Changing physical conditions in star-forming galaxies between redshifts
  0 < z < 4: [OIII]/Hb evolution
  We investigate the redshift evolution of the [OIII]/Hb nebular emission line
ratio for a sample of galaxies spanning the redshift range 0 < z < 4. We
compare the observed evolution to a set of theoretical models which account for
the independent evolution of chemical abundance, ionization parameter and
interstellar-medium (ISM) pressure in star-forming galaxies with redshift.
Accounting for selection effects in the combined datasets, we show that the
evolution to higher [OIII]/Hb ratios with redshift is a real physical effect
which is best accounted for by a model in which the ionization parameter is
elevated from the average values typical of local star-forming galaxies, with a
possible simultaneous increase in the ISM pressure. We rule out the possibility
that the observed [OIII]/Hb evolution is purely due to metallicity evolution.
We discuss the implications of these results for using local empirical
metallicity calibrations to measure metallicities at high redshift, and briefly
discuss possible theoretical implications of our results.
