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We present high spatial resolution (FWHM$\sim$0.14'') observations of theCO($8-7$) line in GDS-14876, a compact star-forming galaxy at $z=2.3$ withtotal stellar mass of $\log(M_{\star}/M_{\odot})=10.9$. The spatially resolvedvelocity map of the inner $r\lesssim1$~kpc reveals a continous velocitygradient consistent with the kinematics of a rotating disk with $v_{\rmrot}(r=1\rm kpc)=163\pm5$ km s$^{-1}$ and $v_{\rm rot}/\sigma\sim2.5$. Thegas-to-stellar ratios estimated from CO($8-7$) and the dust continuum emissionspan 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 theuncertainties in the conversion factors. The dynamical modeling yields adynamical mass of$\log(M_{\rm dyn}/M_{\odot})=10.58^{+0.5}_{-0.2}$ which islower, but still consistent with the baryonic mass, $\log$(M$_{\rm bar}$=M$_{\star}$ + M$^{\rm CO}_{\rm gas}$/M$_{\odot}$)$=11.0$, if the smallestCO-based gas fraction is assumed. Despite a low, overall gas fraction, thesmall physical extent of the dense, star-forming gas probed by CO($8-7$),$\sim3\times$ smaller than the stellar size, implies a strong concentrationthat increases the gas fraction up to $f^{\rm CO, 1\rm kpc}_{\rm gas}\sim 85\%$in the central 1 kpc. Such a gas-rich center, coupled with a highstar-formation rate, SFR$\sim$ 500 M$_{\odot}$ yr$^{-1}$, suggests thatGDS-14876 is quickly assembling a dense stellar component (bulge) in a strongnuclear starburst. Assuming its gas reservoir is depleted withoutreplenishment, GDS-14876 will quickly ($t_{\rm depl}\sim27$ Myr) become acompact quiescent galaxy that could retain some fraction of the observedrotational support.

Spatially resolved kinematics in the central 1 kpc of a compact star-forming galaxy at z=2.3 from ALMA CO observations