Astronomy & Celestial Mechanics
the sky, computed
Concept

Baryon — where it appears

A particle made of three quarks, such as the protons and neutrons that make up atomic nuclei. In astronomy the word stands for ordinary matter as a whole — stars, gas and dust — as distinct from dark matter and radiation.

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

The mass-to-light ratio of NGC 3198, radius by radius. The dynamical mass inside each radius divided by the light inside it, in solar units. Inside two disc scale lengths it is nearly flat at about 2.2 — a galaxy made of stars, weighing what stars weigh. Outside the disc the light stops and the mass does not, so the ratio climbs to 10 by 30 kpc with no sign of turning over. The curve is not fitted: it is the rotation curve's enclosed mass divided by the light profile's enclosed light, both drawn elsewhere in this collection.

The mass that is not the light

A galaxy's mass-to-light ratio is not a number, it is a curve, and it rises without turning over. What stellar populations can plausibly weigh sets a ceiling; the dynamics sit far above it, and the gap is a shape rather than a discrepancy.

galaxies · Dark matter The observed acceleration against the acceleration the visible matter explains. The acceleration measured from a rotation curve, v²/r, against the acceleration the stars and gas alone would produce at the same radius, both on logarithmic axes in metres per second squared. The solid curve is the fitting function to the measured relation for 153 disc galaxies, g(obs) = g(bar) / (1 − e^(−√(g(bar)/a₀))) with a₀ = 1.2 × 10⁻¹⁰ m s⁻², and the band is its observed scatter of ±0.13 dex, most of which is measurement error. Above a₀ the relation follows the dashed line of equality, where the baryons account for everything; below it, it bends onto the dotted line g(obs) = √(g(bar)·a₀). At ten times a₀ the observed acceleration exceeds the baryonic by a factor of 1.06; at a₀, by 1.58; at a hundredth of a₀, by 10.5. The discrepancy the dark matter is invoked to explain is a function of one variable, and the variable is an acceleration.

One acceleration, and every galaxy's missing mass

Plot the acceleration measured at each radius of a rotation curve against the acceleration its stars and gas would produce there, for every galaxy at once, and the points fall on a single curve. Where the discrepancy begins is set not by a radius or a mass but by an acceleration.

galaxies · Dark matter

Named alongside it

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

The baryonic Tully–Fisher relationDark matterDark matter haloDynamical massEnclosed massFeedbackHaloInitial mass functionLuminosity densityMachoMass discrepancyMass-to-light ratio

All concepts