# The Absence of [CII] 158$\mu$m Emission in Spectroscopically-Confirmed Galaxies at $z>8$

## TL;DR

This study investigates the lack of [CII] 158μm emission in two high-redshift galaxies, revealing a potential redshift-dependent [CII] deficit and emphasizing the need for multiple emission diagnostics to understand early galaxy interstellar media.

## Contribution

The paper presents new ALMA observations of two galaxies at z>8, providing the first stringent upper limits on [CII] emission at these redshifts and discussing possible explanations for the [CII] deficit.

## Key findings

- Both galaxies show strong [OIII] 88μm emission but weak or absent [CII] emission.
- The results suggest a redshift-dependent [CII] deficit in early galaxies.
- Using multiple emission lines is crucial for understanding the interstellar medium at high redshift.

## Abstract

The scatter in the relationship between the strength of [CII] 158$\mu$m emission and the star formation rate at high-redshift has been the source of much recent interest. Although the relationship is well-established locally, several intensely star-forming galaxies have been found whose [CII] 158$\mu$m emission is either weak, absent or spatially offset from the young stars. Here we present new ALMA data for the two most distant, gravitationally-lensed and spectroscopically-confirmed galaxies, A2744\_YD4 at $z=$8.38 and MACS1149\_JD1 at $z=$9.11, both of which reveal intense [OIII] 88$\mu$m emission. In both cases we provide stringent upper limits on the presence of [CII] 158$\mu$m with respect to [OIII] 88$\mu$m. We review possible explanations for this apparent redshift-dependent [CII] deficit in the context of our recent hydrodynamical simulations. Our results highlight the importance of using several emission line diagnostics with ALMA to investigate the nature of the interstellar medium in early galaxies.