The ALPINE-ALMA [CII] Survey: the population of [CII]-undetected galaxies and their role in the $\mathrm{L_{[CII]}}$-SFR relation

TL;DR

This study investigates the population of [CII]-undetected high-redshift galaxies in the ALPINE survey, revealing their contribution to the [CII]-SFR relation and supporting the use of [CII] as a star formation tracer up to early cosmic times.

Contribution

It characterizes [CII] non-detections in ALPINE, stacking spectra to reveal their average [CII] luminosity and confirming the [CII]-SFR relation's consistency at high redshift.

Findings

Stacked spectra yield a significant [CII] detection for non-detected galaxies.

The [CII]-SFR relation slope is consistent with local universe measurements.

No evidence of [CII] deficit in high-redshift galaxies was found.

Abstract

The [CII] 158$~\mu$m emission line represents so far one of the most profitable tools for the investigation of the high-redshift galaxies in the early Universe. Being one of the brightest cooling lines in the rest-frame far-infrared regime of star-forming galaxies, it has been successfully exploited as a tracer of star-formation rate (SFR) in local sources. The picture is more complex at higher redshifts, where its usability in this context is still under investigation. Recent results from the ALMA Large Program to INvestigate [CII] at Early times (ALPINE) survey suggest that there is no (or weak) evolution of the L$\mathrm{_{[CII]}}$-SFR relation up to $z\sim6$ but their reliability is hampered by the presence of a large population of [CII] non-detected galaxies. In this work, we characterize the population of [CII] non-detections in ALPINE. By stacking their ALMA spectra, we obtain a signal detected at $\sim5.1\sigma$, resulting in a [CII] luminosity of $\mathrm{log(L_\mathrm{[CII]}}/\mathrm{L_{\odot}})$ $\sim7.8$. When combining this value with those from the [CII] detections, we find a $\mathrm{L_{[CII]}}$-SFR relation with a slope $b=1.14\pm0.11$, in agreement within the uncertainties both with the linear relation found in the local Universe, and with the previous findings from ALPINE at $z\sim5$. This suggests that the [CII] line can be considered a good tracer of star formation up to the distant Universe. Finally, we show that the galaxies of our sample that most deviate from the observed L$_\mathrm{[CII]}$-SFR relation could suffer from a less precise redshift estimation, perhaps artificially reducing their [CII] luminosity. In this respect, we claim that there is no evidence in favour of a deficit of [CII] content in high-z galaxies, in contrast with earlier studies.