Spatially resolved kinematics in the central 1 kpc of a compact star-forming galaxy at z=2.3 from ALMA CO observations [GA]

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 $\log(M_{\star}/M_{\odot})=10.9$. The spatially resolved velocity map of the inner $r\lesssim1$~kpc reveals a continous velocity gradient consistent with the kinematics of a rotating disk with $v_{\rm rot}(r=1\rm kpc)=163\pm5$ km s$^{-1}$ 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, $f^{\rm CO}{\rm gas}=M{\rm gas}/M_{\star}=13-45\%$ and $f^{\rm cont}{\rm gas}=50-67\%$, but are nonetheless consistent given the uncertainties in the conversion factors. The dynamical modeling yields a dynamical mass of$\log(M{\rm dyn}/M_{\odot})=10.58^{+0.5}{-0.2}$ which is lower, but still consistent with the baryonic mass, $\log$(M${\rm bar}$= M${\star}$ + M$^{\rm CO}{\rm gas}$/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($8-7$), $\sim3\times$ smaller than the stellar size, implies a strong concentration that increases the gas fraction up to $f^{\rm CO, 1\rm kpc}{\rm gas}\sim 85\%$ 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 ($t{\rm depl}\sim27$ Myr) become a compact quiescent galaxy that could retain some fraction of the observed rotational support.

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G. Barro, M. Kriek, P. Perez-Gonzalez, et. al.
Wed, 6 Dec 17

Comments: Accepted for Publication in ApJL. Kinematic maps are shown in Figures 2 and 4