Spiral and Disk Galaxies

Spiral and disk galaxies are flattened, rotation-supported systems whose gas-rich disks host ongoing star formation and the characteristic spiral arms.

Definition

Spiral and disk galaxies are galaxies dominated by a flattened, rotationally supported disk of stars and gas, often displaying spiral arms and sometimes a central bar, in which gas cooling sustains ongoing star formation.

Scope

This topic covers the structure of galactic disks, the origin and nature of spiral arms including density-wave theory, the role of bars in disk galaxies, the relation between luminosity and rotation speed expressed by the Tully-Fisher relation, and the connection between disks and star formation.

Core questions

• What is the dynamical structure of a galactic disk?
• What produces spiral arms, and are they long-lived patterns or transient features?
• How do bars affect the evolution of disk galaxies?
• How does the Tully-Fisher relation connect a spiral's rotation to its luminosity?

Key theories

Density-wave theory of spiral arms

Lin and Shu proposed that spiral arms are long-lived density waves through which stars and gas pass, rather than fixed material structures, explaining how arms persist despite differential rotation.

The Tully-Fisher relation

A tight correlation links a spiral galaxy's luminosity to its rotation speed, providing both a distance indicator and a constraint relating luminous and dark matter in disks.

Bar-driven disk evolution

Stellar bars redistribute angular momentum, channel gas toward the center, and can build pseudobulges, making them key drivers of the secular evolution of disk galaxies.

Clinical relevance

Disk galaxies like the Milky Way are the dominant sites of present-day star formation, and the Tully-Fisher relation has been a workhorse of the extragalactic distance scale and a probe of the dark matter content of disks.

History

The persistence of spiral arms despite differential rotation, the so-called winding problem, motivated Lin and Shu's 1964 density-wave theory. The 1977 Tully-Fisher relation gave disks a powerful distance indicator, while later numerical simulations clarified the roles of bars and transient arms.

Key figures

• Chia-Chiao Lin
• Frank Shu
• Brent Tully
• Alar Toomre

Seminal works

• linshu1964
• tully1977
• binney2008

Frequently asked questions

If galaxies rotate differentially, why don't spiral arms wind up?

Density-wave theory resolves this winding problem by treating arms as wave patterns that rotate more slowly than the stars and gas, which stream through the arms rather than being permanently locked into them.

Are all spirals barred?

No, but a large fraction are. The Milky Way has a bar, and surveys find that roughly half to two-thirds of nearby disk galaxies show a bar, the proportion depending on wavelength and definition.