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

Abstract

We present high spatial resolution (FWHM ∼ 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 ⊙ ) = 10.9. The spatially resolved velocity map of the inner r ≲ 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 / σ  ∼ 2.5. The gas-to-stellar ratios estimated from CO(8–7) and the dust continuum emission span a broad range, and , but are nonetheless consistent given the uncertainties in the conversion factors. The dynamical modeling yields a dynamical mass of , which is lower, but still consistent with the baryonic mass, , 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), ∼3× smaller than the stellar size, implies a strong relative concentration that increases the gas fraction up to in the central 1 kpc. Such a gas-rich center, coupled with a high star formation rate (SFR) ∼ 500  M ⊙ 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 ∼ 27 Myr) become a compact quiescent galaxy that could retain some fraction of the observed rotational support.