Evidence for Gas-phase Metal Deficiency in Massive Protocluster Galaxies at z ∼ 2.2

We study the mass-metallicity relation for 19 members of a spectroscopically confirmed protocluster in the COSMOS field at z = 2.2 (CC2.2), and compare it with that of 24 similarly selected field galaxies at the same redshift. Both samples are Ha emitting sources, chosen from the HiZELS narrowband survey, with metallicities derived from the N2 ([NII] lambda 6584/H alpha) line ratio. For the mass-matched samples of protocluster and field galaxies, we Ha find that protocluster galaxies with 10(9.9) M-circle dot <= M-* <= 10(10.9) M-circle dot are metal deficient by 0.10 +/- 0.04 dex (2.5 sigma significance) compared to their coeval field galaxies. This metal deficiency is absent for low-mass galaxies, M-* < 10(9.9) M-circle dot. Moreover, relying on both spectral energy distribution derived and Ha (corrected for dust extinction based on M,) star formation rates (SFRs), we find no strong environmental dependence of the SFR-M-* relation; however, we are not able to rule out the existence of small dependence due to inherent uncertainties in both SFR estimators. The existence of 2.5 sigma significant metal deficiency for massive protocluster galaxies favors a model in which funneling of the primordial cold gas through filaments dilutes the metal content of protoclusters at high redshifts (z greater than or similar to 2). At these redshifts, gas reservoirs in filaments are dense enough to cool down rapidly and fall into the potential well of the protocluster to lower the gas-phase metallicity of galaxies. Moreover, part of this metal deficiency could be originated from galaxy interactions that are more prevalent in dense environments.