The dust and [CII] morphologies of redshift ~4.5 sub-millimeter galaxies
  at ~200pc resolution: The absence of large clumps in the interstellar medium
  of high-redshift galaxies
  We present deep high resolution (0.03", 200pc) ALMA Band 7 observations
covering the dust continuum and [CII] $\lambda157.7\mu$m emission in four
$z\sim4.4-4.8$ sub-millimeter galaxies (SMGs) selected from the ALESS and
AS2UDS surveys. The data show that the rest-frame 160$\mu$m (observed 345 GHz)
dust emission is consistent with smooth morphologies on kpc scales for three of
the sources. One source, UDS47.0, 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 high angular
resolution (0.03") and depth of these observations ($\sigma_{345\text{GHz}}
\sim27-47\mu$Jy beam$^{-1}$). The four SMGs are bright [CII] emitters, and we
extract [CII] spectra from the high resolution data, and recover $\sim20-100$%
of the [CII] flux and $\sim40-80$% of the dust continuum emission, compared to
the previous lower resolution observations. When tapered to 0.2" resolution our
maps recover $\sim80-100$% of the continuum emission, indicating that $\sim60$%
of the emission is resolved out on $\sim200$pc scales. We find that the [CII]
emission in high-redshift galaxies is more spatially extended than the
rest-frame 160$\mu$m dust continuum by a factor of $1.6\pm0.4$. By considering
the $L_{\text{[CII]}}$/$L_{\text{FIR}}$ ratio as a function of the
star-formation rate surface density ($\Sigma_{\text{SFR}}$) we revisit the
[CII] deficit, and suggest that the decline in the
$L_{\text{[CII]}}$/$L_{\text{FIR}}$ ratio as a function of
$\Sigma_{\text{SFR}}$ 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.
