Exploring the Galaxy Mass-Metallicity Relation at z ∼ 3−5

Abstract

Long-duration gamma-ray bursts (GRBs) provide a premier tool for studying high-redshift star-forming galaxies thanks to their extreme brightness and association with massive stars. Here we use GRBs to study the galaxy mass-metallicity (M∗-Z) relation at z ∼ 3 − 5, where conventional direct metallicity measurements are extremely challenging. We use the interstellar medium metallicities of long-duration GRB hosts derived from afterglow absorption spectroscopy (Z ≈ 0.01−1 Z⊙), in conjunction with host galaxy stellar masses determined from deep Spitzer 3.6 µm observations of 20 GRB hosts. We detect about 1/4 of the hosts with MAB(I) ≈ −21.5 to −22.5 mag, and place a limit of MAB(I) & −19 mag on the remaining hosts from a stacking analysis. Using a conservative range of mass-to-light ratios for simple stellar populations (with ages of 70 Myr to ∼ 2 Gyr), we infer the host stellar masses and present the galaxy mass-metallicity measurements at z ∼ 3 − 5 (hzi ≈ 3.5). We find that the detected GRB hosts, with M∗ ≈ 2×1010 M⊙, display a wide range of metallicities, but that the mean metallicity at this mass scale, Z ≈ 0.1 Z⊙, is lower than measurements at z . 3. Combined with stacking of the nondetected hosts (with M∗ . 4×109 M⊙ and Z . 0.03 Z⊙), we find evidence for the existence of an M∗-Z relation at z ∼ 3.5 and continued evolution of this relation to systematically lower metallicities from z ∼ 2.