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

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 $\hat{I}¼=11.4\pm 1.9$) shows a line detection with $L_{[\mathrm{CII}]}=8.9\times 10^6$ L${}_{\odot }$, making it the lowest $L_{[\mathrm{CII}]}$ detection at $z>6$. For MS0451-H (SFR = 0.4 M${}_{\odot }$ yr${}^{-1}$ and $\hat{I}¼=100\pm 20$) we provide an upper limit of $L_{[\mathrm{CII}]}<3\times 10^5$ L${}_{\odot }$, which is 1\,dex below the local SFR-$L_{[\mathrm{CII}]}$ 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 $L_{[\mathrm{CII}]}$. The detection of A383-5.1 is encouraging and suggests that detections are possible, but much fainter than initially predicted.

5 pages, accepted for publication in MNRAS Letters