  Star forming galaxies at high redshift show ubiquitously high ionization
parameters, as measured by the ratio of optical emission lines. We demonstrate
that local (z < 0.2) sources selected as Lyman break analogues also manifest
high line ratios with a typical [O III]/H beta = 3.36(+0.14,-0.04) - comparable
to all but the highest ratios seen in star forming galaxies at z ~ 2-4. We
argue that the stellar population synthesis code BPASS can explain the high
ionization parameters required through the ageing of rapidly formed star
populations, without invoking any AGN contribution. Binary stellar evolution
pathways prolong the age interval over which a starburst is likely to show
elevated line ratios, relative to those predicted by single stellar evolution
codes. As a result, model galaxies at near-Solar metallicities and with ages of
up to ~100 Myr after a starburst typically have a line ratio [O III]/H beta~3,
consistent with those seen in Lyman break galaxies and local sources with
similar star formation densities. This emphasises the importance of including
binary evolution pathways when simulating the nebular line emission of young or
bursty stellar populations.
