Bridging Optical and Far-infrared Emission-line Diagrams of Galaxies from Local to the Epoch of Reionization: Characteristic High [O iii] 88 μm/SFR at z > 6
Yuma Sugahara, Akio Inoue, Yoshinobu Fudamoto, Takuya Hashimoto, Yuichi Harikane, S. Yamanaka
Abstract
Abstract We present photoionization modeling of galaxy populations at z ∼ 0, 2, and >6 to bridge optical and far-infrared (FIR) emission-line diagrams. We collect galaxies with measurements of optical and/or FIR ([O iii ] 88 μ m and [C ii ] 158 μ m) emission-line fluxes and plot them on the [O iii ] λ 5007/H β –[N ii ] λ 6585/H α (BPT) and L ([O iii ] 88 )/SFR– L ([C ii ] 158 )/SFR diagrams, where SFR is the star formation rate and L ([O iii ] 88 ) and L ([C ii ] 158 ) are the FIR line luminosities. We aim to explain the galaxy distributions on the two diagrams with photoionization models that employ three nebular parameters: the ionization parameter U , hydrogen density n H , and gaseous metallicity Z gas . Our models successfully reproduce the nebular parameters of local galaxies, and then predict the distributions of the z ∼ 0, 2, and >6 galaxies in the diagrams. The predicted distributions illustrate the redshift evolution on all the diagrams; e.g., [O iii ]/H β and [O iii ] 88 /[C ii ] 158 ratios continuously decrease from z > 6 to 0. Specifically, the z > 6 galaxies exhibit ∼0.5 dex higher U than low-redshift galaxies at a given Z gas and show predicted flat distributions on the BPT diagram at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">log</mml:mi> </mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mrow> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> <mml:mspace width="0.25em"/> <mml:mrow> <mml:mi mathsize="small" mathvariant="normal">III</mml:mi> </mml:mrow> <mml:mo stretchy="false">]</mml:mo> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mi>β</mml:mi> <mml:mspace width="0.25em"/> <mml:mo>=</mml:mo> <mml:mspace width="0.25em"/> </mml:math> 0.5–0.8. We find that some of the z > 6 galaxies exhibit high L ([O iii ] 88 )/SFR ratios. To explain these high ratios, our photoionization models require a low stellar-to-gaseous-metallicity ratio or bursty/increasing star formation history at z > 6. JWST will test the predictions and scenarios for the z > 6 galaxies proposed by our photoionization modeling.