Starburst‐driven Outflows in Low‐Luminosity Galaxies:ASCAandROSATObservations of NGC 4449

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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 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 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 very 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 soft" component), kT ~ 0.8 keV (the "soft" component), and kT ~ 3.6 keV (the "hard" 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 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 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.