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Recent surveys have identified a seemingly ubiquitous population of galaxieswith elevated [OIII]/H$\beta$ emission line ratios at $z > 1$, though thenature of this phenomenon continues to be debated. The [OIII]/H$\beta$ lineratio is of interest because it is a main component of the standard diagnostictools used to differentiate between active galactic nuclei (AGN) andstar-forming galaxies, as well as the gas-phase metallicity indicators $O_{23}$and $R_{23}$. Here, we investigate the primary driver of increased[OIII]/H$\beta$ ratios by median-stacking rest-frame optical spectra for asample of star-forming galaxies in the 3D-HST survey in the redshift range$z\sim1.4-2.2$. Using $N = 4220$ star-forming galaxies, we stack the data inbins of mass and specific star formation rates (sSFR) respectively. Afteraccounting for stellar Balmer absorption, we measure[OIII]$\lambda5007$\AA/H$\beta$ down to $\mathrm{M} \sim 10^{9.2} \\mathrm{M_\odot}$ and sSFR $\sim 10^{-9.6} \ \mathrm{yr}^{-1}$, more than anorder of magnitude lower than previous work at similar redshifts. We find anoffset of $0.59\pm0.05$ dex between the median ratios at $z\sim2$ and $z\sim0$at fixed stellar mass, in agreement with existing studies. However, withrespect to sSFR, the $z \sim 2$ stacks all lie within 1$\sigma$ of the medianSDSS ratios, with an average offset of only $-0.06\pm 0.05$. We find that theexcitation properties of galaxies are tightly correlated with their sSFR atboth $z\sim2$ and $z\sim0$, with a relation that appears to be roughly constantover the last 10 Gyr of cosmic time.

The Relation Between [OIII]/H$\beta$ and Specific Star Formation Rate in Galaxies at $z \sim 2$