The Relation between [O III]/Hβ and Specific Star Formation Rate in Galaxies at z ∼ 2

Abstract

Recent surveys have identified a seemingly ubiquitous population of galaxies with elevated [O III]/Hβ emission line ratios at z > 1, although the nature of this phenomenon continues to be debated. The [O III]/Hβ line ratio is of interest because it is a main component of the standard diagnostic tools used to differentiate between active galactic nuclei and star-forming galaxies as well as the gas-phase metallicity indicators O ₂₃ and R ₂₃. Here, we investigate the primary driver of increased [O III]/Hβ ratios by median-stacking rest-frame optical spectra for a sample of star-forming galaxies in the 3D-HST survey in the redshift range z ∼ 1.4-2.2. Using N = 4220 star-forming galaxies, we stack the data in bins of mass and specific star formation rates (sSFRs), respectively. After accounting for stellar Balmer absorption, we measure [O III]λ5007 Å/Hβ down to M ∼ 10^9.2 M _⊙ and sSFR ∼ 10^-9.6 yr^-1, greater than an order of magnitude lower than previous work at similar redshifts. We find an offset of 0.59 ± 0.05 dex between the median ratios at z ∼ 2 and z ∼ 0 at fixed stellar mass, in agreement with existing studies. However, with respect to sSFR, the z ∼ 2 stacks all lie within 1σ of the median SDSS ratios, with an average offset of only -0.06 ± 0.05. We find that the excitation properties of galaxies are tightly correlated with their sSFR at both z ∼ 2 and z ∼ 0, with a relation that appears to be roughly constant over the last 10 Gyr of cosmic time.