Local massive early-type galaxies are believed to have completed most of their star formation ∼10Gyr ago and evolved without having substantial star formation since. If so, their progenitors should have roughly solar stellar metallicities (Z∗), comparable to their values today. We report the discovery of two lensed massive (logM∗/Msun ∼11), z∼2.2 dead galaxies, that appear markedly metal deﬁcient given this scenario. Using 17-band HST+Ks+Spitzer photometry and deep HST grism spectra from the GLASS and SN Refsdal followup campaigns covering features near λrest ∼4000 ˚A, we ﬁnd these systems to be dominated by A-type stars with logZ∗/Zsun =−0.40±0.02 and −0.49±0.03 (30-40% solar) under standard assumptions. The second system’s lower metallicity is robust to isochrone changes, though this choice can drive the ﬁrst system’s from logZ∗/Zsun =−0.6 to 0.1. If these two galaxies are representative of larger samples, this ﬁnding suggests that evolutionary paths other than dry minor-merging are required for these massive galaxies. Future analyses with direct metallicity measurements—e.g., by the James Webb Space Telescope—will provide critical insight into the nature of such phenomena.
Morishita, T.; Abramson, L. E.; Treu, T.; Wang, X.