THEORETICAL EVOLUTION OF OPTICAL STRONG LINES ACROSS COSMIC TIME

Abstract

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