We present deep, high-resolution (0.″03, 200 pc) ALMA Band 7
observations covering the dust continuum and [C II] λ157.7 μm
emission in four z ˜ 4.4-4.8 sub-millimeter galaxies (SMGs)
selected from the ALESS and AS2UDS surveys. The data show that the
rest-frame 160 μm (observed 345 GHz) dust emission is consistent with
smooth morphologies on kpc scales for three of the sources. One source,
UDS 47.0, displays apparent substructure, but this is also consistent
with a smooth morphology—as indicated by simulations showing that
smooth exponential disks can appear clumpy when observed at the high
angular resolution (0.″03) and depth of these observations
({σ }345{GHz}˜ 27{--}47 μJy
beam-1). The four SMGs are bright [C II] emitters. We
extract [C II] spectra from the high-resolution data, and recover
˜20%-100% of the [C II] flux and ˜40%-80% of the
dust continuum emission, compared to the previous lower-resolution
observations. When tapered to 0.″2 resolution, our maps recover
˜80%-100% of the continuum emission, indicating that ˜60%
of the emission is resolved out on ˜200 pc scales. We find that the
[C II] emission in high-redshift galaxies is more spatially extended
than the rest-frame 160 μm dust continuum by a factor of 1.6 ±
0.4. By considering the {L}[{{C}{{II}}]}/{L}FIR}
ratio as a function of the star formation rate surface density
({{{Σ }}}SFR}), we revisit the [C II] deficit and
suggest that the decline in the
{L}[{{C}{{II}}]}/{L}FIR} ratio as a function of
{{{Σ }}}SFR} is consistent with local processes. We
also explore the physical drivers that may be responsible for these
trends and can give rise to the properties found in the densest regions
of SMGs.
