COOL-LAMPS: Characterizing the brightest known galaxy in the redshift > 5 Universe

We share the discovery of COOL-J1241+2219, a lensed galaxy at z = 5.04 that is the brightest galaxy known at z > 5. This galaxy and the cluster lens were discovered as part of the COOL-LAMPS project — ChicagO Optically-selected strong Lenses - Located At the Margins of Public Surveys — initiated to find strongly lensed systems in recent ground-based public imaging survey data, consisting primarily of a team of undergraduate students. COOL-J1241+2219 is five times brighter than the prior record-holder at these redshifts, at z_AB = 20.5. We characterize the lensed galaxy, as well as the brightest cluster galaxy (BCG) of the lens cluster (at z=1.00), using ground-based grizJH band photometry and rest-frame UV and optical spectroscopy. We use stellar population synthesis (SPS) modeling to characterize galaxy properties such as the stellar mass and star formation rate in the lensed galaxy. Using COOL-J1241+2219 as an anchor, we discuss the distribution of bright and lensed galaxies at z > 5. Despite the loss of an entire observing season due to COVID-19, the COOL-LAMPS project has had continued success in 2020; we highlight some standout lensed sources discovered and studied so far.