DYNAMICS AND X-RAY-EMISSION OF A GALACTIC SUPERWIND INTERACTING WITH DISK AND HALO GAS

There is a general agreement that the conspicuous extranuclear X-ray, optical-line, and radio-continuum emission of starbursts is associated with powerful galactic superwinds blowing from their centers. However , despite the significant advances in observational studies of superwi nds, there is no consensus on the nature of the emitting material and even on the emission mechanisms themselves. This is to a great extent a consequence of a poor understanding of dynamical processes in the st arburst superwind regions. To address this issue, we have conducted tw o-dimensional hydrodynamical simulations of galactic superwinds. While previous similar studies have used a single (disk) component to repre sent the ISM of the starburst galaxy, we analyze the interaction of th e wind with a two-component disk-halo ambient interstellar medium and argue that this two-component representation is crucial for adequate m odeling of starbursts. The emphasis of this study is on the geometry a nd structure of the wind region and the X-ray emission arising in the wind material and the shocked gas in the disk and the halo of the gala xy. The simulation results have shown that a clear-cut bipolar wind ca n easily develop under a range of very different conditions. On the ot her hand, a complex ''filamentary'' structure associated with the entr ained dense disk material is found to arise within the hot bubble blow n out by the wind. The flow pattern within the bubble is dominated equ ally by the central biconic outflow and a system of whirling motions r elated to the origin and development of the ''filaments.'' The filamen t parameters make them a good candidate for optical-emission-line fila mentary gas observed in starburst halos. We find that the history of m ass and energy deposition in the starburst region of the galaxy is cru cial for wind dynamics. A ''mild'' early wind, which arises as a resul t of the cumulative effect of stellar winds from massive stars, produc es a bipolar vertical cavity in the disk and halo gas without strongly affecting the gaseous disk, thus creating conditions for virtually fr ee vertical escape of the hot gas at the later, much more violent supe rnova-dominated phases of the starburst. We calculate the luminosity, mass, and effective temperature of the X-ray emitting gas in the ''sof t'' (0.1-0.7 keV, 0.7-2.2 keV, and 0.1-2.2 keV) and ''hard'' (1.6-8.3 keV) energy bands and estimate the contribution of different gaseous c omponents to the X-ray flux in these bands. Analysis of these paramete rs enables us to make conclusions regarding the nature of the X-ray-em itting material . We have inferred that the bulk of the soft thermal X -ray emission from starbursts arises in the wind-shocked material of t he disk and halo gas rather than in the wind material itself. This ena bles us to predict that the integrated soft X-ray spectra of starburst s need not show an overabundance of heavy elements which are believed to be produced copiously in the centers of starbursts. Unlike soft X-r ay emission, the hard component of thermal X-ray emission is found to originate in the wind material ejected from the starburst region. Howe ver, the derived ratio of hard-to-soft X-ray luminosities is too small compared to that observed in starbursts. We conclude therefore that t he observed hard X-ray emission of starbursts is probably wt associate d with the thermal emission of hot wind or ambient shocked gas. Typica l temperatures of the bulk of the soft X-ray-emitting material in our very different models have been found to agree well with the ones esti mated on the basis of the ROSAT data for the soft component of X-ray e mission of nearby starbursts. We predict that temperatures of the extr anuclear soft X-ray-emitting gas in starburst galaxies with heavy elem ent abundances near solar should be close to T(Xs) = 2-5 x 10(6) K.