Astronomers think today's giant elliptical galaxies began as spinning disks
No laboratory can build a galaxy, so astronomers test their ideas by checking whether a theory predicts the galaxies we actually see. The leading picture now runs backwards from intuition: flat, spinning disk galaxies probably came first, and the great round ellipticals grew later out of galactic collisions.
Observed galaxies fall into two broad families. Blue ones, mostly spirals, are still making stars; red ones, mostly ellipticals, have largely stopped. Spirals are thin, dense and rotate fairly quickly, while stars in an elliptical move on orbits pointing every which way. Most giant galaxies hide a supermassive black hole weighing millions to billions of suns, and its mass tracks the mass of the galaxy's central bulge. Galaxies that are heavier also tend to be richer in elements beyond hydrogen and helium.
How the first disks took shape is still argued over. In 1962 Olin Eggen, Donald Lynden-Bell and Allan Sandage proposed that a single huge gas cloud collapsed. Clumps of mostly dark matter tugged on one another and picked up spin; as ordinary gas cooled and sank inward, it spun faster, flattening into a disk much as tossed pizza dough does. Newer bottom-up models start instead with small clumps, roughly the mass of globular star clusters, that merge into galaxies, which in turn gather into clusters.
Neither version fully explains why the collapse halts, and disk models struggle to reproduce real rotation speeds and sizes. Suggested brakes include radiation from young stars or an active galactic core, or the pull of the dark matter halo. Dark matter itself is required: galaxies could not have formed or spin as observed without far more mass than is visible.
The standard Lambda-CDM cosmology correctly predicts how common each galaxy type is, yet it foresees too few thin disks, because it calls for so many mergers. A collision with a galaxy of at least 15 percent of one's own mass usually wrecks a disk. Classification is being rethought too: Hubble's tuning fork of 1926 describes shapes but not transformations, and Alister Graham's newer Triangal scheme maps routes between types, including three distinct origins for lens-shaped galaxies.
Source: Galaxy formation and evolution
