ROSAT OBSERVATIONS OF NGC-2146 - EVIDENCE FOR A STARBURST-DRIVEN SUPERWIND

We have imaged the edge-on starburst galaxy NGC 2146 with the Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI) on board ROSAT and have compared these data to optical images an d long-slit spectra. NGC 2146 possesses a very large X-ray nebula with a half-light radius of 1' (4 kpc) and a maximum diameter of similar t o 4', or 17 kpc. The X-ray emission is resolved by the PSPC and prefer entially oriented along the minor axis, with a total flux of 1.1 x 10( -12) ergs cm(-2) s(-1) over 0.2-2.4 keV and a luminosity of similar to 3 x 10(40) ergs s(-1). The inner X-ray nebula is reserved by the HRI into at least four bright knots together with strong diffuse emission responsible for at least 50% of the flux within a radius of 0'.5 (simi lar to 2 kpc). The brightest knot has a luminosity of (2-3) x 10(39) e rgs s(-1). The X-ray nebula has a spatial extent much larger than the starburst ridge seen at centimeter wavelengths by Kronberg and Bierman n (1981) and is oriented in a ''X-like'' pattern along the galaxy mino r axis at a position angle of similar to 30 degrees. This minor-axis X -ray emission is associated with a region of Ha and dust filaments see n in optical images. Optical spectra show that the emission-line gas a long the minor axis is characterized by relatively broad lines (simila r to 250 km s(-1) FWHM) and by ''shocklike'' emission-line flux ratios . Together with the blue-asymmetric nuclear emission-line and NaD inte rstellar absorption-line profiles, these optical data strongly suggest the presence of a starburst-driven superwind. The X-ray spectrum extr acted from the central 5' contains a strong Fe L emission-line complex at 0.6-1.0 keV and a hard excess above 1.0 keV. The spectrum is best described with a two-component model, containing a soft (kT similar to 400-500 eV) Raymond-Smith thermal plasma together with either a Gamma = 1.7 power-law or a kT > 2.2 keV bremsstrahlung component. The soft thermal component provides similar to 30% of the total luminosity over 0.2-2.4 keV, or similar to 10(40) ergs s(-1). The pressure derived fr om the soft component of the X-ray spectrum is consistent with that pr edicted from a starburst-driven superwind if the filling factor of the warm gas is similar to 1%-10%. If the hard X-ray component is thermal gas associated with the galactic outflow, the filling factor must be close to unity. Predictions of the luminosity, temperature, and size o f an adiabatic starburst-generated windblown bubble are consistent wit h those measured for the soft thermal X-ray emission in NGC 2146. The hard X-ray component, however, has a luminosity much larger than predi cted by the superwind model if this component is thermal emission from gas heated by an internal shock in the expanding bubble. We briefly r eview various possibilities as to the nature of the hard X-ray compone nt in NGC 2146.