CLEAR: Emission-line Ratios at Cosmic High Noon
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AbstractWe 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 })$?> log(LH), while [S ii]/(H + [N ii]) is significantly anticorrelated with m{H}}\beta })$?> log(LH). 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.
