Theoretical Evolution of Optical Strong Lines across Cosmic Time

TL;DR

This paper predicts the evolution of optical emission-line ratios in galaxies over cosmic time using advanced simulations and models, focusing on the BPT diagram and the impact of shocks and metallicity.

Contribution

It introduces new theoretical models for galaxy emission lines across redshifts, incorporating shock effects and metallicity evolution, to interpret upcoming near-infrared spectroscopic data.

Findings

Galaxies' position on the BPT diagram reflects ISM conditions and redshift.

AGN and star-forming galaxies form a mixing sequence that evolves with cosmic time.

Galactic wind shocks are distinguishable from AGN at high redshift.

Abstract

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