Comment Forming accretion disks (Score 2) 56
Retrospectively it could have been guessed long ago that disk galaxies need at least a few tens of rotation periods to look progressively like symmetric accretion disks in other astrophysical contexts (disks around black-holes, stars or planets). The difference between galaxies and smaller disks is mainly the number of rotations they could make, a few tens of rotations for spiral galaxies, millions or billions for smaller disks.