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

TL;DR

This paper investigates the conditions under which steady, isothermal galactic winds become transonic within different dark matter density profiles, revealing the necessity of steeper central densities for outflows from the core.

Contribution

It introduces a criterion for transonic galactic outflows based on dark matter density gradients, highlighting the need for steeper central densities than standard CDM predictions.

Findings

Two types of transonic solutions identified: from the center and from the outskirts.

Steeper central density gradients are required for outflows from the galactic center.

Outflows from the outskirts are slow and do not require energetic events.

Abstract

We study fundamental properties of steady, spherically symmetric, isothermal galactic outflow 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 exists two types of transonic solutions: outflows from the central region and from distant region with a finite radius, depending upon the density distribution of the system. The system with sufficiently steep density gradient at the center is allowed to have the transonic outflows from the center. The resultant criterion intriguingly indicates that the density gradient at the center must be steeper than that of the prediction of conventional CDM model including Navarro, Frenk & White (1997) and Moore et al. (1999). 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 outflow from the central region. On the other hand, we predict the outflow, which is started at the outskirts of the galactic center 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.