The mass-metallicity-star formation rate relation at z & 2 with 3D-HST

We present new accurate measurements of the mass, metallicity and star-formation rate of a statistically-significant-sample of 93 galaxies at z&2 using near-infrared spec-troscopy taken as part of the 3D-HST survey. We derive a mass-metallicity relation (MZR) for our sample with metallicities based on the oxygen and Hβ nebular emis-sion lines. We find the MZR derived from our data to have the same trend as previous determinations in the range 0<z<3 with metallicity decreasing with stellar mass. How-ever, we find that our MZR is offset from a previous determination at z&2 which used metallicities derived from the [NII]/Hα ratio. Incorporating star formation rate in-formation, we find that our galaxies are also offset from the fundamental metallicity relation (FMR) by ∼0.3dex. Using the Baldwin-Phillips-Terlevich (BPT) diagram we argue that, if the physi-cal conditions of star-forming regions evolve with redshift, metallicity indicators based on [NII] and Hα, calibrated in the local Universe, may not be consistent with the ones based on oxygen lines and Hβ. Our results thus suggest that the evolution of the FMR previously reported at z∼2-3 may be an artefact of the differential evolution in metallicity indicators, and caution against using locally-calibrated empirical metal-licity relations at high redshift, which do not account for evolution in the physical conditions of star-forming regions