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 [ O iii ] /H \beta and [ O iii ] / [ O ii ] ratios rise , the [ N ii ] /H \alpha and [ S ii ] /H \alpha ratios fall significantly over the 0.2 < z < 0.35 redshift range for stellar masses between 9.2 < \log ( { M / M } _ { \odot } ) < 10.6 .
The [ O iii ] /H \beta and [ O iii ] / [ O ii ] ratios continue to rise across the full 0.2 < z < 0.6 redshift range for stellar masses between 9.8 < \log ( { M / M } _ { \odot } ) < 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 .