THE RELATION BETWEEN $[{\rm{O}}\,{\rm{III}}]/{\rm{H}}\beta $ AND SPECIFIC STAR FORMATION RATE IN GALAXIES AT z ∼ 2

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 A/Hβ down to M ~ 109.2 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.