The Cosmic BPT Diagram: Confronting Theory with Observations
Lisa J. Kewley (ANU, U.Hawaii), Christian Maier (U. Vienna), Kiyoto, Yabe (NAOJ), Kouji Ohta (Kyoto University), Masayuki Akiyama (Tohoku, University), Michael A. Dopita (ANU, King Abdulaziz University), Tiantian, Yuan (ANU)

TL;DR
This paper compares galaxy emission line ratios across redshifts with theoretical models, revealing that high-redshift galaxies have more extreme ISM conditions and proposing a new redshift-dependent classification scheme for star-forming galaxies and AGN.
Contribution
The study introduces a new redshift-dependent optical classification line based on theoretical models, improving galaxy classification accuracy up to z~3.5.
Findings
High-redshift galaxies (z>1.5) have more extreme ISM conditions.
A transition to local-like conditions occurs between 0.8<z<1.5.
The new classification line is consistent with independent AGN identifications.
Abstract
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…
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