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