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.