The Cosmic BPT Diagram: Confronting Theory with Observations

We compare a large sample of galaxies between 0.5 < z < 2.6 with theoretical predictions for how the optical diagnostic line ratios in galaxy ensembles change as a function of cosmic time. We show that star-forming galaxies at high redshift (z > 1.5) are consistent with a model in which the interstellar medium conditions are more extreme at high redshift than seen in the global spectra of local galaxies. We speculate that global spectra of our high-redshift galaxies may be dominated by H II regions similar to the extreme clumpy, dense star-forming complexes in the Antennae and M82. The transition to local-type conditions occurs between 0.8 < z < 1.5. We conclude that classification schemes developed for local samples should not be applied at high redshift (z >= 1.5). We use our theoretical models to derive a new redshift-dependent classification line that utilizes the standard optical diagnostic line ratios [O III]/Hβ and [N II]/Hα. Our new line can be used to separate star-forming galaxies from active galactic nuclei (AGN) between z = 0 to z ~ 3.5. We anticipate that our redshift-dependent optical classification line will be useful for future large surveys with near-infrared multi-object spectrographs. We apply our classification line to a sample of gravitationally lensed galaxies at z ~ 2.5. Although limited by small numbers, we show that our classification line is consistent with the position of AGN that have been independently confirmed via other methods.