Active galactic nuclei emission line diagnostics and the mass-metallicity relation up to redshift z ∼ 2: the impact of selection effects and evolution
Résumé
Emission line diagnostic diagrams probing the ionization sources in galaxies, such as the BaldwinPhillips-Terlevich (BPT) diagram, have been used extensively to distinguish AGN from purely starforming galaxies. Yet, they remain poorly understood at higher redshifts. We shed light on this issue with an empirical approach based on a z ∼ 0 reference sample built from ∼300,000 SDSS galaxies, from which we mimic selection effects due to typical emission line detection limits at higher redshift.
We combine this low-redshift reference sample with a simple prescription for luminosity evolution
of the global galaxy population to predict the loci of high-redshift galaxies on the BPT and MassExcitation (MEx) diagnostic diagrams. The predicted bivariate distributions agree remarkably well with direct observations of galaxies out to z ∼ 1.5, including the observed stellar mass-metallicity (MZ) relation evolution. As a result, we infer that high-redshift star-forming galaxies are consistent with having normal ISM properties out to z ∼ 1.5, after accounting for selection effects and line luminosity evolution. Namely, their optical line ratios and gas-phase metallicities are comparable to that of low-redshift galaxies with equivalent emission-line luminosities. In contrast, AGN narrow-line regions may show a shift toward lower metallicities at higher redshift. While a physical evolution of the ISM conditions is not ruled out for purely star-forming galaxies, and may be more important starting at z $\geq$2, we find that reliably quantifying this evolution is hindered by selections effects.
The recipes provided here may serve as a basis for future studies toward this goal. Code to predict
the loci of galaxies on the BPT and MEx diagnostic diagrams, and the MZ relation as a function of
emission line luminosity limits, is made publicly available.
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