Missing [CII] emission from early galaxies

ALMA observations have revealed that [CII]158µm line emission in high-z galaxies is ≈ 2 − 3× more extended than the UV continuum emission. Here we explore whether surface brightness dimming (SBD) of the [CII] line is responsible for the reported [CII] deficit, and the large L[OIII]/L[CII] luminosity ratio measured in early galaxies. We first analyse archival ALMA images of nine z > 6 galaxies observed in both [CII] and [OIII]. After performing several uv-tapering experiments to optimize the identification of extended line emission, we detect [CII] 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% of the extended [CII] component might be missed at an angular resolution of 0.800, implying that L[CII] is underestimated by a factor ≈ 2 in data at low (< 7) signal-to-noise ratio . By combining these results, we conclude that L[CII] of z > 6 galaxies lies, on average, slightly below the local L[CII] − SFR relation (∆ z=6−9 = −0.07 ± 0.3), but within the intrinsic dispersion of the relation. SBD correction also yields L[OIII]/L[CII] < 10, i.e. more in line with current hydrodynamical simulations.