ALMA [CII] detection of a redshift 7 lensed galaxy behind RXJ1347.1-1145

Abstract

We present the results of ALMA spectroscopic follow-up of a $z=6.765$ Lyman-$α$ emitting galaxy behind the cluster RXJ1347-1145. We report the detection of [CII]158$μ$m line fully consistent with the Lyman-$α$ redshift and with the peak of the optical emission. Given the magnification of $μ=5.0 \pm 0.3$ the intrinsic (corrected for lensing) luminosity of the [CII] line is $L_{[CII]} =1.4^{+0.2}_{-0.3} \times 10^7L_{\odot}$, which is ${\sim}5$ times fainter than other detections of $z\sim 7$ galaxies. The result indicates that low $L_{[CII]}$ in $z\sim 7$ galaxies compared to the local counterparts might be caused by their low metallicities and/or feedback. The small velocity off-set ($Δv = 20_{-40}^{+140} \rm km/s$) between the Lyman-$α$ and [CII] line is unusual, and may be indicative of ionizing photons escaping.

Acepted for publication to ApJLetters