Missing [CII] emission from early galaxies

TL;DR

This study investigates whether surface brightness dimming causes the observed [CII] deficit in high-redshift galaxies, finding that correcting for this effect slightly adjusts their [CII] luminosity and aligns the [OIII]/[CII] ratio with simulations.

Contribution

The paper demonstrates that surface brightness dimming significantly affects [CII] luminosity estimates in early galaxies, providing a correction method and refining the understanding of their emission properties.

Findings

Extended [CII] emission is common in z>6 galaxies.

Surface brightness dimming can cause underestimation of [CII] luminosity by about a factor of 2.

Corrected [CII] luminosities are consistent with local relations and simulations.

Abstract

ALMA observations have revealed that [CII] 158$\mu$m line emission in high-z galaxies is ~2-3$\times$ 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 $L_{\rm [OIII]}/L_{\rm [CII]}$ 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 [CII] 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 0.8$^{\prime\prime}$, implying that $L_{\rm [CII]}$ is underestimated by a factor $\approx2$ in data at low (<7) signal-to-noise ratio . By combining these results, we conclude that $L_{\rm [CII]}$ of z>6 galaxies 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 of the relation. SBD correction also yields $L_{\rm [OIII]}/L_{\rm [CII]}<10$, i.e. more in line with current hydrodynamical simulations.