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Galaxies occupy different regions of the[OIII]$\lambda5007$/H$\beta$-versus-[NII]$\lambda6584$/H$\alpha$ emission-lineratio diagram in the distant and local Universe. We investigate the origin ofthis intriguing result by modelling self-consistently, for the first time,nebular emission from young stars, accreting black holes (BHs) and older,post-asymptotic-giant-branch (post-AGB) stellar populations in galaxy formationsimulations in a full cosmological context. In post-processing, we couplenew-generation nebular-emission models with high-resolution, cosmologicalzoom-in simulations of massive galaxies to explore which galaxy physicalproperties drive the cosmic evolution of the optical-line ratios[OIII]$\lambda5007$/H$\beta$, [NII]$\lambda6584$/H$\alpha$,[SII]$\lambda\lambda6717,6731$/H$\alpha$ and [OI]$\lambda6300$/H$\alpha$. Theline ratios of simulated galaxies agree well with observations of bothstar-forming and active local SDSS galaxies. Towards higher redshifts, at fixedgalaxy stellar mass, the average [OIII]/H$\beta$ increases and [NII]/H$\alpha$,[SII]/H$\alpha$ and [OI]/H$\alpha$ decrease -- widely consistent withobservations. At fixed stellar mass, we identify star formation history, whichcontrols nebular emission from young stars via the ionization parameter, as theprimary driver of the cosmic evolution of [OIII]/H$\beta$ and [NII]/H$\alpha$.For [SII]/H$\alpha$ and [OI]/H$\alpha$, this applies only to redshifts above$z=1.5$, the evolution at lower redshift being driven in roughly equal parts bynebular emission from AGN and post-AGB stars. Instead, changes in the hardnessof ionizing radiation, ionized-gas density, the prevalence of BH accretionrelative to star formation and the dust-to-metal mass ratio (whose impact onthe gas-phase N/O ratio we model at fixed O/H) play at most a minor role in thecosmic evolution of simulated galaxy line ratios.

Synthetic nebular emission from massive galaxies I: origin of the cosmic evolution of optical emission-line ratios