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 ISM 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 HII 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 [OIII]/H-beta and
[NII]/H-alpha. Our new line can be used to separate star-forming galaxies from
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.
