THE MOSDEF SURVEY: ELECTRON DENSITY AND IONIZATION PARAMETER AT z ∼ 2.3

Using observations from the MOSFIRE Deep Evolution Field survey, we investigate the physical conditions of star-forming regions in z similar to 2.3 galaxies, specifically the electron density and ionization state. From measurements of the [O II]lambda lambda 3726,3729 and [S II]lambda lambda 6716,6731 doublets, we find a median electron density of similar to 250 cm(-3) at z similar to 2.3, an increase of an order of magnitude compared to measurements of galaxies at z similar to 0. While z similar to 2.3 galaxies are offset toward significantly higher O-32 values relative to local galaxies at fixed stellar mass, we find that the high-redshift sample follows a similar distribution to the low-metallicity tail of the local distribution in the O-32 versus R-23 and O3N2 diagrams. Based on these results, we propose that z similar to 2.3 star-forming galaxies have the same ionization parameter as local galaxies at fixed metallicity. In combination with simple photoionization models, the position of local and z similar to 2.3 galaxies in excitation diagrams suggests that there is no significant change in the hardness of the ionizing spectrum at fixed metallicity from z similar to 0 to z similar to 2.3. We find that z similar to 2.3 galaxies show no offset compared to low-metallicity local galaxies in emission line ratio diagrams involving only lines of hydrogen, oxygen, and sulfur, but show a systematic offset in diagrams involving [N II]lambda 6584. We conclude that the offset of z similar to 2.3 galaxies from the local star-forming sequence in the [N II] BPT diagram is primarily driven by elevated N/O at fixed O/H compared to local galaxies. These results suggest that the local gas-phase and stellar metallicity sets the ionization state of star-forming regions at z similar to 0 and z similar to 2.