The $M_{\rm BH}-M_{*}$ Relationship at $3<z<7$: Big Black Holes in Little Red Dots

Brenda L. Jones, Dale D. Kocevski, Fabio Pacucci, Anthony J. Taylor, Steven L. Finkelstein, Johannes Buchner, Jonathan R. Trump, Rachel S. Somerville, Michaela Hirschmann, L. Y. Aaron Yung, Guillermo Barro, Eric F. Bell, Laura Bisigello, Antonello Calabro, Nikko J. Cleri, Avishai Dekel, Mark Dickinson, Giovanni Gandolfi, Mauro Giavalisco, Norman A. Grogin, Kohei Inayoshi, Jeyhan S. Kartaltepe, Anton M. Koekemoer, Lorenzo Napolitano, Masafusa Onoue, Swara Ravindranath, Giulia Rodighiero, Stephen M. Wilkins

First listed 2025-10-08 | Last updated 2025-10-08

Abstract

JWST has identified a large population of faint, broad-line active galactic nuclei (AGN) in the early universe that are powered by black holes (BHs) that often appear overmassive relative to their host galaxies. In this study, we examine the relationship between BH mass and galaxy stellar mass at $3<z<7$ using a sample of 70 broad-line AGN identified using NIRSpec/G395M spectroscopy from the CEERS, JADES, and RUBIES surveys. Roughly half (43\%) of our sample appear heavily reddened and are classified as little red dots (LRDs). We estimate BH masses ($M_{\rm BH}$) using single-epoch virial techniques, while host stellar masses ($M_{\star}$) are inferred using a combination of two-dimensional surface brightness profile fitting and spectral energy distribution modeling. We find that a majority of our sources (50/70) have $M_{\rm BH}/M_{\star}$ ratios that are 1-2 dex higher than that observed in AGN locally. Using a forward-modeling Bayesian framework that accounts for uncertainties, intrinsic scatter, and selection effects, we infer a $M_{\rm BH}-M_{\star}$ relationship that is $>3σ$ above the relationship measured for local broad-line AGN. We derive an intrinsic scatter in this relationship of $0.9$ dex, which does not vary over the redshift range of our sample. We also find that the $M_{\rm BH}/M_{\star}$ ratio increases by $2.3$ dex from $z = 3.5$ and $z = 6.5$ with a confidence level of $ > 3σ$. We attribute this trend with the increasing fraction of LRDs in our sample at $z>4$ as their host masses are $\sim1$ dex lower than the non-LRD AGN in our sample. These results support a picture in which the BHs powering JWST's broad-line AGN are genuinely overmassive and become increasingly so with redshift. We discuss the implications of our findings on early BH growth relative to that of their host galaxies and the constraints it places on BH seeding models.

Short digest

Using NIRSpec/G395M spectra from CEERS, JADES, and RUBIES, the authors assemble 70 broad-line AGN at 3<z<7 (43% classified as little red dots) and derive MBH via single-epoch virial methods with host M* from 2D profile fitting plus SED modeling. Most sources (50/70) have MBH/M* ratios elevated by 1–2 dex relative to local AGN, and forward-modeling that includes uncertainties, scatter, and selection effects yields an MBH–M* relation >3σ above the local one with an intrinsic scatter of 0.9 dex that does not evolve. MBH/M* increases by 2.3 dex from z=3.5 to z=6.5, driven by a higher LRD fraction at z>4 whose hosts are ~1 dex less massive than non‑LRDs. The results argue for genuinely overmassive early black holes and sharpen constraints on early growth and seeding scenarios.

Key figures to inspect

• Figure 1: Inspect how the LRD and non‑LRD redshift histograms differ; the LRD peak sits at higher z, illustrating the rising LRD fraction beyond z>4 that underpins the increasing MBH/M* trend.
• Figure 2: Bolometric luminosity versus redshift with LRDs highlighted; check whether LRDs cluster at particular Lbol–z loci and whether luminosity differences could bias the inferred MBH–M* relation.
• Figure 3: Example G395M broad-line decompositions; verify FWHM values (instrument-broadening corrected), the treatment of absorption components, and overall fit quality that feeds the single-epoch virial MBH estimates.
• Figure 4: GALFIT image–model–residual panels; compare point-source–dominated LRDs versus extended hosts to see how host detection (or lack thereof) influences M* and thus MBH/M*.