Central concentration of warm and dense molecular gas in a strongly lensed submillimeter galaxy at z=6

Abstract

We report the detection of the CO(12-11) line emission toward G09-83808 (or H-ATLAS J090045.4+004125), a strongly-lensed submillimeter galaxy at $z=6.02$, with Atacama Large Millimeter/submillimeter Array observations. Combining previously detected [O III]$88\hat{\mathrm{I}}¼\mathrm{m}$, [N II]$205\hat{\mathrm{I}}¼\mathrm{m}$, and dust continuum at 0.6mm and 1.5mm, we investigate the physical properties of the multi-phase interstellar medium in G09-83808. A source-plane reconstruction reveals that the region of the CO(12-11) emission is compact ($R_{\mathrm{e},\mathrm{CO}}=0.49_{-0.19}^{+0.29}\mathrm{kpc}$) and roughly coincides with that of the dust continuum. Non-local thermodynamic equilibrium radiative transfer modeling of CO spectral-line energy distribution reveals that most of the CO(12-11) emission comes from a warm (kinetic temperature of $T_{\mathrm{kin}}=320\pm 170$K) and dense ($\log (n_{H2}/\mathrm{c}{\mathrm{m}}^{-3})=5.4\pm 0.6$) gas, indicating that the warm and dense molecular gas is concentrated in the central 0.5-kpc region. The luminosity ratio in G09-83808 is estimated to be $L_{\mathrm{CO}(12-11)}/L_{\mathrm{CO}(6-5)}=1.1\pm 0.2$. The high ratio is consistent with those in local active galactic nuclei (AGNs) and $6<z<7$ quasars, the fact of which implies that G09-83808 would be a good target to explore dust-obscured AGNs in the epoch of reionization. In the reconstructed [O III]$88\hat{\mathrm{I}}¼\mathrm{m}$ and [N II]$205\hat{\mathrm{I}}¼\mathrm{m}$ cubes, we also find that a monotonic velocity gradient is extending over the central starburst region by a factor of two and that star-forming sub-components exist. High-resolution observations of bright [C II]$158\hat{\mathrm{I}}¼\mathrm{m}$ line emissions will enable us to characterize the kinematics of a possible rotating disk and the nature of the sub-components.

13 pages, 11 figures, accepted for publication in PASJ