2018ApJ...859...12G

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_345GHz∼27–47µJy 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_[CII]/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_[CII]/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.