A RISE IN THE IONIZING PHOTONS IN STAR-FORMING GALAXIES OVER THE PAST 8 BILLION YEARS

摘要

We investigate the change in ionizing photons in galaxies in the range 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 in the range 9.2 < log(M/M-circle dot) < 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 in the range 9.8 < log(M/M-circle dot) < 10.0. We conclusively rule out active galactic nucleus 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 in the range 0.2 < z < 0.35. We find that the ionization parameter rises significantly with redshift (by 0.1-0.25 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 mu 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.