Spatially Resolved Kinematics in the Central 1 kpc of a Compact Star-forming Galaxy at z ∼ 2.3 from ALMA CO Observations

We present high spatial resolution (FWHM similar to 0.14) observations of the CO(8-7) line in GDS-14876, a compact star-forming galaxy at z = 2.3 with a total stellar mass of log(M-*/M-circle dot) = 10.9. The spatially resolved velocity map of the inner r less than or similar to 1 kpc reveals a continuous velocity gradient consistent with the kinematics of a rotating disk with v(rot)(r = 1 kpc) = 163. +/-. 5 km.s(-1) and v(rot)/s similar to 2.5. The gas-to-stellar ratios estimated from CO(8-7) and the dust continuum emission span a broad range, f(gas)(CO) = M-gas/M-* = 13%-45% and f(gas)(CO) = 50%-67% but are nonetheless consistent given the uncertainties in the conversion factors. The dynamical modeling yields a dynamical mass of log(M-dyn/M-circle dot.) = 10.5(-0.2)(+0.5), which is lower, but still consistent with the baryonic mass, log(M-bar = M-circle dot + M-gas(CO)/M-circle dot)= 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), similar to 3x smaller than the stellar size, implies a strong relative concentration that increases the gas fraction up to f(gas)(CO) similar to 85% , 1 kpc in the central 1 kpc. Such a gas-rich center, coupled with a high star formation rate (SFR) similar to 500. M-circle dot 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 (t(depl) similar to 27 Myr) become a compact quiescent galaxy that could retain some fraction of the observed rotational support.