2016MNRAS.462L...6K

The far-infrared fine-structure line [C II] 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 submillimetre 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_☉ yr^–1 and magnification of µ = 11.4 ± 1.9) shows a line detection with L_ [C  II]_ = 8.9×10⁶ L_☉, making it the lowest L_ [C  II]_ detection at z > 6. For MS0451-H (SFR = 0.4 M_☉ yr^–1 and µ = 100 ± 20) we provide an upper limit of L_ [C  II]_ < 3×10⁵ L_☉, which is 1 dex below the local SFR-L_ [C  II]_ 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_[CII]. The detection of A383-5.1 is encouraging and suggests that detections are possible, but much fainter than initially predicted.