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arXiv · 2109.08147

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
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Abstract

We use Hubble Space Telescope WFC3 G102 and G141 grism spectroscopy to measure rest-optical emission-line ratios of 533 galaxies at $z\sim1.5$ in the CANDELS Ly$\alpha$ Emission at Reionization (CLEAR) survey. We compare $\frac{[OIII]}{H\beta}$ vs. $\frac{[SII]}{(H\alpha +[NII])}$ as an "unVO87" diagram for 461 galaxies and $\frac{[OIII]}{Hb}$ vs. $\frac{[NeIII]}{[OII]}$ as an "OHNO" diagram for 91 galaxies. The unVO87 diagram does not effectively separate active galactic nuclei (AGN) and $[NeV]$ 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 effectively separates X-ray AGN and $[NeV]$-emitting galaxies from the rest of the population. We find that the $\frac{[OIII]}{H\beta}$ line ratios are significantly anti-correlated with stellar mass and significantly correlated with $\log(L_{H\beta})$, while $\frac{[SII]}{(H\alpha +[NII])}$ is significantly anti-correlated with $\log(L_{H\beta})$. Comparison with MAPPINGS~V photoionization models indicates that these trends are consistent with lower metallicity and higher ionization in low-mass and high-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 $\frac{[OIII]}{Hb}$ vs. $\frac{[NeIII]}{[OII]}$ 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.

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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. 2021-09-15. CLEAR: Emission Line Ratios at Cosmic High Noon. https://doi.org/10.3847/1538-4357/ac3919

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