From Cosmic Clouds to Galactic Shapes
Galaxies are collections of stars, gas, dust, and dark matter, held together by gravity. While they come in various forms, many, like our own Milky Way, exhibit a flat, disk-like shape. This flatness is a fundamental outcome of their formation and evolution. Their flatness results from initial conditions, rotation, energy loss, and dark matter.
The Role of Rotation and Angular Momentum
Galaxies begin as clouds of gas, dust, and dark matter in the early universe. These clouds are not uniform; density fluctuations allow gravity to pull matter inward. As material draws together, these areas become denser, forming protogalaxies. This transforms a diffuse cloud into a concentrated early galactic structure.
These primordial clouds possess angular momentum. As the cloud collapses under gravity, this angular momentum must be conserved. Like a figure skater spinning faster when pulling their arms inward, the galactic cloud’s rotation speeds up as it contracts.
Increasing rotation creates an outward force perpendicular to the axis. This force resists outward collapse. Along the axis, there is no opposing force, allowing matter to fall inward. This differential resistance drives flattening, as material is prevented from collapsing radially but can contract axially.
Gravity’s Influence and Energy Dissipation
While rotation initiates flattening, gravity pulls matter inward. Gas particles within the collapsing cloud collide. These collisions allow gas to lose kinetic energy through energy dissipation. Unlike stars or dark matter particles, which rarely collide, gas particles transfer energy through inelastic collisions.
As gas particles lose energy, they fall towards the gravitational center. They settle into a common plane of rotation. Motion perpendicular to this plane results in collisions that dissipate energy, moving particles closer to the central plane. This continuous energy loss and settling forms a thin, dense disk.
The Unseen Influence of Dark Matter
Galaxies contain visible stars, gas, and dust, but much of their mass is dark matter. Dark matter does not interact with light or collide like ordinary gas. It does not flatten into a disk because it doesn’t interact through collisions. Instead, it forms a spherical “halo” enveloping the visible galaxy.
Dark matter exerts a strong gravitational pull. This gravitational force creates a potential well where visible matter resides. The dark matter halo holds the galaxy together, allowing dissipative gas to settle into a flat disk within its spherical embrace. The visible flat structure is embedded within this larger, non-flat dark matter distribution.
Why Not All Galaxies Are Flat
While many galaxies exhibit a flat, disk-like structure, not all galaxies are flat. These mechanisms explain spiral and disk galaxies, characterized by organized rotation and gas content. Elliptical galaxies have different shapes and formation histories. They are more spherical and lack flat disks.
Non-flat galaxies often form through major mergers between existing disk galaxies. Mergers randomize angular momentum, preventing a new flat disk. Some elliptical galaxies formed from gas clouds with little rotation or insufficient gas for dissipation.