THE RELATION BETWEEN [O III] Hβ AND SPECIFIC STAR FORMATION RATE IN GALAXIES AT z ∼ 2

Recent surveys have identified a seemingly ubiquitous population of galaxies with elevated [O III]/H beta emission line ratios at z > 1, although the nature of this phenomenon continues to be debated. The [O III]/H beta 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]/H beta 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 similar to 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]lambda 5007 angstrom/H beta down to M similar to 10(9.2) M-circle dot and sSFR similar to 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 similar to 2 and z similar to 0 at fixed stellar mass, in agreement with existing studies. However, with respect to sSFR, the z similar to 2 stacks all lie within 1 sigma 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 similar to 2 and z similar to 0, with a relation that appears to be roughly constant over the last 10 Gyr of cosmic time.