We present high spatial resolution ( FWHM \sim 0 \farcs 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 r \lesssim 1 kpc reveals a continous velocity gradient consistent with the kinematics of a rotating disk with v _ { rot } ( r = 1 kpc ) = 163 \pm 5 km s ^ { -1 } and v _ { rot } / \sigma \sim 2.5 .
The gas-to-stellar ratios estimated from CO ( 8 - 7 ) and the dust continuum emission span a broad range , f ^ { CO } _ { gas } = M _ { gas } / M _ { \star } = 13 - 45 \% and f ^ { cont } _ { gas } = 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 _ { \odot } ) = 10.58 ^ { +0.5 } _ { -0.2 } which is lower , but still consistent with the baryonic mass , \log ( M _ { bar } =M _ { \star } + M ^ { CO } _ { 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 ) , \sim 3 \times smaller than the stellar size , implies a strong relative concentration that increases the gas fraction up to f ^ { CO, 1 kpc } _ { 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 _ { depl } \sim 27 Myr ) become a compact quiescent galaxy that could retain some fraction of the observed rotational support .