CLEAR: Emission-line Ratios at Cosmic High Noon

Bren E. Backhaus, Jonathan R. Trump, Nikko J. Cleri, Raymond Simons, Ivelina Momcheva, Casey Papovich, Vicente Estrada-Carpenter, Steven L. Finkelstein, Jasleen Matharu, Zhiyuan Ji, Benjamin Weiner, Mauro Giavalisco, Intae Jung

Research output: Contribution to journalArticlepeer-review

23 Scopus citations


We use Hubble Space Telescope Wide Field Camera 3 G102 and G141 grism spectroscopy to measure rest-frame optical emission-line ratios of 533 galaxies at z ∼1.5 in the CANDELS Lyα Emission at Reionization survey. We compare [O iii]/Hβ versus [S ii]/(Hα + [N ii]) as an "unVO87"diagram for 461 galaxies and [O iii]/Hβ versus [Ne iii]/[O ii] as an "OHNO"diagram for 91 galaxies. The unVO87 diagram does not effectively separate active galactic nuclei (AGN) and [Ne v] sources from star-forming galaxies, indicating that the unVO87 properties of star-forming galaxies evolve with redshift and overlap with AGN emission-line signatures at z > 1. The OHNO diagram does effectively separate X-ray AGN and [Ne v]-emitting galaxies from the rest of the population. We find that the [O iii]/Hβ line ratios are significantly anticorrelated with stellar mass and significantly correlated with log(LHβ), while [S ii]/(Hα + [N ii]) is significantly anticorrelated with log(LHβ). Comparison with MAPPINGS V photoionization models indicates that these trends are consistent with lower metallicity and higher ionization in low-mass and high-star formation rate (SFR) galaxies. We do not find evidence for redshift evolution of the emission-line ratios outside of the correlations with mass and SFR. Our results suggest that the OHNO diagram of [O iii]/Hβ versus [Ne iii]/[O ii] will be a useful indicator of AGN content and gas conditions in very high-redshift galaxies to be observed by the James Webb Space Telescope.

Original languageEnglish (US)
Article number161
JournalAstrophysical Journal
Issue number2
StatePublished - Feb 1 2022

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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