Kavli Affiliate: Linhua Jiang
| First 5 Authors: Zhiwei Pan, Linhua Jiang, Wei-Jian Guo, Shengxiu Sun, Małgorzata Siudek
| Summary:
We present a study on the possible overestimation of single-epoch
supermassive black hole (SMBH) masses in previous works, based on more than
55,000 type 1 quasars at $0.25 < z < 0.8$ from the Dark Energy Spectroscopic
Instrument (DESI). We confirm that iron emission strength serves as a good
tracer of the Eddington ratio, and estimate SMBH masses using an iron-corrected
$R$-$L$ relation for H$beta$, where $R$ is the broad line region size and $L$
is the continuum luminosity. Compared to our measurements, previous canonical
measurements without the iron correction are overestimated by a factor of 1.5
on average. The overestimation can be up to a factor of 5 for super-Eddington
quasars. The fraction of super-Eddington quasars in our sample is about 5%,
significantly higher than 0.4% derived from the canonical measurements. Using a
sample featuring both H$beta$ and MgII emission lines, we calibrate MgII-based
SMBH masses using iron-corrected, H$beta$-based SMBH masses and establish an
iron-corrected $R$-$L$ relation for MgII. The new relation adds an extra term
of $-0.34R_{mathrm{Fe}}$ to the $R$-$L$ relation, where $R_{mathrm{Fe}}$
denotes the relative iron strength. We use this formula to build a catalog of
about 0.5 million DESI quasars at $0.6<z<1.6$. If these iron-corrected $R$-$L$
relations for H$beta$ and MgII are valid at high redshift, current mass
measurements of luminous quasars at $zge6$ would have been overestimated by a
factor of 2.3 on average, alleviating the tension between SMBH mass and growth
history in the early universe.
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