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We investigate the redshift evolution of the [OIII]/Hb nebular emission lineratio for a sample of galaxies spanning the redshift range 0 < z < 4. Wecompare the observed evolution to a set of theoretical models which account forthe independent evolution of chemical abundance, ionization parameter andinterstellar-medium (ISM) pressure in star-forming galaxies with redshift.Accounting for selection effects in the combined datasets, we show that theevolution to higher [OIII]/Hb ratios with redshift is a real physical effectwhich is best accounted for by a model in which the ionization parameter iselevated from the average values typical of local star-forming galaxies, with apossible simultaneous increase in the ISM pressure. We rule out the possibilitythat the observed [OIII]/Hb evolution is purely due to metallicity evolution.We discuss the implications of these results for using local empiricalmetallicity calibrations to measure metallicities at high redshift, and brieflydiscuss possible theoretical implications of our results.

Changing physical conditions in star-forming galaxies between redshifts 0 < z < 4: [OIII]/Hb evolution