Astronomy & Celestial Mechanics
the sky, computed
Concept

Star formation efficiency — where it appears

The fraction of a gas reservoir turned into stars, either over a cloud's lifetime or per free-fall time. It is low, typically a few per cent per free-fall time, and the reasons it is low — turbulence, feedback, magnetic support — are disputed.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

The star-formation law, and the clock inside it. 46 model discs on the Kennicutt–Schmidt plane, built from Σ_SFR = 2.5e-4 Σ_gas^1.4 with 0.28 dex of scatter, with the exponent then fitted back off the drawn points at 1.42. The diagonal dashed lines are constant depletion times: gas divided by the rate consuming it. Every disc on this plane consumes its gas within a few gigayears, and a galaxy that has been forming stars for ten of them therefore cannot have been working from the gas it started with. The slope steeper than one is what makes the depletion time shorter where the gas is richer, which is the opposite of the intuition that a full tank lasts longer.

The gas runs out before the galaxy does

Divide a disc's gas by the rate it is turning that gas into stars and the answer is about two billion years — a seventh of the age of the disc. Every spiral still forming stars is therefore being fed from outside, and the star-formation law says the richer the disc, the sooner it starves.

galaxies · Star formation Turbulence against star formation, driven two ways. Gas velocity dispersion against star-formation rate per unit area, both on logarithmic scales, for model discs of increasing gas surface density: a nearby spiral (circular speed 200 km/s at 8 kpc, gas a fifth of the disc's mass) and a disc at redshift two (the same speed at 4 kpc, gas half). Dashed: turbulence driven by supernovae alone, a tenth of their energy staying in the gas — it settles between 11 and 13 km/s however fast the disc forms stars, because more gas means both more stars and faster dissipation. Solid: the same discs when gravity can also stir them, holding the gas at marginal stability once supernovae fall short — which in the redshift-two disc happens above a gas surface density of 72 M☉ pc⁻², after which the dispersion climbs with the star-formation rate to 76 km/s. The shaded boxes are the approximate ranges in which nearby spirals and redshift-two discs are observed. Feedback alone reaches the first and not the second; with gravity the model reaches the second at its lower edge in star formation, a sign that its simple star-formation law, one per cent of the gas per free-fall time of the whole layer, is slow for the clumpy gas of those discs.

A stirring supernovae cannot pay for

The gas in a disc forming stars at redshift two moves at fifty kilometres a second in random directions, five times faster than in a spiral today, and turbulence that fast decays in a few tens of millions of years. Supernovae are the obvious source of the energy, and they run out at about thirteen kilometres a second however fast the disc forms stars. What holds the discs up is gravity — gas stirring itself by falling inward — and a disc stirred that way is also a disc pouring its gas into its own centre.

galaxies · Tully–Fisher

Named alongside it

The objects these essays reach for when they reach for this one.

Depletion timeFree-fall timeGas accretionGas surface densityGravitational instabilityInitial mass functionThe Kennicutt–Schmidt lawMolecular cloudPressure supportQuenchingRadial inflowStar formation rate

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