Astronomy & Celestial Mechanics
the sky, computed
Concept

Pseudo bulge — where it appears

A flattened, rotating concentration of stars at a disc galaxy's centre, built slowly by gas driven inward by bars and spirals. It differs from a classical bulge, which is round, supported by random motions and made early in mergers.

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

The angular momentum a halo has and the angular momentum a disc needs. The fraction of the mass with specific angular momentum below j, against j in units of the mean, for a halo's material — the universal profile fitted to simulated haloes, with shape parameter μ of 1.1, 1.25, 1.5 — and for an exponential disc with a flat rotation curve (black). Both have the same mean, as a disc that kept its halo's angular momentum would. They do not have the same shape. Below a tenth of the mean a disc holds 2% of its mass; a halo with μ = 1.25 holds 11%, and one with μ = 1.1, 15%. The halo also stops at a largest j, 4.0 times the mean for μ = 1.25, beyond which the disc still has a thin tail. A disc built from a halo's gas without any exchange of angular momentum would be too concentrated at its centre and cut off at its edge.

A halo with too much slow gas for a disc

A disc's size follows from its halo's average spin, and the average is all the simplest account of disc formation uses. The whole distribution does not match. A halo holds five times as much of its material at low angular momentum as an exponential disc can accommodate, so gas that kept its angular momentum exactly would build a dense centre, a thin body and a hard edge — and the bulgeless discs that are common in the nearby universe say that something removed a fifth to a quarter of the slowest gas before it settled.

galaxies · Galaxy spin The share of discs that should bar. The fraction of haloes whose spin is too low to keep a disc stable, against the disc's share of the halo's mass, for spin distributions of width 0.4, 0.5, 0.6. At a share of 1 per cent it is 1 per cent; at 3 per cent, 50; at 5 per cent, 85. Observed bar fractions among nearby discs, a third to two-thirds (shaded), fall where discs hold 2.5 to 4 per cent of their haloes' mass.

A spin too low to keep a disc from barring

A disc galaxy's size is set by how fast its dark halo happened to be turning. A slowly turning halo makes a small, dense disc, and a disc dense enough to supply much of its own rotation is unstable: it rearranges itself into a bar. With halo spins scattered as simulations find them, the fraction of discs that should bar depends almost entirely on one number — how much of the halo's mass ended up in the disc — and the bar fractions astronomers count say it is about three per cent.

galaxies · Galaxy spin

Named alongside it

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

Dark matter haloExponential discGalaxy spinSpecific angular momentumSpin parameterBulgeDisc scale lengthGalactic outflow

All concepts