MASS LOADING AND COLLIMATION OF GALACTIC SUPERWINDS

摘要

We attempt to constrain (1) the site and amount of mass loading and (2) the collimator geometry for a galactic superwind observed in the starburst galaxy M82 by comparing predictions for the superwind pressure, temperature, and X-ray emission deduced from hydrodynamical simulations of the bipolar conical hot gas outflow with the observed central pressures and temperatures as well as X-ray properties of this galaxy. We conclude that the superwind in M82 must be a mass-loading wind: the total mass flux turns out to be 3-6 times higher than the mass deposition rate due to supernova ejecta only, thus implying that the resulting wind is "loaded" with the mass of evaporated gas clouds. Such a wind means that M82 must have ejected of ～10~8 solar mass gas into the halo over the lifetime of the starburst event, 2-3 x 10~7 yr. We have found a strong dependence of the wind X-ray emission properties on the amount of mass loading. We therefore have concluded that the wind X-ray emission provides a sensitive tool to probe ISM transformations associated with the supernova explosions in the center of starbursts similar to M82. Comparison of the predicted pressures and temperatures with the estimates inferred from observations for the center of M82 indicates that essentially all mass loading must be confined to a thin layer in the center of the starburst where supernova explosions occur rather than being distributed throughout the vertical extension of the disk. The simulation results have also shown that both a wide-angle, fanlike wind suggested by the optical data (the cone opening angle θ ～ 60°) and a much narrower, jetlike wind inferred from recent ROSAT data (θ = 22°) appear to be compatible with the observed X-ray constraints. However, the two cases imply quite different geometries for the central cavity where wind collimation occurs. The collimator must be wide (r ～ 500 pc) if the wind above the central gaseous disk has a wide cone opening angle, such as suggested by the optical data, θ ≈ 60°. For a jetlike wind of the type inferred recently from ROSAT data (θ = 22°), the collimator radius at the base of the wind must be close to that of the starburst region, r ～ 150 pc. For the X-ray properties to be compatible with those observed in M82, it is essential that the outflow is strongly collimated over the central ～ 1.5 kpc but becomes much less collimated in the halo. This model predicts an outward softening of the X-ray flux such that the effective temperature of the X-ray gas in the outer part of the emitting region is a factor of ～2 smaller than the temperature within the central ～1.5 kpc.