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
TL;DR
The paper analyzes rest-optical emission-line ratios in z~1.5 galaxies from the CLEAR survey using HST/WFC3 grism spectra. It evaluates two diagnostics, unVO87 and OHNO, to separate star formation from AGN and to constrain metallicity and ionization at high redshift. The unVO87 diagram fails to cleanly separate AGN at z>1, while OHNO reliably identifies X-ray AGN and NeV-emitting systems; observed trends show lower metallicity and higher ionization in low-mass, high-SFR galaxies, with line ratios largely explained by MAPPINGS V models. No strong redshift evolution beyond mass and SFR trends is found, and OHNO is highlighted as a useful tool for JWST-era studies of gas conditions in the early universe.
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$α$ Emission at Reionization (CLEAR) survey. We compare $\frac{[OIII]}{Hβ}$ vs. $\frac{[SII]}{(Hα+[NII])}$ 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 $\frac{[OIII]}{Hβ}$ line ratios are significantly anti-correlated with stellar mass and significantly correlated with $\log(L_{Hβ})$, while $\frac{[SII]}{(Hα+[NII])}$ is significantly anti-correlated with $\log(L_{Hβ})$. 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.