Interpreting high [O III]/H β ratios with maturing starbursts

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]/Hβ =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]/Hβ ˜ 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.