  We report on the discovery of a merger-driven starburst at z = 1.52,
PACS-787, based on high signal-to-noise ALMA observations. CO(5-4) and
continuum emission (850um) at a spatial resolution of 0.3" reveal two compact
(r_1/2 ~ 1 kpc) and interacting molecular gas disks at a separation of 8.6 kpc
thus indicative of an early stage in a merger. With a SFR of 991 Msun/yr, this
starburst event should occur closer to final coalescence, as usually seen in
hydrodynamical simulations. From the CO size, inclination, and velocity profile
for both disks, the dynamical mass is calculated through a novel method that
incorporates a calibration using simulations of galaxy mergers. Based on the
dynamical mass, we measure (1) the molecular gas mass, independent from the CO
luminosity, (2) the ratio of the total gas mass and the CO(1 - 0) luminosity
(alpha_CO = M_gas/L'_CO(1-0)), and (3) the gas-to-dust ratio, with the latter
two being lower than typically assumed. We find that the high star formation,
triggered in both galaxies, is caused by a set of optimal conditions: a high
gas mass/fraction, a short depletion time (t_depl=85 and 67 Myrs) to convert
gas into stars, and the interaction of likely counter-rotating molecular disks
that may accelerate the loss of angular momentum. The state of interaction is
further established by the detection of diffuse CO and continuum emission,
tidal debris that bridges the two nuclei and is associated with stellar
emission seen by HST/WFC3. This observation demonstrates the power of ALMA to
study the dynamics of galaxy mergers at high redshift.
