ASCAObservations 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), nearlyedge-on, starburst galaxy NGC 2146. These X-ray spectral data complement ROSATPSPC and HRI imaging discussed by Armus et al., 1995. The broad band (0.6-9keV) X-ray spectrum of NGC 2146 is best described by a two component model: thesoft X-ray emission with a Raymond-Smith thermal plasma model having atemperature of kT $\sim 0.8$ keV; the hard X-ray emission with a thermal plasmamodel having kT $\sim 8$ keV or a power-law model having a photon index of$\sim 1.7$. We do not find compelling evidence of substantial excess absorptionabove the Galactic value. The soft (hard) thermal component provides about 30%(70%) of the total luminosity in the 0.5 - 2.0 keV energy band, while in the2-10 keV energy range only the hard component plays a major role. The spectralresults allow us to set tighter constraints on the starburst-driven superwindmodel, which we show can satisfactorily account for the luminosity, mass, andenergy content represented by the soft X-ray spectral component. We estimatethat the mass outflow rate ($\sim$ 9 M$_{\odot}$ per year) is about an order ofmagnitude greater than the predicted rate at which supernovae and stellar windsreturn mass into the interstellar medium and, therefore, argue that the flow isstrongly "mass-loaded" with material in and around the starburst. The estimatedoutflow velocity of the hot gas is close to the escape velocity from thegalaxy, so the fate of the gas is not clear. We suggest that the hard X-rayspectral component is due to the combined emission of X-ray binaries and/oryoung supernovae remnants associated with the starburst.Comment: 26 pages plus 4 figures, LaTex manuscript, Accepted for publication in the Astrophysical Journa