Astronomy & Celestial Mechanics
the sky, computed
Concept

Radial migration — where it appears

The movement of stars in a disc galaxy to new orbital radii, driven by spiral arms and bars, while their orbits stay nearly circular. It carries stars born at one radius, with that radius's chemistry, to another, blurring the record of the disc's abundance gradient.

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

An arm that moves angular momentum outwards, and is spent doing it. Where a 2-armed spiral pattern of 25 km/s per kiloparsec takes angular momentum from the disc and where it gives it back, against galactocentric radius, for the Milky Way's rotation curve. The resonances are found on that curve rather than placed: the inner Lindblad resonance at 1.91 kiloparsecs, corotation at 8.67, the outer Lindblad resonance at 13.80. The lower curve is the angular momentum removed per unit radius and the upper one what the wave delivers; between them runs the flux the wave carries, which is flat across the whole region where nothing is resonant, because a wave that is not interacting with anything simply travels. The two deposits are required to cancel to a part in a million — the pattern is a conveyor and keeps nothing — so the disc's total angular momentum is unchanged while its distribution is not. That is the sense in which a spiral is a machine: it moves mass inwards by moving angular momentum outwards, which is the same operation an accretion disc performs and the same one a protostellar disc must perform to make a star. The cost is paid in random motion. Every exchange heats the stellar disc, a hotter disc supports a weaker wave, and the pattern that did the work is the thing the work destroys. What the figure cannot show is the pattern's own lifetime, which is the unsettled part of the subject.

An arm that is undone by the work it does

A spiral pattern takes angular momentum from the inner disc and delivers it to the outer, which lets mass move inwards without violating anything. It is paid for in random motion, and a disc with too much random motion cannot carry a wave — so the pattern destroys the conditions it needs to exist.

galaxies · Spiral structure A disc built from the inside out, with a gradient of −0.066 dex per kiloparsec at 12 Gyr. The metallicity of the gas, in solar units on a logarithmic scale, against galactocentric radius, at ages of 2, 6, 12 Gyr, for a disc in which every ring is its own box with infall: pristine gas arrives on a timescale that grows with radius, from 1 Gyr in the centre to 7 Gyr at 8 kpc, turns into stars on the depletion time of a Kennicutt law, which is shorter where the gas is denser and much longer below a threshold of 7 M☉ pc⁻², and keeps everything it makes. The slopes fitted between 4 and 14 kpc: −0.259 dex/kpc at 2 Gyr, −0.124 dex/kpc at 6 Gyr, −0.066 dex/kpc at 12 Gyr. The inner disc has had its gas early and turned it over many times, so it is near the yield; the outer disc is still accreting and forming stars slowly, so its gas is diluted and young in the chemical sense. At 8 kpc the model's present abundance is 1.16 of the yield. Every ring is independent here: no gas flows between them and no star moves, which are the two processes that real discs add and which both act to flatten what is drawn.

A gradient the old stars have walked away from

The gas in a disc galaxy is richer in metals near the centre than at the edge, by about six-hundredths of a dex per kiloparsec in the Milky Way. Two ingredients of disc growth make that slope, a disc that grows from the inside out makes it flatten with time — and the old stars that should carry the steeper history have moved several kiloparsecs from where they were born.

galaxies · Chemical evolution

Named alongside it

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

Abundance gradientAge metallicity relationAngular momentumAngular momentum transportChemical evolutionCorotationCorotation resonanceDensity waveDepletion timeDisc heatingEpicyclic frequencyGalactic outflow

All concepts