NGDEEP: The Star Formation and Ionization Properties of Galaxies at 1.7 < z < 3.4

We use JWST/NIRISS slitless spectroscopy from the Next Generation Deep Extragalactic Exploratory Public (NGDEEP) Survey to investigate the physical condition of 178 star-forming galaxies at 1.7 < z < 3.4. At these redshifts, the deep NGDEEP NIRISS slitless spectroscopy covers the [O ii]λλ3726,3729, [O iii]λλ4959,5007, Hβ and Hα emission features for galaxies with stellar masses log(M*/M⊙)≳7 , nearly a factor of 100 lower than previous studies. We focus on the [O ii]/[O iii] (O32) ratio which is primarily sensitive to the ionization state and with a secondary dependence on the gas-phase metallicity of the interstellar medium. We find significant (≳5σ) correlations between the O32 ratio and galaxy properties as O32 increases with decreasing stellar mass, decreasing star formation rate (SFR), increasing specific SFR (sSFR ≡ SFR/M *), and increasing equivalent width (EW) of Hα and Hβ. These trends suggest a tight connection between the ionization parameter and these galaxy properties. Galaxies at z ∼ 2–3 exhibit a higher O32 than local normal galaxies with the same stellar masses and SFRs, indicating that they have a higher ionization parameter and lower metallicity than local normal galaxies. In addition, we observe a mild evolutionary trend in the O32–EW(Hβ) relation from z ∼ 0 to z ≳ 5, where higher redshift galaxies show increased O32 and EW, with possibly higher O32 at fixed EW. We argue that both the enhanced recent star formation activity and the higher star formation surface density may contribute to the increase in O32 and the ionization parameter.