Author(s): , ,
Institution(s): 1. Tsukuba University of Technology, 2. University of Tsukuba
We present fundamental properties of transonic galactic outflows in the gravitational potential of a cold dark matter halo with a central super-massive black hole assuming an isothermal, steady and spherically symmetric state. We classify transonic solutions of galactic outflows according to the perspective of their topological features. As a result, we find that there are mainly two types of transonic solutions arising from a magnitude relationship between the gravity of a cold dark matter halo and that of a super-massive black hole. These two types of transonic solutions have different locus of the transonic point. One transonic point forms at a central region (< 0.01kpc); and another is at a very distant region (> 100kpc). Also, these two transonic solutions have different mass fluxes and acceleration processes. Thus, they may have different influences on the evolution of galaxies and the release of metals into intergalactic space. In addition, we prove that the transonic solution is entropy-maximum independent of the form of the gravitational potential. Therefore, we conclude that the transonic flow is universal in galactic outflows.
Furthermore, we apply our model to the observed hot gas in the Sombrero Galaxy, which has an unsolved paradox. In this galaxy, the Chandra X-ray Observatory detected the wide-spread hot gas as the trace of galactic outflows while the star-formation rate is low, and the observed gas density distribution presumably indicates the hydrostatic equilibrium. To solve this discrepancy, we propose a solution that this galaxy has a transonic outflow, however, the transonic point forms in a very distant region (~ 120 kpc) from the galactic center. In this slowly accelerated transonic outflow, the subsonic region widely spreads beyond the stellar distribution, and the gas density distribution is similar to the hydrostatic state in the subsonic region. Such slowly accelerated outflows are essentially different from the conventional supersonic outflows observed in star-forming galaxies.