Rotating starburst cores in massive galaxies at z=2.5
  We present spatially resolved ALMA observations of the CO J=3-2 emission line
in two massive galaxies at z=2.5 on the star-forming main sequence. Both
galaxies have compact dusty star-forming cores with effective radii of Re=1.3
kpc and Re=1.2 kpc in the 870 um continuum emission. The spatial extent of
star-forming molecular gas is also compact with Re=1.9 kpc and Re=2.3 kpc, but
more extended than the dust emission. Interpreting the observed
position-velocity diagrams with dynamical models, we find the starburst cores
to be rotation-dominated with the ratio of the maximum rotation velocity to the
local velocity dispersion of v/sigma=7.0 (v=386 km/s) and v/sigma_0=4.1 (v=391
km/s). Given that the descendants of these massive galaxies in the local
universe are likely ellipticals with v/sigma nearly an order of magnitude
lower, the rapidly rotating galaxies would lose significant net angular
momentum in the intervening time. The comparisons among dynamical, stellar,
gas, and dust mass suggest that the starburst CO-to-H2 conversion factor of
alpha_CO=0.8 Msun/(K km/s/pc2) is appropriate in the spatially resolved cores.
The dense cores are likely to be formed in extreme environments similar to the
central regions of local ultraluminous infrared galaxies. Our work also
demonstrates that a combination of medium-resolution CO and high-resolution
dust continuum observations is a powerful tool for characterizing the dynamical
state of molecular gas in distant galaxies.
