of \[\]/FIR deficit in two strongly lensed galaxies at $z$=1.7 and 5.6 were recently presented by @Lamarche2018 and @Litke2018, respectively, showing a pronounced \[\]/FIR deficit ($10^{-4} - 10^{-3}$) on (sub)kpc scales.

Despite the recent progress, it is still unclear how well the results and relations derived from local observations hold for the high-redshift population, especially the intensely star-forming high-redshift SMGs, with $\Sigma_\mathrm{SFR}$ two to three orders of magnitude higher than the local star-forming galaxies ($10^{-3}-1$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$, e.g.,@Smith2017). To directly compare the \[\] emission in high-redshift SMGs to the local galaxies and study its connection to the star-formation, high-resolution (kpc-scale) observations of the \[\] emission, alongside the rest-frame FIR continuum (tracing the obscured star formation) and the low-$J$ CO emission tracing the molecular gas are necessary.

At high redshift, such resolved, multi-tracer studies are limited by the angular extent of the source (few arcseconds at most) and the need for robust redshifts to ensure that both \[\] and low-$J$ CO emission are observable from the ground. For example, @Stacey2010 compared unresolved \[\]/FIR/CO($J_{upp}\leq4$) observations of a heterogeneous sample of $z=1-2$ galaxies. @Gullberg2015 compared \[\], FIR and CO(2–1)/(1–0) observations in 20 strongly lensed SMGs ($z_\mathrm{source}$=2.1–5.7); they found the \[\] and CO 