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$\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 $\log (M_{\star }\mathrm{/}{M}_{\odot })=10.9$. The spatially resolved velocity map of the inner $r\lesssim 1$~kpc reveals a continous velocity gradient consistent with the kinematics of a rotating disk with $v_{\rm rot}(r=1\rm kpc)=163\pm5$kms$^{-1}$and$v_{\rm rot}/\sigma\sim2.5$. The gas-to-stellar ratios estimated from CO($8-7$) and the dust continuum emission span a broad range, $f_{\mathrm{g}\mathrm{a}\mathrm{s}}^{\mathrm{C}\mathrm{O}}=M_{\mathrm{g}\mathrm{a}\mathrm{s}}\mathrm{/}{M}_{\star }=13-45\mathrm{\%}$ and $f_{\mathrm{g}\mathrm{a}\mathrm{s}}^{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}}=50-67\mathrm{\%}$, but are nonetheless consistent given the uncertainties in the conversion factors. The dynamical modeling yields a dynamical mass of$\log (M_{\mathrm{d}\mathrm{y}\mathrm{n}}\mathrm{/}{M}_{\odot })=10.58_{-0.2}^{+0.5}$ which is lower, but still consistent with the baryonic mass, $\log$(M${}_{\mathrm{b}\mathrm{a}\mathrm{r}}$= M${}_{\star }$ + M${}_{\mathrm{g}\mathrm{a}\mathrm{s}}^{\mathrm{C}\mathrm{O}}$/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($8-7$), $\sim 3\times$ smaller than the stellar size, implies a strong concentration that increases the gas fraction up to $f_{\mathrm{g}\mathrm{a}\mathrm{s}}^{\mathrm{C}\mathrm{O},1\mathrm{k}\mathrm{p}\mathrm{c}}\sim 85\mathrm{\%}$ 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 ($t_{\mathrm{d}\mathrm{e}\mathrm{p}\mathrm{l}}\sim 27$ Myr) become a compact quiescent galaxy that could retain some fraction of the observed rotational support.