  The central structure in three of the brightest unlensed z=3-4 submillimeter
galaxies are investigated through 0.015" - 0.05" (120 -- 360~pc) 860 micron
continuum images obtained using the Atacama Large Millimeter/submillimeter
Array (ALMA). The distribution in the central kpc in AzTEC1 and AzTEC8 are
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 Msun/yr/kpc^2, suggesting regions with extreme star formation near the
Eddington Limit. By comparing the flux obtained by ALMA and Submillimeter Array
(SMA), we find that 68-90% of the emission is extended (> 1 kpc) in AzTEC 4 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 to 30% of the 860 micron continuum is concentrated in clumpy
structures in the central kpc 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.
