Kaasinen, MelanieKewley, LisaBian, FuyanGroves, Brent AllanKashino, DaichiSilverman, J. D.Kartaltepe, Jeyhan S.2020-07-102020-07-100035-8711http://hdl.handle.net/1885/205992We investigate the evolution of the ionization parameter of star-forming galaxies using a high-redshift (z~ 1.5) sample from the FMOS-COSMOS (Fibre Multi-Object Spectrograph- COSMic evOlution Survey) and matched low-redshift samples from the Sloan Digital Sky Survey. By constructing samples of low-redshift galaxies for which the stellar mass (M*), star formation rate (SFR), and specific star formation rate (sSFR) are matched to the high-redshift sample, we remove the effects of an evolution in these properties. We also account for the effect of metallicity by jointly constraining the metallicity and ionization parameter of each sample. We find an evolution in the ionization parameter for main-sequence, star-forming galaxies and show that this evolution is driven by the evolution of sSFR. By analysing the matched samples as well as a larger sample of z < 0.3, star-forming galaxies we show that high ionization parameters are directly linked to high sSFRs and are not simply the by-product of an evolution in metallicity. Our results are physically consistent with the definition of the ionization parameter, a measure of the hydrogen ionizing photon flux relative to the number density of hydrogen atoms.LK gratefully acknowledges support from an ARC Laureate Fellowship (FL150100113). BG gratefully acknowledges the support of the Australian Research Council as the recipient of a Future Fellowship (FT140101202). Parts of this research were conducted by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013.application/pdfen-AU© 2018 The Author(s)galaxies: evolutiongalaxies: high-redshiftgalaxies: ISM2018-04-2410.1093/MNRAS/STY10122020-07-06