Galactic outflow

A galactic outflow carries gas, metals, energy, and momentum away from a galaxy under stellar or active-galactic-nucleus feedback. Whether the material escapes or later returns depends on its speed and the galactic potential.

A galactic wind is a large-scale flow carrying gas, energy and momentum away from a galaxy. Stellar winds and supernovae can maintain a hot, pressurized component that expands out of a star-forming region; an active galactic nucleus can supply another driving mechanism. Cooler entrained gas can coexist with the hot component.

Observations must distinguish the wind from unrelated point sources and foreground emission. In M82, ROSAT measurements separated extended soft X-ray emission from compact sources and traced emission away from the galactic plane. The emitting gas need not be the freely expanding wind itself: shocked clouds can dominate the detected radiation.^[ref]

Gas lifted out of a galactic disk can remain bound and return rather than escape into the intergalactic medium. This recycling carries metals and angular momentum between the disk and its surroundings and delays the availability of gas for subsequent star formation.

Self-gravitating energy is nonlocal. Shell assembly assigns $-GM(<r)dM/r$ and counts pairs once; the density $\rho \Phi /2$ is another total-energy allocation. Removing a test parcel from a fixed potential instead requires $-\Phi dM$. These prescriptions need not give the same radial feedback threshold, even though their units agree.

If stellar feedback supplies energy $e_{\ast }$ per unit formed stellar mass, an energy-driven outflow requires $\nu v_{\mathrm{esc}}^2/2\lesssim e_{\ast }$. Fixed energy coupling therefore suggests mass-loading factor $\nu \propto v_{\mathrm{esc}}^{-2}$. A fixed momentum supply $p_{\ast }$ instead gives $\nu v_{\mathrm{esc}}\lesssim p_{\ast }$ and momentum-driven outflow scaling $\nu \propto v_{\mathrm{esc}}^{-1}$. These are launch and escape estimates, not universal wind laws; subsequent entrainment, cooling and fallback change the escaping mass.

The mass-loading factor is the ratio of outflow rate to star-formation rate, $\lambda ={\dot{M}}_{\mathrm{out}}/{\dot{M}}_{\ast }$. Its value can depend on galaxy mass, radius, gas phase, and the surface through which the flow is measured.