each sup ernova must heat an average of a few hundred M. of ambient interstella r gas (the flow must be strongly ''mass loaded''). In principle, the X -ray gas is hot enough to escape the gravitational potential well of t he galaxy, and in so doing could carry away much of the newly metal-en riched gas. However, it is not clear whether this hot gas can actually ''blow out'' of the extensive H I envelope of NGC 4449.