A rise in the ionizing photons in star-forming galaxies over the past 5 billion years

TL;DR

This study reveals a significant increase in ionizing photon output in star-forming galaxies over the past 5 billion years, driven by rising ionization parameters linked to younger stellar populations.

Contribution

It provides the first detailed analysis of how ionization parameters and line ratios evolve in galaxies between redshifts 0.2 and 0.6, ruling out AGN and other factors as causes.

Findings

Ionization parameter increases with redshift.

Line ratios [OIII]/Hb and [OIII]/[OII] rise over 0.2<z<0.6.

Strong correlation between ionization parameter and young stellar fraction.

Abstract

We investigate the change in ionizing photons in galaxies between 0.2<z<0.6 using the F2 field of the SHELS complete galaxy redshift survey. We show, for the first time, that while the [OIII]/Hb and [OIII]/[OII] ratios rise, the [NII]/H-alpha and [SII]/H-alpha ratios fall significantly over the 0.2<z<0.35 redshift range for stellar masses between 9.2<log(M/Msun)<10.6. The [OIII]/H-beta and [OIII]/[OII] ratios continue to rise across the full 0.2<z<0.6 redshift range for stellar masses between 9.8<log(M/Msun)<10.0. We conclusively rule out AGN contamination, a changing ISM pressure, and a change in the hardness of the EUV radiation field as the cause of the change in the line ratios between 0.2<z<0.35. We find that the ionization parameter rises significantly with redshift (by 0.1 to 0.25 dex depending on the stellar mass of the sample). We show that the ionization parameter is strongly correlated with the fraction of young-to-old stars, as traced by the H-beta equivalent width. We discuss the implications of this result on higher redshift studies, and we consider the implications on the use of standard optical metallicity diagnostics at high redshift.