Theoretical Evolution of Optical Strong Lines Across Cosmic Time

We use the chemical evolution predictions of cosmological hydrodynamic simulations with our latest theoretical population synthesis, photoionization, and shock models to predict the strong line evolution of ensembles galaxies from z = 3 to the present day. In this paper, we focus on the brightest optical emission-line ratios, ii/Hα and O iii/Hβ. We use the optical diagnostic Baldwin–Phillips–Terlevich (BPT) diagram as a tool for the spectral properties of ensembles of active galaxies. We use four redshift windows chosen to new near-infrared multi-object spectrographs. We predict how the BPT diagram will appear in these four windows given different sets of assumptions. We show that the position of star-forming galaxies on the diagram traces the interstellar medium conditions and radiation field in galaxies at a given redshift. Galaxies active galactic nucleus (AGN) form a mixing sequence with purely star-forming galaxies. This mixing may change dramatically with cosmic time, due to the metallicity sensitivity of the optical emission-lines. , the position of the mixing sequence may probe metallicity gradients in galaxies as a function of, depending on the size of the AGN narrow-line region. We apply our latest slow shock models for gas by galactic-scale winds. We show that at high redshift, galactic wind shocks are clearly separated from in line ratio space. Instead, shocks from galactic winds mimic high metallicity starburst galaxies. We discuss models in the context of future large near-infrared spectroscopic surveys.