In general relativity, the presence of matter can curve spacetime, and the path of a light ray will be deflected as a result. This is strong gravitational lensing.

An unexpected discovery of the last decade is the remarkable compactness of the quiescent progenitors of today's most massive elliptical galaxies. Although some consensus exists about how these early galaxies grow through merging to become present-day ellipticals, the details of the formation of such dense objects are hotly debated. Although formation models predict unique observational signatures that are imprinted on the stellar populations and structures, these galaxies are often barely resolved, even with the Hubble Space Telescope (HST) and similarly in the future with the James Webb Space Telescope.

Strong gravitational lensing can provide an unmatched improvement in spatial resolution, and, despite these objects' low space density, we have identified a unique sample of ten strongly lensed high-redshift (1.6<z<3.2), massive (10.5<logM*<11.7) quiescent galaxies at this pivotal epoch. Our survey includes new HST WFC3/G141 grism spectroscopy to measure age gradients in the inner cores (<1kpc) of the to-date most comprehensive sample of strongly lensed massive quiescent galaxies. Our analysis will combine deep G141 grism spectroscopy with a wealth of existing rest-frame FUV-optical HST and Spitzer/IRAC imaging. Additionally, we will add ultra-deep dust continuum imaging from a Cycle 6 and 7 REQUIEM-ALMA program.

Exploiting the unrivaled spatially resolved, multi-wavelength REQUIEM dataset, we will characterize the stellar populations and dust content of the premier sample of quiescent galaxies to provide strong constraints on the physical mechanisms driving massive galaxy evolution in this key phase of their formation.