Why does gas form stars at only one percent per free-fall time? · revision history

Revision 2

Accepted · core change · record

Changes against revision 1:

Title
Why does gas form stars at only one percent per free-fall time?
Plain
If gas clouds simply collapsed under their own weight, galaxies would form stars about a hundred times faster than they do. Turbulence, magnetic fields and the push of newborn stars all slow it down, but which one sets the value is unknown.
Precise · changed · core
Revision 1: Molecular clouds convert gas into stars at efficiency per free-fall time eps_ff = SFR t_ff / M_gas ~ 0.01, with integrated cloud efficiencies of $\sim 1-10\,\mathrm{percent}$ before dispersal. Explain the value and its scatter as a function of cloud surface density and virial parameter, separating the roles of supersonic turbulence, magnetic support and stellar feedback (photoionization, winds, radiation pressure, jets, supernovae).
Revision 2: Molecular clouds convert gas into stars at efficiency per free-fall time eps_ff = SFR t_ff / M_gas ~ 0.01, with integrated cloud efficiencies of $\sim 1-10\%$ before dispersal. Explain the value and its scatter as a function of cloud surface density and virial parameter, separating the roles of supersonic turbulence, magnetic support and stellar feedback (photoionization, winds, radiation pressure, jets, supernovae).
Settled by
Simulations with all feedback channels that reproduce the measured eps_ff distribution across cloud populations, plus observations that measure eps_ff and its dependence on surface density in many galaxies.
Kind
phenomenon
Answer type
mechanism
References
(empty)
Literature status
open
Status note
(empty)
Posed since
1974

Decisions