A dynamically cold disk galaxy in the early Universe
  The extreme astrophysical processes and conditions that characterize the
early Universe are expected to result in young galaxies that are dynamically
different from those observed today. This is because the strong effects
associated with galaxy mergers and supernova explosions would lead to most
young star-forming galaxies being dynamically hot, chaotic and strongly
unstable. Here we report the presence of a dynamically cold, but highly
star-forming, rotating disk in a galaxy at redshift ($z$) 4.2, when the
Universe was just 1.4 billion years old. Galaxy SPT-S J041839-4751.9 is
strongly gravitationally lensed by a foreground galaxy at $z = 0.263$, and it
is a typical dusty starburst, with global star-forming and dust properties that
are in agreement with current numerical simulations and observations of its
galaxy population. Interferometric imaging at a spatial resolution of about 60
pc reveals a ratio of rotational-to-random motions of $V/\sigma = 9.7\pm 0.4$,
which is at least four times larger than expected from any galaxy evolution
model at this epoch, but similar to the ratios of spiral galaxies in the local
Universe. We derive a rotation curve with the typical shape of nearby massive
spiral galaxies, which demonstrates that at least some young galaxies are
dynamically akin to those observed in the local Universe, and only weakly
affected by extreme physical processes.
