Extreme Values of Black Hole to Stellar Mass Ratio for High-Redshift Galaxies

Cameron Heather, Teeraparb Chantavat, Siri Chongchitnan, Joseph Silk

First listed 2026-04-02 | Last updated 2026-04-02

Abstract

With recent data from the \emph{James Webb Space Telescope} (JWST), it is possible to calculate the mass of the supermassive black holes at the centre of galaxies, and the stellar mass of the host galaxies at $z \gtrsim 5$. In this work, we apply the method of extreme-value statistics to calculate the distributions of extreme black hole and stellar mass for the redshift range $3.5 \lesssim z \lesssim 8.5$. We sample these distributions to obtain a prediction for the black hole to stellar mass ratio of $\sim0.24$ over this redshift range, with the median in each bin varying in the range $0.18-0.35$. Our predictions are consistent with the highest observed values of the ratio from JWST observations of high-redshift galaxies.

Short digest

This paper asks how extreme the black-hole-to-stellar-mass ratio of high-redshift JWST galaxies should be if one looks at the tail of the underlying mass distributions rather than at a typical object. The main result is that extreme-value statistics predict BH-to-stellar-mass ratios around 0.2 to 0.35 across roughly z=3.5 to 8.5, consistent with the most extreme currently reported JWST systems. The paper matters because it reframes apparently overmassive black holes as expected extreme outliers of the early-galaxy population rather than automatically requiring a special new class.

Key figures to inspect

• Figure 2 is the must-see plot: it shows the predicted extreme BH-to-stellar-mass ratio as a function of redshift and compares it directly to the observed JWST points, which is the paper's central result in one figure.
• Figure 3 is the next figure to inspect: it puts black-hole mass against stellar mass and overlays the predicted extreme-ratio relation, so this is where you can see how the statistical envelope compares with the existing observations.
• Figure 1 is mainly the setup figure: it shows the adopted black-hole and stellar mass functions that feed the extreme-value calculation, so read it if you want to understand where the later ratio prediction comes from.