[CII] emission in z ~ 6 strongly lensed, star-forming galaxies

TL;DR

This study reports ALMA observations of two high-redshift, lensed star-forming galaxies, detecting [CII] emission in one and setting upper limits in the other, highlighting challenges and prospects for using [CII] as a star-formation tracer at z~6.

Contribution

First detection of [CII] emission in a z~6 galaxy with low luminosity, providing insights into [CII] emission in early, low-metallicity galaxies and informing future observations.

Findings

Detected [CII] in one galaxy at z~6 with low luminosity.

Set upper limit on [CII] emission in a second galaxy.

Results align with low-metallicity galaxy predictions at high redshift.

Abstract

The far-infrared fine-structure line [CII] at 1900.5\,GHz is known to be one of the brightest cooling lines in local galaxies, and therefore it has been suggested to be an efficient tracer for star-formation in very high-redshift galaxies. However, recent results for galaxies at $z>6$ have yielded numerous non-detections in star-forming galaxies, except for quasars and submillimeter galaxies. We report the results of ALMA observations of two lensed, star-forming galaxies at $z = 6.029$ and $z=6.703$. The galaxy A383-5.1 (star formation rate [SFR] of 3.2 M$_\odot$ yr$^{-1}$ and magnification of $\mu = 11.4\pm1.9$) shows a line detection with $L_{\rm [CII]} = 8.9\times10^{6}$ L$_\odot$, making it the lowest $L_{\rm [CII]}$ detection at $z>6$. For MS0451-H (SFR = 0.4 M$_\odot$ yr$^{-1}$ and $\mu = 100\pm20$) we provide an upper limit of $L_{\rm [CII]} < 3\times10^{5}$ L$_\odot$, which is 1\,dex below the local SFR-$L_{\rm [CII]}$ relations. The results are consistent with predictions for low-metallicity galaxies at $z>6$, however, other effects could also play a role in terms of decreasing $L_{\rm [CII]}$. The detection of A383-5.1 is encouraging and suggests that detections are possible, but much fainter than initially predicted.