Evidence for gas-phase metal deficiency in massive protocluster galaxies at z~2.2

Abstract

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 $\mathrm{H}\alpha$ emitting sources, chosen from the HiZELS narrow-band survey, with metallicities derived from $N2(\frac{[\mathrm{NII}]\lambda 6584}{\mathrm{H}\alpha })$ line ratio. For the mass-matched samples of protocluster and field galaxies, we find that protocluster galaxies with $10^{9.9}{\mathrm{M}}_{\odot }\le {\mathrm{M}}_{\ast }\le 10^{10.9}{\mathrm{M}}_{\odot }$ are metal deficient by $0.10\pm 0.04$ dex ($2.5\sigma$ significance) compared to their coeval field galaxies. This metal deficiency is absent for low mass galaxies, ${\mathrm{M}}_{\ast }<10^{9.9}{\mathrm{M}}_{\odot }$. Moreover, relying on both SED-derived and $\mathrm{H}\alpha$ (corrected for dust extinction based on ${\mathrm{M}}_{\ast }$) SFRs, we find no strong environmental dependence of SFR-${\mathrm{M}}_{\ast }$ 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 ($z\gtrsim 2$). 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 which are more prevalent in dense environments.

10 pages, 5 figures, 2 tables, accepted for publication in ApJ

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