2016ApJ...829L..10I

The central structure in three of the brightest unlensed z=3-4 submillimeter galaxies is investigated through 0.''015-0.''05 (120-360 pc) 860 µm continuum images obtained using the Atacama Large Millimeter/submillimeter Array (ALMA). The distribution in the central kiloparsec in AzTEC1 and AzTEC8 is extremely complex, and they are composed of multiple ∼200 pc clumps. AzTEC4 consists of two sources that are separated by ∼1.5 kpc, indicating a mid-stage merger. The peak star formation rate densities in the central clumps are ∼300-3000 M_☉ yr^-1 kpc^-2, suggesting regions with extreme star formation near the Eddington limit. By comparing the flux obtained by ALMA and Submillimeter Array, we find that 68%-90% of the emission is extended (>=1 kpc) in AzTEC4 and 8. For AzTEC1, we identify at least 11 additional compact (∼200 pc) clumps in the extended 3-4 kpc region. Overall, the data presented here suggest that the luminosity surface densities observed at <=150 pc scales are roughly similar to that observed in local ULIRGs, as in the eastern nucleus of Arp 220. Between 10% and 30% of the 860 µm continuum is concentrated in clumpy structures in the central kiloparsec, while the remaining flux is distributed over >=1 kpc regions, some of which could also be clumpy. These sources can be explained by a rapid inflow of gas such as a merger of gas-rich galaxies, surrounded by extended and clumpy starbursts. However, the cold mode accretion model is not ruled out.