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Published May 1, 2016 | Published + Submitted
Journal Article Open

Rest-UV Absorption Lines as Metallicity Estimator: the Metal Content of Star-Forming Galaxies at z ~ 5


We measure a relation between the depth of four prominent rest-UV absorption complexes and metallicity for local galaxies and verify it up to z ~ 3. We then apply this relation to a sample of 224 galaxies at 3.5 < z < 6.0 ( 〈z〉 = 4.8) in the Cosmic Evolution Survey (COSMOS), for which unique UV spectra from the Deep Imaging Multi-object Spectrograph (DEIMOS) and accurate stellar masses from the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH) are available. The average galaxy population at z ~ 5 and log(M/M_☉) > 9 is characterized by 0.3–0.4 dex (in units of 12 + log O/H lower metallicities than at z ~ 2, but comparable to z ~ 3.5. We find galaxies with weak or no Lyα emission to have metallicities comparable to z ~ 2 galaxies and therefore may represent an evolved subpopulation of z ~ 5 galaxies. We find a correlation between metallicity and dust in good agreement with local galaxies and an inverse trend between metallicity and star-formation rate consistent with observations at z ~ 2. The relation between stellar mass and metallicity (MZ relation) is similar to z ~ 3.5, but there are indications of it being slightly shallower, in particular for the young, Lyα-emitting galaxies. We show that, within a "bathtub" approach, a shallower MZ relation is expected in the case of a fast (exponential) build-up of stellar mass with an e-folding time of 100–200 Myr. Because of this fast evolution, the process of dust production and metal enrichment as a function of mass could be more stochastic in the first billion years of galaxy formation compared to later times.

Additional Information

© 2016 The American Astronomical Society. Received 2015 November 19; accepted 2016 February 13; published 2016 May 2. We would like to acknowledge the support of the Keck Observatory staff who made these observations possible as well as Micaela Bagley, Janice Lee, and David Sobral for valuable discussions. We also would like to thank the referee for valuable comments that have improved this paper. A.F. acknowledges support from the Swiss National Science Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

Attached Files

Published - apj_822_1_29.pdf

Submitted - 1512.00018v1.pdf


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