ASCA observations of the starburst-driven superwind galaxy NGC 2146:: Broadband (0.6-9 keV) spectral properties

We report ASCA GIS and SIS observations of the nearby (D = 11.6 Mpc), nearly edge-on, starburst galaxy NGC 2146. These X-ray spectral data complement ROSAT PSPC and HRI imaging already discussed in the literature. The broadband (0.6-9 keV) X-ray spectrum of NGC 2146 is best described by a two-component model, the soft X-ray emission being modeled by a Raymond-Smith thermal plasma model with a temperature of kT similar to 0.8 keV, while the hard X-ray emission is modeled by a thermal plasma model with kT similar to 8 keV, or by a power-law model with a photon index of similar to 1.7. We do not find compelling evidence of substantial excess absorption above the Galactic value. The total luminosities of NGC 2146 in the soft (0.5-2.0 keV), hard (2-10 keV), and broad (0.5-10.0 keV) energy bands are similar to 1.3 x 10(40), similar to 1.8 x 10(40), and similar to 3.1 x 10(40) ergs s(-1), respectively. The soft and hard thermal components provide about 30% and 70%, respectively, of the total luminosity in the 0.5-2.0 keV energy band, while in the 2-10 keV energy range only the hard component plays a major role. The spectral results allow us to set tighter constraints on the starburst-driven superwind model, which we show can satisfactorily account for the luminosity, mass, and energy content represented by the soft X-ray spectral component. We estimate that the mass outflow rate (similar to 9 M-. yr(-1)) is about an order of magnitude greater than the predicted rate at which supernova and stellar winds return mass into the interstellar medium, and therefore we argue that the flow is strongly "mass loaded" with material in and around the starburst. The estimated outflow velocity of the hot gas is close to the escape velocity from the galaxy, so the fate of the gas is not clear. We suggest that the hard X-ray spectral component is due to the combined emission of X-ray binaries and/or young supernova remnants associated with the starburst.