Recent Star Formation in a Massive Slowly Quenched Lensed Quiescent Galaxy at z = 1.88

In this Letter, we reconstruct the formation pathway of MRG-S0851, a massive, $\mathrm{log}{M}_{* }/{M}_{\odot }=11.02\pm 0.04$ , strongly lensed, red galaxy at z = 1.883 ± 0.001. While the global photometry and spatially resolved outskirts of MRG-S0851 imply an early formation scenario with a slowly decreasing or constant star formation history, a joint fit of 2D grism spectroscopy and photometry reveals a more complex scenario: MRG-S0851 is likely to be experiencing a centrally concentrated rejuvenation in the inner ∼1 kpc in the last ∼100 Myr of evolution. We estimate 0.5 ± 0.1% of the total stellar mass is formed in this phase. Rejuvenation episodes are suggested to be infrequent for massive galaxies at z ∼ 2, but as our analyses indicate, more examples of complex star formation histories may yet be hidden within existing data. By adding an FUV color criterion to the standard U-V/V-J diagnostic—thereby heightening our sensitivity to recent star formation—we show that we can select populations of galaxies with similar spectral energy distributions to that of MRG-S0851, but note that deep follow-up spectroscopic observations and/or spatially resolved analyses are necessary to robustly confirm the rejuvenation of these candidates. Using our criteria with MRG-S0851 as a prototype, we estimate that ∼1% of massive quiescent galaxies at 1 < z < 2 are potentially rejuvenating.