Metal Deficiency in Two Massive Dead Galaxies at $z\sim2$

TL;DR

This study discovers two massive, quiescent galaxies at z~2.2 that are unexpectedly metal deficient, challenging the assumption that such galaxies should have solar metallicity due to their early star formation history.

Contribution

It provides the first evidence of metal deficiency in massive, dead galaxies at high redshift, suggesting alternative evolutionary pathways beyond dry minor-merging.

Findings

Galaxies have metallicities of 30-40% solar.

Metallicity robust to isochrone variations for one galaxy.

Implication that other evolutionary processes are involved.

Abstract

Local massive early-type galaxies are believed to have completed most of their star formation $\sim10$Gyr 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 ($\log M_*/M_\odot\sim11$), $z\sim2.2$ 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 $\lambda_{\rm rest}\sim4000$\AA, we find these systems to be dominated by A-type stars with $\log Z_*/Z_\odot=-0.40\pm0.02$ and $-0.49\pm0.03$ ($30$-$40\%$ 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 $\log Z_*/Z_\odot=-0.6$ 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 $James\ Webb\ Space\ Telescope$-will provide critical insight into the nature of such phenomena.