Abstract
NGC 4449 is a nearby (D = 5.55 Mpc for H0 = 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 (E 2.5 σ) up to about 3' (~4.8 kpc) from the center of the galaxy and appears to be embedded inside the filamentary Hα envelope in which NGC 4449 is immersed. The overall 0.2-6 keV X-ray spectrum of NGC 4449 is complex. Assuming an absorbing column density along the line of sight equal to the Galactic value (NHGal ≡ 1.4×1020 cm-2), it is best described by a model with at least three thermal components with temperatures of kT ~ 0.25 keV (the very component), kT ~ 0.8 keV (the soft component), and kT ~ 3.6 keV (the component). The total unabsorbed X-ray luminosity of NGC 4449 is about 3.1 × 1039 ergs s-1 in the 0.5-2.0 keV energy band and about 1.8 × 1039 ergs s-1 in the 2.0-10.0 keV energy band. The and the 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 and 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 107 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.
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