The mass-metallicity relation of high-z type-2 active galactic nuclei
  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) and
X-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 Subaru
Telescope 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-2
AGNs, whose host stellar masses have been estimated in literature. We divided
our sample in three stellar mass bins and calculated averaged emission-line
flux ratios of CIV1549/HeII1640 and CIII]1909/CIV1549. Comparing observed
emission-line flux ratios with photoionization model predictions, we estimated
narrow line region (NLR) metallicities for each mass bin. We found that there
is a positive correlation between NLR metallicities and stellar masses of
type-2 AGNs at z ~ 3. This is the first indication that AGN metallicities are
related to their hosts, i.e., stellar mass. Since NLR metallicities and stellar
masses follow a similar relation as the MZR in star-forming galaxies at similar
redshifts, our results indicate that NLR metallicities are related to those of
the host galaxies. This study highlights the importance of considering
lower-mass X-ray selected AGNs in addition to radio galaxies to explore the
metallicity properties of NLRs at high redshift.
