The Brightest Galaxy in the z>5 Universe: Observing Distant Lensed Galaxies with RST

The Roman Space Telescope (RST), with its wide area NIR coverage at HST-like spatial resolution, will be a unique resource for the discovery and characterization of strongly-lensed galaxies in the early universe. It will fill the broad gap in discovery space between the brightest systems found in all-sky shallower data, and the faintest lensed systems found already in the Frontier Fields and expected from JWST. We share here an example of the former. COOLJ1241+2219, a lensed galaxy at z=5.043, is the brightest galaxy known at z > 5. This galaxy was discovered via COOL-LAMPS (ChicagO Optically-selected strong Lenses - Located At the Margins of Public Surveys), a project initiated to find strongly-lensed systems in recent ground-based public imaging survey data, consisting of a team of undergraduates. COOLJ1241+2219 is 5x brighter than the prior record-holder at these redshifts, at z mag (AB)=20.5. We characterize the lensed galaxy, and the brightest cluster galaxy (BCG) of the lens cluster (at z=1.002), using ground-based grizJH band photometry and rest-frame UV+optical spectroscopy. We use stellar population synthesis to characterize its stellar mass and star formation rate. Using COOLJ1241+2219 as an anchor, we discuss the enormous benefits of RST’s resolution combined with the resolving power of strong gravitational lensing, to systematically observe the interiors of distant galaxies, and to capture rest-frame UV signatures from stellar populations in high-z galaxies.