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

Abstract

We use JWST/NIRISS slitless spectroscopy from the Next Generation Deep Extragalactic Exploratory Public (NGDEEP) Survey to investigate the physical condition of star-forming galaxies at $1.7<z<3.4$. At these redshifts, the deep NGDEEP NIRISS slitless spectroscopy covers the [O II]$\lambda \lambda$3726,3729, [O III]$\lambda \lambda$4959,5007, H$\beta$ and H$\alpha$ emission features for galaxies with stellar masses $\log ({\mathrm{M}}_{\ast }/{\mathrm{M}}_{\odot })\gtrsim 7$, nearly a factor of a hundred lower than previous studies. We focus on the [O III]/[O II] (O${}_{32}$) 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 ($\gtrsim 5\sigma$) correlations between the O${}_{32}$ ratio and galaxy properties as O${}_{32}$ increases with decreasing stellar mass, decreasing star formation rate (SFR), increasing specific SFR (sSFR$\equiv \mathrm{SFR}/M_{\ast }$), and increasing equivalent width (EW) of H$\beta$ and H$\alpha$. These trends suggest a tight connection between the ionization parameter and these galaxy properties. Galaxies at $z\sim 2-3$ exhibit a higher O${}_{32}$ 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 O${}_{32}$ -- EW(H$\beta$) relation from $z\sim 0$ to $z\gtrsim 5$, where higher redshift galaxies show increased O${}_{32}$ and EW, with possibly higher O${}_{32}$ 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 O${}_{32}$ and the ionization parameter.

31 pages, 12 figures

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