The FeII/MgII flux ratio of low-luminosity quasars at z~3
  The FeII/MgII line flux ratio has been used to investigate the chemical
evolution of high redshift active galactic nuclei (AGNs). No strong evolution
has been found out to z~6, implying that the type 1a supernova activity has
been already occurred in early universe. However, the trend of no evolution can
be caused by the sample selection bias since the previous studies have utilized
mostly very luminous AGNs, which may be already chemically matured at the
observed redshift. As motivated by the previously reported correlation between
AGN luminosity and metallicity, we investigate the FeII/MgII flux ratio over a
large dynamic range of luminosity, by adding a new sample of 12 quasars at z~3,
of which the lower luminosity limit is more than 1 dex smaller than that of the
previously studied high-z quasars. Based on the Gemini/GNIRS observations, we
find that the seven low-luminosity quasars with a mean bolometric luminosity
log L_bol~46.5 \pm 0.2 has an average FeII/MgII ratio of 0.68 \pm 0.11 dex.
This ratio is comparable to that of high-luminosity quasars (log L_bol~47.3 \pm
0.3) in our sample (i.e., FeII/MgII ratio of 0.59 \pm 0.15 dex) and that of the
previously studied high-luminosity quasars at higher redshifts. One possible
scenario is that the low-luminosity quasars in our sample are still relatively
luminous and already chemically matured. To search for chemically-young AGNs,
and fully understand the chemical evolution based on the FeII/MgII flux ratio,
it is required to investigate much lower-luminosity AGNs.
