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, f_gas^CO=M_gas/M_\star =13%-45% and f_gas^cont=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_⊙)=10.58_-0.2^+0.5, which is lower, but still consistent with the baryonic mass, log(M_bar=M_\star+M_gas^CO/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 relative concentration that increases the gas fraction up to f_gas^CO,1 kpc∼ 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.