Program: GN-2020A-DD-110

We have recently discovered the z=5.039 strongly lensed galaxy COOL J1241+2219. It is the OIR-brightest galaxy known at z>5, surpassing the four next brightest systems (all also strongly lensed) by factors of 5-13. As such we anticipate that this will become a key object for detailed study with JWST and other facilities. Our current spectroscopy and imaging establish the redshift of the lensed source, and the lens (at z=1.002), as well as providing a reasonably confined stellar mass. These data also indicate that this galaxy has a blue UV continuum, which is fit by a sub-solar metallicity population with low extinction. The star formation rate, however, is poorly constrained. At such a high redshift, not long after the epoch of reionization, the only standard optical emission line accesible from the ground is the [OII] doublet, at 2.25 microns observed - all other standard lines are in redder JWST territory. We propose to measure this one available line with a short GNIRS observation. This will improve the relative SFR uncertainty by a factor of 30-50x, and inform upcoming JWST proposals, we will publish this measurement in a discovery paper that is already drafted, as an additional section that broadens and strongly improves that paper. Further analyses of these data will also occur in conjunction with SMA observations now underway, to be published later this year.