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 and 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 ~4', or 17 kpc. The X-ray emission is resolved by the PSPC and preferentially 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 ~3 x 10^40^ ergs s^-1^. The inner X-ray nebula is resolved 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' (~2 kpc). The brightest knot has a luminosity of (2-3) x 10^39^ ergs s^-1^. The X-ray nebula has a spatial extent much larger than the starburst ridge seen at centimeter wavelengths by Kronberg & Biermann (1981) and is oriented in a "X-like" pattern along the galaxy minor axis at a position angle of ~30^deg^. This minor-axis X-ray emission is associated with a region of Hα and dust filaments seen in optical images. Optical spectra show that the emission-line gas along the minor axis is characterized by relatively broad lines (~250 km s^-1^ FWHM) and by "shocklike" emission-line flux ratios. Together with the blue-asymmetric nuclear emission-line and NaD interstellar absorption-line profiles, these optical data strongly suggest the presence of a starburst-driven superwind. The X-ray spectrum extracted 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~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 30% of the total luminosity over 0.2-2.4 keV, or ~10^40^ ergs s^-1^. The pressure derived from the soft component of the X-ray spectrum is consistent with that predicted from a starburst-driven superwind if the filling factor of the warm gas is ~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 of an adiabatic starburst-generated windblown bubble are consistent with those measured for the soft thermal X-ray emission in NGC 2146. The hard X-ray component, however, has a luminosity much larger than predicted by the superwind model if this component is thermal emission from gas heated by an internal shock in the expanding bubble. We briefly review various possibilities as to the nature of the hard X-ray component in NGC 2146.