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

TL;DR

This study examines how the [OIII]/Hb emission line ratio in star-forming galaxies evolves from redshift 0 to 4, revealing that increased ionization and possibly ISM pressure, rather than metallicity, drive this change.

Contribution

It provides evidence that the evolution of [OIII]/Hb ratios is primarily due to elevated ionization parameters and ISM pressure at high redshift, challenging metallicity-based explanations.

Findings

[OIII]/Hb ratio increases with redshift

Ionization parameter elevation explains the ratio evolution

Metallicity evolution alone cannot account for the observations

Abstract

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.