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The mass-metallicity relation (MZR) of type-2 active galactic nuclei (AGNs)at 1.2 < z < 4.0 is investigated by using high-z radio galaxies (HzRGs) andX-ray selected radio-quiet AGNs. We combine new rest-frame ultraviolet (UV)spectra of two radio-quiet type-2 AGNs obtained with FOCAS on the SubaruTelescope with existing rest-frame UV emission lines, i.e., CIV1549, HeII1640,and CIII]1909, of a sample of 16 HzRGs and 6 additional X-ray selected type-2AGNs, whose host stellar masses have been estimated in literature. We dividedour sample in three stellar mass bins and calculated averaged emission-lineflux ratios of CIV1549/HeII1640 and CIII]1909/CIV1549. Comparing observedemission-line flux ratios with photoionization model predictions, we estimatednarrow line region (NLR) metallicities for each mass bin. We found that thereis a positive correlation between NLR metallicities and stellar masses oftype-2 AGNs at z ~ 3. This is the first indication that AGN metallicities arerelated to their hosts, i.e., stellar mass. Since NLR metallicities and stellarmasses follow a similar relation as the MZR in star-forming galaxies at similarredshifts, our results indicate that NLR metallicities are related to those ofthe host galaxies. This study highlights the importance of consideringlower-mass X-ray selected AGNs in addition to radio galaxies to explore themetallicity properties of NLRs at high redshift.

The mass-metallicity relation of high-z type-2 active galactic nuclei