Missing [C II] emission from early galaxies

ALMA observations have revealed that [C II] 158 mu m line emission in high-z galaxies is approximate to 2-3 x more extended than the UV continuum emission. Here we explore whether surface brightness dimming (SBD) of the [C II] line is responsible for the reported [C II] deficit, and the large L-[O III]/L-[C II] luminosity ratio measured in early galaxies. We first analyse archival ALMA images of nine z > 6 galaxies observed in both [C II] and [O III]. After performing several uv-tapering experiments to optimize the identification of extended line emission, we detect [C II] emission in the whole sample, with an extent systematically larger than the [OIII] emission. Next, we use interferometric simulations to study the effect of SBD on the line luminosity estimate. About 40 per cent of the extended [C II] component might be missed at an angular resolution of 0.8 arcsec, implying that L-[C II] is underestimated by a factor approximate to 2 in data at low (<7) signal-to-noise ratio. By combining these results, we conclude that L-[C II] of z > 6 galaxies lies, on average, slightly below the local L-[C II] - SFR relation (Delta(z) = 6-9 = -0.07 +/- 0.3), but within the intrinsic dispersion of the relation. SBD correction also yields L-[O III]/L-[C II] < 10, i.e. more in line with current hydrodynamical simulations.