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

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Barro, G.
Kriek, M.
Díaz Santos, T.
Price, S. H.
Rujopakarn, W.
Pandya, V.
Koo, D. C.
Faber, S. M.
Dekel, A.
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We present high spatial resolution (FWHM similar to 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-circle dot) = 10.9. The spatially resolved velocity map of the inner r less than or similar to 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)/s similar to 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-* = 13%-45% and f(gas)(CO) = 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-circle dot.) = 10.5(-0.2)(+0.5), which is lower, but still consistent with the baryonic mass, log(M-bar = M-circle dot + M-gas(CO)/M-circle dot)= 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), similar to 3x smaller than the stellar size, implies a strong relative concentration that increases the gas fraction up to f(gas)(CO) similar to 85% , 1 kpc in the central 1 kpc. Such a gas-rich center, coupled with a high star formation rate (SFR) similar to 500. M-circle dot 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) similar to 27 Myr) become a compact quiescent galaxy that could retain some fraction of the observed rotational support.
© 2017. The American Astronomical Society. All rights reserved. G.B. and M.K. acknowledge support from HST-AR-12847 and HST-AR-14552. P.G.P.-G. acknowledge support from grants AYA2015-63650-P and AYA2015-70815-ERC. W.R. is supported by JSPS KAKENHI grant number JP15K17604 and the Thailand Research Fund/Office of the Higher Education Commission grant number MRG6080294. This Letter makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.00907.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
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