Early Galaxy Assembly Uncovered with ALMA and JWST: A Remarkably UV and [CII] Bright, Strongly Lensed Sub-L* Galaxy at z=6.072

We propose NIRCam imaging and NIRSpec IFU spectroscopy for two multiple images of a strongly lensed Lyman break galaxy at z[CII]=6.072 discovered by a 100-hr ALMA Lensing Cluster Survey. Among normal star-forming galaxies at z>5, both of the multiple images are the brightest lensed sources known, with the H-band continuum of H~23.7 mag and the [CII]158um line flux of ~20 mJy due to the high magnification factors. One of the lensed images is stretched over a 6''-scale arc crossing the critical line, making a z=6 sub-L* galaxy (Mstar~10^9 Msun) with extremely faint dust emission (L_FIR~10^9 Lsun) highly visible. Exploiting these unique ALMA lensed sources with well characterised cold inter-stellar medium (ISM) properties, this JWST program unveils the remaining key elements of their hot ISM and stellar properties by 1) mapping out star-forming regions and stellar mass distributions down to ~20-80 pc scales and ~32-35 mag after the lens correction, 2) obtaining rest-frame optical lines of Halpha, Hbeta, [OIII]5007, [NII]6583, [SII]6716,6731 to determine ionizing sources on the BPT diagram and to estimate electron density, metal gradients, and dust attenuation, and 3) probing kinematics of star-forming clumps and the rotation curve of the host galaxy disk up to ~2.5xeffective radius to test the existence of inflow/outflowing gas clumps and a potential declination of the rotation curve as reported in baryon-dominated massive galaxies at z~1-2. This joint ALMA and JWST study is an essential step in providing a representative view of the early galaxy assembly at z=6 with building blocks of both dark matter and baryon down to globular cluster scales.