Polytropic transonic galactic outflows in a dark matter halo with a central black hole

TL;DR

This study explores polytropic transonic galactic outflows influenced by dark matter halos and central black holes, revealing new solution types, conditions for slowly accelerating outflows, and applying the model to the Sombrero Galaxy with improved fit over isothermal models.

Contribution

It introduces a comprehensive polytropic model for galactic outflows considering dark matter and black holes, classifies solution topologies, and applies the model to real galaxy data.

Findings

Transonic solutions depend on mass distribution, energy, polytropic index, and slope.

A critical slope value causes dramatic changes in solution types.

The model predicts slowly accelerating outflows in quiescent galaxies.

Abstract

Polytropic transonic solutions of spherically symmetric and steady galactic winds in the gravitational potential of a dark matter halo (DMH) with a supermassive black hole (SMBH) are studied. The solutions are classified in terms of their topological features, and the gravitational potential of the SMBH adds a new branch to the transonic solutions generated by the gravity of the DMH. The topological types of the transonic solutions depend on the mass distribution, the amount of supplied energy, the polytropic index $\gamma$, and the slope $\alpha$ of the DMH mass distribution. When $\alpha$ becomes larger than a critical value $\alpha_\mathrm{c}$, the transonic solution types change dramatically. Further, our model predicts that it is possible for a slowly accelerating outflow to exist, even in quiescent galaxies with small $\gamma$. This slowly accelerating outflow differs from those considered in many of the previous studies focusing on supersonic outflows in active star-forming galaxies. In addition, our model indicates that outflows in active star-forming galaxies have only one transonic point in the inner region ($\sim$ 0.01 kpc). The locus of this transonic point does not strongly depend on $\gamma$. We apply the polytropic model incorporating mass flux supplied by stellar components to the Sombrero Galaxy, and conclude that it can reproduce the observed gas density and the temperature distribution well. This result differs significantly from the isothermal model, which requires an unrealistically large mass flux (Igarashi et al. 2014). Thus, we conclude that the polytropic model is more realistic than the isothermal model, and that the Sombrero Galaxy can have a slowly accelerating outflow.