ASCA OBSERVATIONS OF STARBURSTING DWARF GALAXIES - THE CASE OF NGC-1569

We present ASCA observations of the nearby star-forming dwarf galaxy N GC 1569. Combining the ASCA data with archival ROSAT PSPC and HRI data , optical broad and narrow Ha images, and new infrared K-band images, we have investigated, in greater detail than previously possible, the X-ray properties of this prototype dwarf starburst. The principal resu lts of this investigation are as follows: 1. The ASCA SIS broad-band ( 0.5-6 keV) X-ray spectrum reveals the presence of at least two spectra l components. The soft component is best described by a thermal model with a temperature of similar to 0.64-0.8 keV, while the hard componen t can be described by a thermal model with a temperature of similar to 3.7 keV or by a power law with photon index of similar to 2.1. The to tal unabsorbed X-ray luminosity is similar to 3.1 x 10(38) ergs s(-1) (0.5-2.0 keV) and similar to 1.3 x 10(38) ergs s(-1) (2-10 keV). The s oft thermal component provides similar to 60% of the total luminosity in the (0.5-2.0) keV energy band but less than similar to 10% of the t otal luminosity in the 2-10 keV energy band.2. The ROSAT PSPC image of NGC 1569 shows that the soft (E < 2 keV) X-ray emission is clearly ex tended, is morphologically associated with the system of H alpha filam ents (in which NGC 1559 is immersed), and seems to extend preferential ly along the optical minor axis of the galaxy. In the azimuthally aver aged and background-corrected radial profile, the soft X-ray emission (from the ROSAT PSPC) can be detected out to a radius of similar to 1. '9 (similar or equal to 1.2 kpc); the half-light radius is similar to 28 degrees (0.29 kpc). The soft X-ray emission profile (from the ASCA SIS) can be described by a simple model consisting of a pointlike X-ra y source along with diffuse X-ray emission described by a bidimensiona l Gaussian distribution with sigma similar to 30 '' (similar or equal to 0.31 kpc). About 40% of the soft X-ray emission derives from the po intlike source and the remainder from the diffuse emission. The hard ( E > 2 keV) X-ray emission is consistent with a pointlike source (the c entral starburst) at the spatial resolution of the ASCA XRT + SIS syst em. 3. The ROSAT HRI image resolves the central starburst into a group of at least four bright knots of X-ray emission. Almost all the flux in the hard spectral component could be associated with the two bright est knots of X-ray emission. If this is the case, the spectral shape t hat we observe for the hard spectral component suggests an origin of t he X-ray emission in young supernova remnants or in low-mass X-ray bin aries. The new ASCA data provide crucial support for the idea that sta rbursts can drive gas out of dwarf galaxies. First, the data establish that the diffuse X-ray emission in NGC 1569 is produced by hot gas. S econd, they show that the temperature of this gas far exceeds the dept h of the galaxy's shallow potential well. The data also suggest that a population of galaxies like NGC 1569 would have X-ray spectra that ar e too soft to contribute significantly to the hard (>2 keV) cosmic X-r ay background. These objects contribute in a major way to the soft X-r ay background only.