Iron-corrected Single-epoch Black Hole Masses of DESI Quasars at low redshift

Zhiwei Pan, Linhua Jiang, Wei-Jian Guo, Shengxiu Sun, Małgorzata Siudek, Jessica Nicole Aguilar, Steven Ahlen, David Brooks, Todd Claybaugh, Axel de la Macorra, Peter Doel, Enrique Gaztañaga, Satya Gontcho A Gontcho, Stephanie Juneau, Theodore Kisner, Andrew Lambert, Martin Landriau, Laurent Le Guillou, Marc Manera, Paul Martini, Aaron Meisner, Ramon Miquel, John Moustakas, Adam Myers, Claire Poppett, Francisco Prada, Graziano Rossi, Eusebio Sanchez, Michael Schubnell, Hee-Jong Seo, David Sprayberry, Gregory Tarlé, Benjamin Alan Weaver, Hu Zou

First listed 2025-02-06 | Last updated 2025-02-06

Abstract

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$β$, 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$β$ and MgII emission lines, we calibrate MgII-based SMBH masses using iron-corrected, H$β$-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$β$ and MgII are valid at high redshift, current mass measurements of luminous quasars at $z\ge6$ 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.

Short digest

Using >55,000 type 1 DESI quasars at 0.25<z<0.8, the authors show that Fe emission strength (R_Fe) tracks Eddington ratio and derive iron-corrected R–L relations for Hβ, then calibrate Mg II masses against the Hβ-based results. Relative to canonical single-epoch scalings, black hole masses are overestimated by ~1.5× on average and up to ~5× for super-Eddington objects, which raises the super-Eddington fraction to ~5% (vs. 0.4%). The Mg II relation adds a −0.34 R_Fe term and is applied to build a ~0.5M-quasar DESI catalog at 0.6<z<1.6. If these iron-corrected relations hold at high z, z≥6 quasar masses would drop by a mean factor of ~2.3, easing early SMBH growth tension.

Key figures to inspect

• Fig. 1 (optical fit with strong Fe): Check how the continuum, Fe II template, Hβ (broad+narrow), and [O III] are decomposed and where the green-band iron window is measured—this underpins the R_Fe metric used in the Hβ R–L correction.
• Fig. 2 (UV fit with strong Fe): Inspect the Mg II region, Fe II template, and Balmer continuum modeling to see how R_Fe is defined in the UV and how Mg II FWHM and continuum luminosity enter the iron-corrected calibration.
• Fig. 3 (sample/parameter distributions): Compare the parent versus Hβ subsample uncertainties and parameter spreads (R_Fe, Hβ FWHM, [O III] EW, Fe II kinematics) to gauge measurement robustness and selection for the Hβ-based calibration.
• Fig. 4 (R–L departures vs Eddington state): Look at the systematic offsets of sub- vs super-Eddington sources and the regression of R–L departure with Eddington ratio—this motivates the −0.34 R_Fe term and the necessity of iron correction.