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Observations have revealed massive (logM*/Msun>11) galaxies that were alreadydead when the universe was only ~2 Gyr. Given the short time before thesegalaxies were quenched, their past histories and quenching mechanism(s) are ofparticular interest. In this paper, we study star formation histories (SFHs) of24 massive galaxies at 1.6<z<2.5. A deep slitless spectroscopy + imaging dataset collected from multiple Hubble Space Telescope surveys allows robustdetermination of their spectral energy distributions and SFHs with nofunctional assumption on their forms. We find that most of our massive galaxieshad formed > 50% of their extant masses by ~1.5 Gyr before the time of observedredshifts, with a trend where more massive galaxies form earlier. Theirstellar-phase metallicities are already compatible with those of localearly-type galaxies, with a median value of logZ*/Zsun=0.25 and scatter of~0.15dex. In combination with the reconstructed SFHs, we reveal their rapidmetallicity evolution from z~5.5 to ~2.2 at a rate of ~0.2dex/Gyr in logZ*/Zsun. Interestingly, the inferred stellar-phase metallicities are, whencompared at half-mass time, ~0.25dex higher than observed gas-phasemetallicities of star forming galaxies. While systematic uncertainties remain,this may imply that these quenched galaxies have continued low-level starformation, rather than abruptly terminating their star formation activity, andkept enhancing their metallicity until recently.

Massive Dead Galaxies at z~2 with HST Grism Spectroscopy I. Star Formation Histories and Metallicity Enrichment