Iron-corrected Single-epoch Black Hole Masses of DESI Quasars at Low Redshift
Zhiwei Pan, Linhua Jiang, Wei-Jian Guo, Shengxiu Sun, M. Siudek, J. Aguilar, S. P. Ahlen, D. Brooks, T. Claybaugh, Axel de la Macorra, P. Doel, E. Gaztañaga, Satya Gontcho A Gontcho, S. Juneau, Theodore Kisner, Andrew Lambert, Martin Landriau, L. Le Guillou, Marc Manera, Paul Martini, Aaron Meisner, R. Miquel, John Moustakas, Adam Myers, Claire Poppett, Francisco Prada, Graziano Rossi, E. Sánchez, M. Schubnell, Hee‐Jong Seo, David Sprayberry, G. Tarlé, Benjamin Alan Weaver, Hu Zou
Abstract
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 with 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 Mg ii emission lines, we calibrate Mg ii -based SMBH masses using iron-corrected, H β -based SMBH masses and establish an iron-corrected R – L relation for Mg ii . The revised relation features a flatter luminosity dependence with a slope of 0.36 and incorporates an additional term of −0.21 R Fe , where R 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 Mg ii are valid at high redshift, current mass measurements of luminous quasars at z ≥ 6 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.