THE cosmic BPT diagram: Confronting theory with observations

Lisa J. Kewley, Christian Maier, Kiyoto Yabe, Kouji Ohta, Masayuki Akiyama, Michael A. Dopita, Tiantian Yuan

Research output: Contribution to journalArticlepeer-review

170 Citations (Scopus)


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.

Original languageEnglish
Article numberL10
JournalAstrophysical Journal Letters
Issue number1
Publication statusPublished - 2013 Sep 1


  • galaxies: abundances
  • galaxies: active
  • galaxies: starburst

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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