Large--Scale Outflows in Galaxies

TL;DR

This paper analyzes the dynamics of large-scale galaxy outflows driven by supermassive black holes, showing a transition from momentum-driven to energy-driven phases and explaining observed outflows without active nuclei.

Contribution

It provides an analytical model of outflow evolution, clarifying the transition from momentum-driven to energy-driven regimes and their implications for galaxy evolution.

Findings

Outflows are momentum-driven at small radii and energy-driven at large radii.

Thermal energy can drive outflows for ~10 times longer than active phase.

Large-radius outflows without active nuclei may be remnants of past activity.

Abstract

We discuss massive outflows in galaxy bulges, particularly ones driven by accretion episodes where the central supermassive black hole reaches the Eddington limit. We show that the quasar radiation field Compton--cools the wind shock until this reaches distances $\sim 1$ kpc from the black hole, but becomes too dilute to do this at larger radii. Radiative processes cannot cool the shocked gas within the flow time at any radius. Outflows are therefore momentum-driven at small radii (as required to explain the $M - \sigma$ relation). At large radii they are energy-driven, contrary to recent claims. We solve analytically the motion of an energy--driven shell after the central source has turned off. This shows that the thermal energy in the shocked wind can drive further expansion for a time $\sim 10$ times longer than the active time of the central source. Outflows observed at large radii with no active central source probably result from an earlier short (few Myr) active phase of this source.