We present high spatial resolution (FWHM∼0.14'') observations of the
CO(8−7) line in GDS-14876, a compact star-forming galaxy at z=2.3 with
total stellar mass of log(M⋆/M⊙)=10.9. The spatially resolved
velocity map of the inner r≲1~kpc reveals a continous velocity
gradient consistent with the kinematics of a rotating disk with vrot(r=1kpc)=163±5 km s−1 and vrot/σ∼2.5. The
gas-to-stellar ratios estimated from CO(8−7) and the dust continuum emission
span a broad range, fgasCO=Mgas/M⋆=13−45% and
fgascont=50−67%, but are nonetheless consistent given the
uncertainties in the conversion factors. The dynamical modeling yields a
dynamical mass oflog(Mdyn/M⊙)=10.58−0.2+0.5 which is
lower, but still consistent with the baryonic mass, log(Mbar=
M⋆ + MgasCO/M⊙)=11.0, if the smallest
CO-based gas fraction is assumed. Despite a low, overall gas fraction, the
small physical extent of the dense, star-forming gas probed by CO(8−7),
∼3× smaller than the stellar size, implies a strong concentration
that increases the gas fraction up to fgasCO,1kpc∼85%
in the central 1 kpc. Such a gas-rich center, coupled with a high
star-formation rate, SFR∼ 500 M⊙ yr−1, suggests that
GDS-14876 is quickly assembling a dense stellar component (bulge) in a strong
nuclear starburst. Assuming its gas reservoir is depleted without
replenishment, GDS-14876 will quickly (tdepl∼27 Myr) become a
compact quiescent galaxy that could retain some fraction of the observed
rotational support.Comment: Accepted for Publication in ApJL. Kinematic maps are shown in Figures
2 and