ALMA reveals potential evidence for spiral arms, bars, and rings in high-redshift submillimeter galaxies (1810.12307v3)

Abstract: We present sub-kpc-scale mapping of the 870 $\mu$m ALMA continuum emission in six luminous ($L_{\rm IR}~\sim~5~\times10^{12}$ L${\odot}$) submillimeter galaxies (SMGs) from the ALESS survey of the Extended Chandra Deep Field South. Our high-fidelity 0.07$''$-resolution imaging ($\sim$500 pc) reveals robust evidence for structures with deconvolved sizes of $\lesssim$0.5-1 kpc embedded within (dominant) exponential dust disks. The large-scale morphologies of the structures within some of the galaxies show clear curvature and/or clump-like structures bracketing elongated nuclear emission, suggestive of bars, star-forming rings, and spiral arms. In this interpretation, the ratio of the ring' andbar' radii (1.9$\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{\rm 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 ($\Sigma_{\rm SFR}$) 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.

An Analysis of Sub-kpc Dust Structures in High-Redshift Submillimeter Galaxies

The research by Hodge et al. presents a detailed examination of the sub-kiloparsec (sub-kpc) structures in luminous high-redshift submillimeter galaxies (SMGs) using high-resolution data from the Atacama Large Millimeter Array (ALMA). The investigation focuses on six SMGs from the ALESS survey, providing insights into their morphological features, the spatial distribution of star-forming regions, and their implications for galaxy evolution.

Key Findings

1. Structural Evidence: The paper provides compelling sub-kpc-scale evidence for structural features such as spiral arms, bars, and rings within the observed SMGs. These structures have deconvolved sizes ranging from less than 500 pc to 1 kpc and account for a significant fraction (2-20%) of a galaxy's total continuum emission. The presence of these features suggests complex dynamics and potentially interaction-driven morphological disturbances.
2. Star Formation Rate Densities: High-resolution maps of the star formation rate density (ΣSFR) reveal substantial variations between the galaxies, with peak densities ranging from approximately 40 to 600 M⊙ yr^-1 kpc^-2. These variations are linked to differences in integrated dust temperatures and luminosities across the galaxies, reflecting diverse physical environments and star-forming efficiencies.
3. Comparison with Local U/LIRGs: The results indicate that SMGs can support high star formation rate densities over more extensive scales compared to local ultra-luminous infrared galaxies (U/LIRGs). Despite their massive star-forming potential, the SMGs studied do not exceed the Eddington limit typically imposed by radiation pressure on dust, even at the resolved 500 pc scales.
4. Potential Morphological Features: The alignment of clump-like structures along the galaxy's major axis, observed in several sources, suggests possible barreled structures with accompanying rings. The ratio of ring to bar radii is consistent with those seen in local galaxies, implying flat inner rotation curves and possible bar-driven enhancements in star formation efficiency.

Implications and Future Directions

The detection of potential bars and spiral features in these high-redshift galaxies challenges the conventional understanding of the evolutionary timeline and formation mechanisms of such structures, typically prevalent in later cosmic phases. These findings can prompt a reevaluation of early galaxy interactions and the conditions conducive to non-axisymmetric features.

Future studies could benefit from high-resolution kinematic data to verify the interpretations of the morphological features and gain further insights into the dynamics governing such starburst regions. Additionally, deeper, multi-wavelength observations might elucidate the role of dust temperature gradients and their impact on Σ_SFR calculations, offering a more granular understanding of star formation processes in these distant systems.

These findings bolster the narrative of SMGs as vital laboratories for exploring early star formation and galaxy evolution, bridging the observational gap between the epoch of peak star formation and the local universe's mature, structured galaxies.