Transonic solutions of isothermal galactic winds in a cold dark matter halo

We study fundamental properties of steady, spherically symmetric, isothermal galactic outflows in appropriate gravitational potential models. We aim at constructing a universal scale-free theory not only for galactic winds, but also for winds from clusters/groups of galaxies. In particular, we consider effects of mass-density distribution on the formation of transonic galactic outflows under several models of the density distribution profile predicted by cosmological simulations of structure formation based on the cold dark matter (CDM) scenario. In this study, we have clarified that there exist two types of transonic solutions: outflows from the central region and from a distant region with a finite radius, depending upon the density distribution of the system. The system with a sufficiently steep density gradient at the centre is allowed to have the transonic outflows from the centre. The resultant criterion intriguingly indicates that the density gradient at the centre must be steeper than the prediction of conventional CDM models including Navarro, Frenk & White and Moore et al. This result suggests that an additional steeper density distribution originated by baryonic systems such as the stellar component and/or the central massive black hole is required to realize transonic outflows from the central region. On the other hand, we predict the outflow, which is started at the outskirts of the galactic centre and is slowly accelerated without any drastic energy injection like starburst events. These transonic outflows may contribute secularly to the metal enrichment of the intergalactic medium.