ALMA Lensing Cluster Survey: A spectral stacking analysis of [CII] in lensed $z\sim6$ galaxies

TL;DR

This study used spectral stacking of ALMA data to search for [CII] emission in gravitationally lensed galaxies at z~6, finding no detection but setting upper limits on [CII] luminosity and dust obscuration, informing early galaxy evolution models.

Contribution

It presents the first stacking analysis of [CII] and dust emission in a large sample of z~6 lensed galaxies, providing new constraints on their interstellar medium properties.

Findings

No [CII] or dust emission detected in the stacked data.

Upper limits suggest [CII] is under-luminous compared to local relations.

Less than half of star formation is obscured by dust at z~6.

Abstract

The properties of galaxies at redshift $z>6$ hold the key to our understanding of the early stages of galaxy evolution and can potentially identify the sources of the ultraviolet radiation that give rise to the epoch of reionisation. The far-infrared cooling line of [CII] at 158$\mu$m is known to be bright and correlate with the star formation rate (SFR) of low-redshift galaxies, and hence is also suggested to be an important tracer of star formation and interstellar medium properties for very high-redshift galaxies. With the aim to study the interstellar medium properties of gravitationally lensed galaxies at $z>6$, we search for [CII] and thermal dust emission in a sample of 52 $z\sim6$ galaxies observed by the ALMA Lensing Cluster Survey (ALCS). We perform our analysis using \textsc{LineStacker}, stacking both [CII] and continuum emission. The target sample is selected from multiple catalogues, and the sample galaxies have spectroscopic redshift or low-uncertainty photometric redshifts ($\sigma_z < 0.02$) in nine galaxy clusters. Source properties of the target galaxies are either extracted from the literature or computed using spectral energy distribution (SED) fitting. Both weighted-average and median stacking are used, on both the full sample and three sub-samples. Our analyses find no detection of either [CII] or continuum. An upper limit on $L_{\rm [CII]}$ is derived, implying that [CII] remains marginally consistent for low-SFR $z>6$ galaxies but likely is under-luminous compared to the local $L_{\rm [CII]}$-SFR relationship. We discuss potential biases and possible physical effects that may be the cause of the non-detection. Further, the upper limit on the dust continuum implies that less than half of the star formation is obscured.