  We present an analysis of high-resolution ALMA interferometry of CO(4-3) line
emission and dust continuum in the "Ruby" (PLCK_G244.8+54.9), a bright,
gravitationally lensed galaxy at z = 3.0 discovered with the Planck all-sky
survey. The Ruby is the brightest of Planck's Dusty GEMS, a sample of 11 of the
brightest gravitationally lensed high-redshift galaxies on the extragalactic
sub-mm sky. We resolve the high-surface-brightness continuum and CO line
emission of the Ruby in several extended clumps along a partial, nearly
circular Einstein ring with 1.4" diameter around a massive galaxy at z = 1.5.
Local star-formation intensities are up to 2000 M$_{\odot}$ yr$^{-1}$
kpc$^{-2}$, amongst the highest observed at high redshift, and clearly in the
range of maximal starbursts. Gas-mass surface densities are a few $\times$
10$^4$ M$_{\odot}$ pc$^{-2}$. The Ruby lies at, and in part even above, the
starburst sequence in the Schmidt-Kennicutt diagram, and at the limit expected
for star formation that is self-regulated through the kinetic energy injection
from radiation pressure, stellar winds, and supernovae. We show that these
processes can also inject sufficient kinetic energy and momentum into the gas
to explain the turbulent line widths, which are consistent with marginally
gravitationally bound molecular clouds embedded in a critically Toomre-stable
disk. The star-formation efficiency is in the range 1-10% per free-fall time,
consistent with the notion that the pressure balance that sets the local
star-formation law in the Milky Way may well be universal out to the highest
star-formation intensities. AGN feedback is not necessary to regulate the star
formation in the Ruby, in agreement with the absence of a bright AGN component
in the infrared and radio regimes.
