Metal Deficiency in Two Massive Dead Galaxies at $z\sim 2$

Abstract

Local massive early-type galaxies are believed to have completed most of their star formation $\sim 10$Gyr ago and evolved without having substantial star formation since. If so, their progenitors should have roughly solar stellar metallicities ($Z_{\ast }$), comparable to their values today. We report the discovery of two lensed massive ($\log M_{\ast }/M_{\odot }\sim 11$), $z\sim 2.2$ dead galaxies, that appear markedly metal deficient given this scenario. Using 17-band $HST$+$K_s$+$Spitzer$ photometry and deep $HST$ grism spectra from the GLASS and SN Refsdal follow-up campaigns covering features near $\hat{I}{»}_{\mathrm{rest}}\sim 4000$Ã , we find these systems to be dominated by A-type stars with $\log Z_{\ast }/Z_{\odot }=-0.40\pm 0.02$ and $-0.49\pm 0.03$ ($30$-$40\%$ 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 $\log Z_{\ast }/Z_{\odot }=-0.6$ 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 $JamesWebbSpaceTelescope$-will provide critical insight into the nature of such phenomena.

Accepted for publication in Astrophysical Journal Letters; 6 pages, 3 figures, 1 table