Past exam of the formation of Structure in the Universe course of the University of Cambridge / 2014 / Paper 1 / 3 / c

Stellar radiation, stellar winds and supernovae return energy, momentum, mass and metals to surrounding gas. Heating, ionization, turbulent support and gas removal can reduce subsequent star formation; swept-up shells can also compress gas locally. Whether thermal energy drives an outflow depends on its radiative gas cooling time relative to expansion: radiative losses can remove energy while leaving substantial momentum transfer. Thus deposited energy alone does not specify the dynamical effect.

The galactic gas regulator has mass budget ${\dot{M}}_g={\dot{M}}_{\mathrm{in}}-(1-R+\eta )\Psi$, where $R$ is the returned fraction and $\eta ={\dot{M}}_{\mathrm{out}}/\Psi$ is the mass-loading factor. In a quasi-steady state, $\Psi \simeq {\dot{M}}_{\mathrm{in}}/(1-R+\eta )$. This connects feedback to the available gas supply. A wind's kinetic power ${\dot{M}}_{\mathrm{out}}{v}_w^2/2$ and momentum flux ${\dot{M}}_{\mathrm{out}}{v}_w$ must fit the source budget and gravitational binding cost. Gas that fails to escape may form a galactic fountain and later return; preferential metal loss changes chemical evolution.

Absorption against a galaxy's stellar continuum provides a concrete outflow test: compare interstellar sodium absorption with the systemic velocity and stellar absorption spectrum. A primary starburst survey found blueshifted interstellar absorption and an inclination dependence consistent with minor-axis outflows. Its saturated lines also illustrate why velocity detection is more direct than a unique column density or mass flux.^[ref] Deprojection, covering fraction, ionization, radius and the unseen hot phase limit total outflow-rate estimates.

An accreting supermassive black hole adds radiation, winds and astrophysical jets. Coupling these to the interstellar or circumgalactic medium can suppress cooling or expel gas, especially in deep potentials where stellar driving alone may be inadequate. Jet cavities, line-emitting gas and multiphase outflows test different channels; coexisting activity and low star formation alone do not prove a causal feedback history.