A High-Resolution Investigation of the Multi-Phase ISM in a Galaxy
  during the First Two Billion Years
  We have carried out the first spatially-resolved investigation of the
multi-phase interstellar medium (ISM) at high redshift, using the z=4.24
strongly-lensed sub-millimetre galaxy H-ATLASJ142413.9+022303 (ID141). We
present high-resolution (down to ~350 pc) ALMA observations in dust continuum
emission and in the CO(7-6), H_2O (2_{1,1} - 2_{0,2}), CI(1-0) and CI(2-1)
lines, the latter two allowing us to spatially resolve the cool phase of the
ISM for the first time. Our modelling of the kinematics reveals that the system
appears to be dominated by a rotationally-supported gas disk with evidence of a
nearby perturber. We find that the CI(1-0) line has a very different
distribution to the other lines, showing the existence of a reservoir of cool
gas that might have been missed in studies of other galaxies. We have estimated
the mass of the ISM using four different tracers, always obtaining an estimate
in the range (3.2-3.8) x 10^{11} M_sol, significantly higher than our dynamical
mass estimate of (0.8-1.3) x 10^{11} M_sol. We suggest that this conflict and
other similar conflicts reported in the literature is because the gas-to-tracer
ratios are ~4 times lower than the Galactic values used to calibrate the ISM in
high-redshift galaxies. We demonstrate that this could result from a top-heavy
initial mass function and strong chemical evolution. Using a variety of
quantitative indicators, we show that, extreme though it is at z=4.24, ID141
will likely join the population of quiescent galaxies that appears in the
Universe at z~3.
