ASCA observations of starbursting dwarf galaxies: The case of NGC 1569

We present ASCA observations of the nearby star-forming dwarf galaxy NGC 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 results 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 spectral 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 component 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 total 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 soft 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 total 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 extended, is morphologically associated with the system of H alpha filaments (in which NGC 1559 is immersed), and seems to extend preferentially along the optical minor axis of the galaxy. In the azimuthally averaged 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-ray source along with diffuse X-ray emission described by a bidimensional 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 pointlike source and the remainder from the diffuse emission. The hard (E > 2 keV) X-ray emission is consistent with a pointlike source (the central starburst) at the spatial resolution of the ASCA XRT + SIS system. 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 brightest knots of X-ray emission. If this is the case, the spectral shape that we observe for the hard spectral component suggests an origin of the X-ray emission in young supernova remnants or in low-mass X-ray binaries. The new ASCA data provide crucial support for the idea that starbursts 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. Second, they show that the temperature of this gas far exceeds the depth 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 are too soft to contribute significantly to the hard (>2 keV) cosmic X-ray background. These objects contribute in a major way to the soft X-ray background only.