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Lookup NU author(s): Dr Anne ArchibaldORCiD
This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
© 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.The observed gravitational-wave background (GWB) spectrum is higher in amplitude than model predictions by a factor of 2–3. Using a semi-analytic model, we evaluate the effect of a high-scatter supermassive black hole (SMBH) scaling relation (MBH–Mbulge) on models of the nanohertz GWB. By implementing an intrinsic scatter of the MBH–Mbulge relation, which is larger at higher redshift, but matches local observations, we find that the amplitude of GWB models increases to be consistent with the low-frequency end of the GWB spectrum. This amplitude increase is not uniform across frequencies, a strongly evolving scatter preferentially increases the number density of the most massive SMBHs which, in the GWB spectrum, minimizes the strength of the low-frequency turnover. Our models with positively evolving intrinsic scatter can reproduce the electromagnetically observed overmassive SMBHs at 4 < z < 6 without changing the MBH–Mbulge normalization though we find that including moderate normalization evolution marginally improves fits to the GWB data. We conclude that the MBH–Mbulge relation which best describes the available GWB and electromagnetic data sets has intrinsic scatter that evolves as (Formula presented) ε(z)=ε0+(0.56±0.4)log10(1+z) and normalization that evolves as α(z) = α0(1 + z)0.84±0.35. The results of this work imply that the MBH–Mbulge relation we see today is not universal throughout cosmic time and that a diversity of seeding models and growth mechanisms may be at play in the early stages of SMBH–galaxy evolution.
Author(s): Matt C, Gultekin K, Agazie G, Agarwal N, Anumarlapudi A, Archibald AM, Arzoumanian Z, Baier JG, Baker PT, Becsy B, Blecha L, Brazier A, Brook PR, Burke-Spolaor S, Burnette R, Case R, Casey-Clyde JA, Charisi M, Chatterjee S, Cohen T, Cordes JM, Cornish NJ, Crawford F, Cromartie HT, Crowter K, DeCesar ME, Demorest PB, Deng H, Dey L, Dolch T, Doskoch GM, Ferrara EC, Fiore W, Fonseca E, Freedman GE, Gardiner EC, Garver-Daniels N, Gentile PA, Gersbach KA, Glaser J, Good DC, Harris CJ, Hazboun JS, Jennings RJ, Johnson AD, Jones ML, Kaplan DL, Sreekumar AK, Kelley LZ, Kerr M, Key JS, Laal N, Lam MT, Lamb WG, Larsen B, Lazio TJW, Lewandowska N, Liu T, Lorimer DR, Luo J, Lynch RS, Ma C-P, Madison DR, Martsen A, McEwen A, McKee JW, McLaughlin MA, McMann N, Meyers BW, Meyers PM, Mingarelli CMF, Mitridate A, Ng C, Nice DJ, Nichols S, Ocker SK, Olum KD, Pennucci TT, Perera BBP, Petrov P, Pol NS, Radovan HA, Ransom SM, Ray PS, Romano JD, Runnoe JC, Saffer A, Sardesai SC, Schmiedekamp A, Schmiedekamp C, Schmitz K, Shapiro-Albert BJ, Siemens X, Simon J, Fiscella SVS, Stairs IH, Stinebring DR, Stovall K, Susobhanan A, Swiggum JK, Taylor J, Taylor SR, Thompson MS, Turner JE, Vallisneri M, van Haasteren R, Vigeland SJ, Wahl HM, Wilson KP, Witt CA, Wright D, Young O
Publication type: Article
Publication status: Published
Journal: Astrophysical Journal
Year: 2026
Volume: 1006
Issue: 1
Online publication date: 15/07/2026
Acceptance date: 18/06/2026
Date deposited: 04/08/2026
ISSN (print): 0004-637X
ISSN (electronic): 1538-4357
Publisher: American Astronomical Society
URL: https://doi.org/10.3847/1538-4357/ae801e
DOI: 10.3847/1538-4357/ae801e
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