A gravitationally unstable gas disk of a starburst galaxy 12 billion
  years ago
  Submillimeter bright galaxies in the early Universe are vigorously forming
stars at ~1000 times higher rate than the Milky Way. A large fraction of stars
is formed in the central 1 kiloparsec region, that is comparable in size to
massive, quiescent galaxies found at the peak of the cosmic star formation
history, and eventually the core of giant elliptical galaxies in the
present-day Universe. However, the physical and kinematic properties inside a
compact starburst core are poorly understood because dissecting it requires
angular resolution even higher than the Hubble Space Telescope can offer. Here
we report 550 parsec-resolution observations of gas and dust in the brightest
unlensed submillimeter galaxy at z=4.3. We map out for the first time the
spatial and kinematic structure of molecular gas inside the heavily
dust-obscured core. The gas distribution is clumpy while the underlying disk is
rotation-supported. Exploiting the high-quality map of molecular gas mass
surface density, we find a strong evidence that the starburst disk is
gravitationally unstable, implying that the self-gravity of gas overcomes the
differential rotation and the internal pressure by stellar radiation feedback.
The observed molecular gas would be consumed by star formation in a timescale
of 100 million years, that is comparable to those in merging starburst
galaxies. Our results suggest that the most extreme starburst in the early
Universe originates from efficient star formation due to a gravitational
instability in the central 2 kpc region.
