Imprints of galaxy evolution on H II regions

Context. H ii regions in galaxies are the sites of star formation, so they are special places for understanding the build-up of stellar mass in the universe. The line ratios of this ionized gas are frequently used to characterize the ionization conditions. In particular, the oxygen abundances are assumed to trace the chemical enrichment of galaxies. Aims. We explore the connections between the ionization conditions and the properties of the overall underlying stellar population (ionizing or not-ionizing) in H ii regions, in order to uncover the actual physical connection between them. Methods. We use the H ii regions catalog from the CALIFA survey, which is the largest in existence with more than 5000 H ii regions, to explore their distribution across the classical [O iii] 5007/H vs. [N ii] 6583/H diagnostic diagram, and the way it depends on the oxygen abundance, ionization parameter, electron density, and dust attenuation. The location of H ii regions within this diagram is compared with predictions from photoionization models. Finally, we explore the dependence of the location within the diagnostic diagram on the properties of the host galaxies, the galactocentric distances, and the properties of the underlying stellar population.