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.

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.