Kavli Affiliate: Linhua Jiang
| First 5 Authors: Danyang Jiang, Masafusa Onoue, Linhua Jiang, Samuel Lai, Eduardo Banados
| Summary:
The Fe II/Mg II emission line flux ratio in quasar spectra serves as a proxy
for the relative Fe to alpha-element abundances in the broad line regions of
quasars. Due to the expected different enrichment timescales of the two
elements, they can be used as a cosmic clock in the early Universe. We present
a study of the Fe II/Mg II ratios in a sample of luminous quasars exploiting
high-quality near-IR spectra taken primarily by the XQR-30 program with VLT
XSHOOTER. These quasars have a median bolometric luminosity of log(L_bol[erg
s^-1])~47.3 and cover a redshift range of z=6.0-6.6. The median value of the
measured Fe II/Mg II ratios is ~7.9 with a normalized median absolute deviation
of ~2.2. In order to trace the cosmic evolution of Fe II/Mg II in an unbiased
manner, we select two comparison samples of quasars with similar luminosities
and high-quality spectra from the literature, one at intermediate redshifts
(z=3.5-4.8) and the other at low redshifts (z=1.0-2.0). We perform the same
spectral analysis for all these quasars, including the usage of the same iron
template, the same spectral fitting method, and the same wavelength fitting
windows. We find no significant redshift evolution in the Fe II/Mg II ratio
over the wide redshift range from z=1 to 6.6. The result is consistent with
previous studies and supports the scenario of a rapid iron enrichment in the
vicinity of accreting supermassive black holes at high redshift.
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