ALMA Lensing Cluster Survey: Bright [CII] 158 $\hat{I}¼$m Lines from a Multiply Imaged Sub-$L^{\star }$ Galaxy at $z=6.0719$

Abstract

We present bright [CII] 158 $\hat{I}¼$m line detections from a strongly magnified and multiply-imaged ($\hat{I}¼\sim 20-160$) sub-$L^{\ast }$ ($M_{\mathrm{UV}}$ = $-19.75_{-0.44}^{+0.55}$) Lyman-break galaxy (LBG) at $z=6.0719\pm 0.0004$ from the ALMA Lensing Cluster Survey (ALCS). Emission lines are identified at 268.7 GHz at $\ge$ 8$\ddot{I}ƒ$ exactly at positions of two multiple images of the LBG behind the massive galaxy cluster RXCJ0600$-$2007. Our lens models, updated with the latest spectroscopy from VLT/MUSE, indicate that a sub region of the LBG crosses the caustic and is lensed into a long ($\sim 6^{\prime \prime }$) arc with a local magnification of $\hat{I}¼\sim 160$, for which the [CII] line is also significantly detected. The source-plane reconstruction resolves the interstellar medium (ISM) structure, showing that the [CII] line is co-spatial with the rest-frame UV continuum at the scale of $\sim$300 pc. The [CII] line properties suggest that the LBG is a rotation-dominated system whose velocity gradient explains a slight difference of redshifts between the whole LBG and its sub region. The star formation rate (SFR)-$L_{[\mathrm{CII}]}$ relations from the sub to the whole regions of the LBG are consistent with those of local galaxies. We evaluate the lower limit of the faint-end of the [CII] luminosity function at $z=6$, and find that it is consistent with predictions from semi analytical models and from the local SFR-$L_{[\mathrm{CII}]}$ relation with a SFR function at $z=6$. These results imply that the local SFR-$L_{[\mathrm{CII}]}$ relation is universal for a wide range of scales including the spatially resolved ISM, the whole region of galaxy, and the cosmic scale, even in the epoch of reionization.

27 pages, 10 figures, 6 tables, ApJ in press