CLEAR: Emission Line Ratios at Cosmic High Noon

Abstract

We use Hubble Space Telescope WFC3 G102 and G141 grism spectroscopy to measure rest-optical emission-line ratios of 533 galaxies at $z\sim 1.5$ in the CANDELS Ly$\hat{I}\pm$ Emission at Reionization (CLEAR) survey. We compare $\frac{[OIII]}{H{\hat{I}}^2}$ vs. $\frac{[SII]}{(H\hat{I}\pm +[NII])}$ 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 $\frac{[OIII]}{H{\hat{I}}^2}$ line ratios are significantly anti-correlated with stellar mass and significantly correlated with $\log (L_{H{\hat{I}}^2})$, while $\frac{[SII]}{(H\hat{I}\pm +[NII])}$ is significantly anti-correlated with $\log (L_{H{\hat{I}}^2})$. 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.

20 pages, 22 figures