Metal Deficiency in Two Massive Dead Galaxies at z ̃ 2

Abstract

Local massive early-type galaxies are believed to have completed most of their star formation ̃10 Gyr ago and evolved without having substantial star formation since that time. If so, their progenitors should have roughly solar stellar metallicities (Z <SUB>*</SUB>), comparable to their values today. We report the discovery of two lensed massive ({log}{M}<SUB>* </SUB>/{M}<SUB>☉ </SUB>̃ 11), z ̃ 2.2 dead galaxies that appear markedly metal deficient given this scenario. Using 17-band HST+K <SUB> s </SUB>+Spitzer photometry and deep Hubble Space Telescope (HST) grism spectra from the Grism Lens Amplified Survey from Space (GLASS) and supernova (SN) Refsdal follow-up campaigns covering features near λ <SUB>rest</SUB> ̃ 4000 Å, we find these systems to be dominated by A-type stars with {log}{Z}<SUB>* </SUB>/{Z}<SUB>☉ </SUB>=-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 {log}{Z}<SUB>* </SUB>/{Z}<SUB>☉ </SUB>=-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.