The FMOS-COSMOS Survey of Star-forming Galaxies at z ~ 1.6. IV. Excitation State and Chemical Enrichment of the Interstellar Medium

Abstract

We investigate the physical conditions of ionized gas in high-z star-forming galaxies using diagnostic diagrams based on the rest-frame optical emission lines. The sample consists of 701 galaxies with an Hα detection at 1.4 ≾ z ≾ 1.7, from the Fiber Multi-Object Spectrograph (FMOS)-COSMOS survey, that represent the normal star-forming population over the stellar mass range 10^(9.6) ≾ M_*/M_⊙ ≾ 10^(11.6), with those at M_* > 10^(11) M_⊙ being well sampled. We confirm an offset of the average location of star-forming galaxies in the Baldwin–Phillips–Terlevich (BPT) diagram ([O III]/Hβ versus [N II]/Hα), primarily toward higher [O III]/Hβ, compared with local galaxies. Based on the [S ii] ratio, we measure an electron density n_e 220_(-130)^(+170) cm^(-3)), which is higher than that of local galaxies. Based on comparisons to theoretical models, we argue that changes in emission-line ratios, including the offset in the BPT diagram, are caused by a higher ionization parameter both at fixed stellar mass and at fixed metallicity, with additional contributions from a higher gas density and possibly a hardening of the ionizing radiation field. Ionization due to active galactic nuclei is ruled out as assessed with Chandra. As a consequence, we revisit the mass–metallicity relation using [N II]/Hα and a new calibration including [N II]/[S II] as recently introduced by Dopita et al. Consistent with our previous results, the most massive galaxies (M_* ≳ 10^(11) M_⊙ are fully enriched, while those at lower masses have metallicities lower than local galaxies. Finally, we demonstrate that the stellar masses, metallicities, and star formation rates of the FMOS sample are well fit with a physically motivated model for the chemical evolution of star-forming galaxies.