  Cosmological models predict that galaxies forming in the early Universe
experience a chaotic phase of gas accretion and star formation, followed by gas
ejection due to feedback processes. Galaxy bulges may assemble later via
mergers or internal evolution. Here we present submillimeter observations (with
spatial resolution of 700 parsecs) of ALESS 073.1, a starburst galaxy at
redshift z~5, when the Universe was 1.2 billion years old. This galaxy's cold
gas forms a regularly rotating disk with negligible noncircular motions. The
galaxy rotation curve requires the presence of a central bulge in addition to a
star-forming disk. We conclude that massive bulges and regularly rotating disks
can form more rapidly in the early Universe than predicted by models of galaxy
formation.
