Spatially Resolved Kinematics in the Central 1 kpc of a Compact Star-forming Galaxy at z ∼ 2.3 from ALMA CO Observations
Astrophysical Journal Letters
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 total stellar mass of (M_/M_)=10.9. The spatially resolved velocity map of the inner r≲1 kpc reveals a continous 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, f^ CO_ gas=M_
... % 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(M_ dyn/M_)=10.58^+0.5_-0.2 which is lower, but still consistent with the baryonic mass, (M_ bar= M_ + M^ CO_ gas/M_)=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), ∼3× smaller than the stellar size, implies a strong concentration that increases the gas fraction up to f^ CO, 1 kpc_ gas∼ 85% 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.