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 178 star-forming galaxies at 1.7 &lt; z &lt; 3.4. At these redshifts, the deep NGDEEP NIRISS slitless spectroscopy covers the [O ii ] λλ 3726,3729, [O iii ] λλ 4959,5007, H β and H α emission features for galaxies with stellar masses <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≳</mml:mo> <mml:mn>7</mml:mn> </mml:math> , nearly a factor of 100 lower than previous studies. We focus on the [O ii ]/[O iii ] (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 (≳5 σ ) 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 ≡ SFR/ M * ), and increasing equivalent width (EW) of H α and H β . 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 a mild evolutionary trend in the O 32 –EW(H β ) relation from z ∼ 0 to z ≳ 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.