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

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 $\mu = 11.4\pm1.9$)showsalinedetectionwith$L_{\rm [CII]} = 8.9\times10^{6}$L$\odot$,makingitthelowest$L{\rm [CII]}$detectionat$z>6$. For MS0451-H (SFR = 0.4 M${}_{\odot }$ yr${}^{-1}$ and $\mu =100\pm 20$) we provide an upper limit of $L_{[\mathrm{C}\mathrm{I}\mathrm{I}]}<3\times {10}^5$ L${}_{\odot }$, which is 1,dex below the local SFR-$L_{[\mathrm{C}\mathrm{I}\mathrm{I}]}$ 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 $L_{[\mathrm{C}\mathrm{I}\mathrm{I}]}$. The detection of A383-5.1 is encouraging and suggests that detections are possible, but much fainter than initially predicted.