We present high spatial resolution (FWHM$sim$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 $log(M_{star}/M_{odot})=10.9$. The spatially resolved velocity map of the inner $rlesssim1$~kpc reveals a continous velocity gradient consistent with the kinematics of a rotating disk with $v_{rm rot}(r=1rm kpc)=163pm5$ km s$^{-1}$ and $v_{rm rot}/sigmasim2.5$. The gas-to-stellar ratios estimated from CO($8-7$) and the dust continuum emission span a broad range, $f^{rm CO}_{rm gas}=M_{rm gas}/M_{star}=13-45%$ and $f^{rm cont}_{rm gas}=50-67%$, but are nonetheless consistent given the uncertainties in the conversion factors. The dynamical modeling yields a dynamical mass of$log(M_{rm dyn}/M_{odot})=10.58^{+0.5}_{-0.2}$ which is lower, but still consistent with the baryonic mass, $log$(M$_{rm bar}$= M$_{star}$ + M$^{rm CO}_{rm gas}$/M$_{odot}$)$=11.0$, 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($8-7$), $sim3times$ smaller than the stellar size, implies a strong concentration that increases the gas fraction up to $f^{rm CO, 1rm kpc}_{rm gas}sim 85%$ in the central 1 kpc. Such a gas-rich center, coupled with a high star-formation rate, SFR$sim$ 500 M$_{odot}$ 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 ($t_{rm depl}sim27$ Myr) become a compact quiescent galaxy that could retain some fraction of the observed rotational support.
Download