ALMA reveals potential evidence for spiral arms, bars, and rings in high-redshift submillimeter galaxies

We present sub-kpc-scale mapping of the 870 $\mu$m ALMA continuum emission in six luminous ($L_{\mathrm{I}\mathrm{R}}\sim 5\times {10}^{12}$ L${}_{\odot }$) submillimeter galaxies (SMGs) from the ALESS survey of the Extended Chandra Deep Field South. Our high-fidelity 0.07${}^{\mathrm{\prime }\mathrm{\prime }}$-resolution imaging ($\sim$500 pc) reveals robust evidence for structures with deconvolved sizes of $\lesssim$0.5-1 kpc embedded within (dominant) exponential dust disks. The large-scale morphologies of the structures within some of the galaxies show clear curvature and/or clump-like structures bracketing elongated nuclear emission, suggestive of bars, star-forming rings, and spiral arms. In this interpretation, the ratio of the ring' and bar’ radii (1.9$\pm$0.3) agrees with that measured for such features in local galaxies. These potential spiral/ring/bar structures would be consistent with the idea of tidal disturbances, with their detailed properties implying flat inner rotation curves and Toomre-unstable disks (Q<1). The inferred one-dimensional velocity dispersions (${\sigma }_{\mathrm{r}}\lesssim$ 70-160 km s${}^{-1}$) are marginally consistent with the limits implied if the sizes of the largest structures are comparable to the Jeans length. We create maps of the star formation rate density (${\mathrm{\Sigma }}_{\mathrm{S}\mathrm{F}\mathrm{R}}$) on $\sim$500 pc scales and show that the SMGs are able to sustain a given (galaxy-averaged) $\Sigma_{\rm SFR}$ over much larger physical scales than local (ultra-)luminous infrared galaxies. However, on 500 pc scales, they do not exceed the Eddington limit set by radiation pressure on dust. If confirmed by kinematics, the potential presence of non-axisymmetric structures would provide a means for net angular momentum loss and efficient star formation, helping to explain the very high star formation rates measured in SMGs.