Astronomy & Celestial Mechanics
the sky, computed
Concept

Satellite galaxy — where it appears

A galaxy orbiting inside a larger halo rather than dominating its own. Becoming one starts a clock of a few billion years after which star formation stops, and the difference between satellites and centrals at fixed mass is the environmental effect.

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

Four defensible choices, and a factor of 2.9 between them. The Coulomb logarithm against the ratio of the two impact parameters it is cut off at. The curve is a logarithm, so it is flat — a factor of ten in the ratio buys 2.3 — and that is usually offered as the reason not to worry. The four marked conventions are all in current use and all defensible, and they give ln Λ from 3.4 to 9.9. Since the drag force is proportional to ln Λ and not to its logarithm, that is a factor of 2.9 in every sinking time computed from it. The flatness protects the answer from a wrong guess about the ratio; it does not protect it from there being no correct guess, which is the actual situation.

A drag computed with a logarithm nobody can pin down

Chandrasekhar's drag formula is exact, derived from first principles, and contains a logarithm of a ratio of two lengths that the derivation does not supply. Every sinking time in astronomy is proportional to that logarithm, and the four conventions in current use differ by a factor of three.

gravitation · Dynamical friction Two ways to stop, and the same curve shifted. The fraction of galaxies that have stopped forming stars, against their stellar mass, at four local densities. Two things are happening at once. Mass quenching turns the curve upward above about 10^10.6 solar masses, and it does so at the same mass whatever the environment — the curves rise in step. Environment quenching lifts the whole curve, and it does so by the same fraction of what is left at every mass, which is why the four curves look like one curve with its floor raised rather than like four different shapes. At 10⁹·⁵ solar masses the red fraction runs from 13 per cent in the field to 97 in a cluster core, and almost all of that difference is environmental; at 10¹¹·⁵ almost everything is red everywhere and the environment has nothing left to do. The two mechanisms are separable, which is the census's central quantitative result and the thing any account of either one has to explain.

Two ways to stop that do not interact

A galaxy's chance of having stopped forming stars depends on its mass and on where it is, and the census says the two dependences multiply. The efficiency with which environment quenches a galaxy is the same number at every mass, across two and a half decades of it — which is surprising, because both mechanisms work on the same gas.

galaxies · Galaxy populations

Named alongside it

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

CalibrationCoulomb logarithmDivergenceDynamical frictionFeedbackGlobular clusterHalo massImpact parameterMorphology density relationOrder of magnitudeQuenchingQuenching efficiency

All concepts