CLEAR: Emission-line Ratios at Cosmic High Noon Academic Article uri icon


  • Abstract 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 Ly Emission at Reionization survey. We compare [O iii]/H versus [S ii]/(H + [N ii]) as an unVO87 diagram for 461 galaxies and [O iii]/H 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]/H line ratios are significantly anticorrelated with stellar mass and significantly correlated with m{H}}\beta })$?> log ( L H ) , while [S ii]/(H + [N ii]) is significantly anticorrelated with m{H}}\beta })$?> log ( L H ) . 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]/H 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.

published proceedings


altmetric score

  • 8.08

author list (cited authors)

  • Backhaus, B. E., Trump, J. R., Clen, N. J., Simons, R., Momcheva, I., Papovich, C., ... Jung, I.

citation count

  • 15

complete list of authors

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

publication date

  • February 2022