CEERS Key Paper VIII: Emission Line Ratios from NIRSpec and NIRCam Wide-Field Slitless Spectroscopy at z>2

TL;DR

This study uses JWST NIRCam and NIRSpec spectroscopy to analyze emission line ratios in 155 galaxies at z>2, revealing correlations with redshift, stellar mass, and star formation rate, and comparing observations with theoretical models.

Contribution

It provides new measurements of emission line ratios at high redshift using JWST data and compares these with models to understand galaxy metallicity and ionization evolution.

Findings

Significant correlation between [OIII]/Hb and redshift.

Emission-line ratios correlate with star formation rate.

High [OIII]/Hb ratios are consistent with low metallicity and high ionization.

Abstract

We use James Webb Space Telescope Near-Infrared Camera Wide Field Slitless Spectroscopy (NIRCam WFSS) and Near-Infrared spectrograph (NIRSpec) in the Cosmic Evolution Early Release survey (CEERS) to measure rest-frame optical emission-line of 155 galaxies at z>2. The blind NIRCam grism observations include a sample of galaxies with bright emission lines that were not observed on the NIRSpec masks. We study the changes of the Ha, [OIII]/Hb, and [NeIII]/[OII] emission lines in terms of redshift by comparing to lower redshift SDSS and CLEAR samples. We find a significant (>3$\sigma$) correlation between [OIII]/Hb with redshift, while [NeIII]/[OII] has a marginal (2$\sigma$) correlation with redshift. We compare [OIII]/Hb and [NeIII]/[OII] to stellar mass and Hb SFR. We find that both emission-line ratios have a correlation with Hb SFR and an anti-correlation with stellar mass across the redshifts 0<z<9. Comparison with MAPPINGS~V models indicates that these trends are consistent with lower metallicity and higher ionization in low-mass and high-SFR galaxies. We additionally compare to IllustriousTNG predictions and find that they effectively describe the highest [OIII]/Hb ratios observed in our sample, without the need to invoke MAPPINGS models with significant shock ionizionation components.