James Webb Space Telescope Feed Post

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

Paper 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\lambda3726,3729, [O III]\lambda\lambda4959,5007, H\beta and H\alpha emission features for galaxies with stellar masses \log(\mathrm{M_\ast/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 (\gtrsim5\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_*), 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~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 an evolutionary trend in the O_{32} -- EW(H\beta) relation from z~0 and z\gtrsim5, such that higher redshift galaxies have higher EW(H\beta) and 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.