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

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.

13 pages, 5 figures, accepted for publication in MNRAS

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• Measuring the Physical Conditions in High-Redshift Star-Forming Galaxies: Insights from KBSS-MOSFIRE
• The VANDELS ESO public spectroscopic survey
• The VANDELS survey: the stellar metallicities of star-forming galaxies at 2.5 < z < 5.0
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• The First Billion Years project: constraining the dust attenuation law of star-forming galaxies at z $\simeq$ 5
• The MOSDEF-LRIS Survey: The Connection Between Massive Stars and Ionized Gas in Individual Galaxies at $z\sim 2$
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• The VANDELS survey: Dust attenuation in star-forming galaxies at $\mathbf{z}=3-4$
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• COALAS III: The ATCA CO(1-0) look at the growth and death of H$\alpha$ emitters in the Spiderweb protocluster at z=2.16