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ALMA observations have revealed that [CII] 158$\mu$m line emission in high-zgalaxies is ~2-3$\times$ more extended than the UV continuum emission. Here weexplore whether surface brightness dimming (SBD) of the [CII] line isresponsible for the reported [CII] deficit, and the large $L_{\rm[OIII]}/L_{\rm [CII]}$ luminosity ratio measured in early galaxies. We firstanalyse archival ALMA images of nine z>6 galaxies observed in both [CII] and[OIII]. After performing several uv-tapering experiments to optimize theidentification of extended line emission, we detect [CII] emission in the wholesample, with an extent systematically larger than the [CII] emission. Next, weuse interferometric simulations to study the effect of SBD on the lineluminosity estimate. About 40% of the extended [CII] component might be missedat an angular resolution of 0.8$^{\prime\prime}$, implying that $L_{\rm [CII]}$is underestimated by a factor $\approx2$ in data at low (<7) signal-to-noiseratio . By combining these results, we conclude that $L_{\rm [CII]}$ of z>6galaxies lies, on average, slightly below the local $L_{\rm [CII]}-SFR$relation ($\Delta^{z=6-9}=-0.07\pm0.3$), but within the intrinsic dispersion ofthe relation. SBD correction also yields $L_{\rm [OIII]}/L_{\rm [CII]}<10$,i.e. more in line with current hydrodynamical simulations.

Missing [CII] emission from early galaxies