The Ages of Passive Galaxies in a z = 1.62 Protocluster
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© 2017. The American Astronomical Society. All rights reserved. We present a study of the relation between galaxy stellar age and mass for 14 members of the z = 1.62 protocluster IRC 0218, using multiband imaging and HST G102 and G141 grism spectroscopy. Using UVJ colors to separate galaxies into star-forming and quiescent populations, we find that, at stellar masses M∗ ≥ 1010.85 M⊙, the quiescent fraction in the protocluster is fQ = 1.0-0.37+0.00, consistent with a ∼ 2× enhancement relative to the field value, fQ = 0.45-0.03+0.03. At masses 1010.2 M⊙ ≤ M∗ ≤ 1010.85 M⊙, fQ in the cluster is fQ = 0.40-0.18+0.20, consistent with the field value of fQ = 0.28-0.02+0.02. Using galaxy Dn (4000)values derived from the G102 spectroscopy, we find no relation between galaxy stellar age and mass. These results may reflect the impact of merger-driven mass redistribution - which is plausible, as this cluster is known to host many dry mergers. Alternately, they may imply that the trend in fQ in IRC 0218 was imprinted over a short timescale in the protocluster's assembly history. Comparing our results with those of other high-redshift studies and studies of clusters at z ∼ 1, we determine that our observed relation between fQ and stellar mass only mildly evolves between z ∼ 1.6 and z ∼ 1, and only at stellar masses M∗ ≤ 1010.85 M⊙. Both the z ∼ 1 and z ∼ 1.6 results are in agreement that the red sequence in dense environments was already populated at high redshift, z ≳ 3, placing constraints on the mechanism(s) responsible for quenching in dense environments at z ≥ 1.5.
author list (cited authors)
Lee-Brown, D. B., Rudnick, G. H., Momcheva, I. G., Papovich, C., Lotz, J. M., Tran, K., ... Farrah, D.