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

TL;DR

This study reconstructs the complex star-formation history of a massive, lensed galaxy at z=1.88, revealing a recent, centrally-concentrated rejuvenation phase involving about 0.5% of its stellar mass.

Contribution

It introduces a combined FUV and optical diagnostic to identify recent star formation, uncovering potential rejuvenation episodes in massive quiescent galaxies at high redshift.

Findings

Rejuvenation episodes are rare but present in massive galaxies at z~2.

Approximately 1% of such galaxies may be undergoing recent star formation.

A new FUV-based criterion improves detection of recent star formation.

Abstract

In this letter, we reconstruct the formation pathway of MRG-S0851, a massive, $\log M_*/M_\odot=11.02\pm0.04$, strongly lensed, red, galaxy at $z=1.883\pm0.001$. 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 $\sim$1 kpc in the last $\sim$100 Myr of evolution. We estimate $0.5\pm0.1\%$ of the total stellar mass is formed in this phase. Rejuvenation episodes are suggested to be infrequent for massive galaxies at $z\sim2$, but as our analyses indicate, more examples of complex star-formation histories may yet be hidden within existing data. By adding a 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 $\sim$1\% of massive quiescent galaxies at $1<z<2$ are potentially rejuvenating.