Evidence for evolutionary pathway-dependent black hole scaling relations

Jonathan H. Cohn, Emmanuel Durodola, Quinn O. Casey, Erini Lambrides, Ryan C. Hickox

First listed 2025-03-31 | Last updated 2025-03-31

Abstract

Recent observations have identified an abundance of high-redshift active galactic nuclei (AGN) with supermassive black holes (BHs) that are over-massive compared to the local BH mass$-$total stellar mass ($M_{\mathrm{BH}}-M_\star$) relation. $M_{\mathrm{BH}}$ measurements at high-$z$ are critical for probing the growth histories of BHs and their host galaxies, including BH seeding and evolution of the $M_{\mathrm{BH}}-M_\star$ relation. However, BH masses in high-$z$ AGN are generally estimated from single-epoch measurements, which are anchored to local relations based on reverberation mapping and carry large systematic uncertainties. Alternate $M_{\mathrm{BH}}$ detection methods such as dynamical measurements are more reliable but currently only possible in the local Universe or with strongly lensed systems. Recently, dynamical $M_{\mathrm{BH}}$ measurements were made in a $z\sim2$ lensed quiescent galaxy as well as a sample of six local galaxies identified as likely relics of common quiescent red nugget galaxies at cosmic noon. We compare the $z\sim2$ red nugget and relic BHs to recent results for $4<z<11$ AGN, quasars, and Little Red Dots. Intriguingly, the $z\sim2$ galaxy and local relic galaxies all lie on both the local $M_{\mathrm{BH}}-M_\star$ relation for bulges and the $4<z<7$ $M_{\mathrm{BH}}-M_\star$ relation. Our results suggest the $M_{\mathrm{BH}}-M_\star$ relation for bulges was likely in place at high-$z$ and indicate careful consideration of different evolutionary pathways is needed when building BH scaling relations. While improvements to $M_{\mathrm{BH}}$ estimates in AGN will increase our confidence in high-$z$ BH masses, detecting BHs in relic galaxies and lensed galaxies presents a complementary probe of the high-$z$ relations.

Short digest

This letter benchmarks secure dynamical black hole masses in a z~2 lensed quiescent galaxy and six local red‑nugget relics against recent 4<z<11 AGN, quasar, and Little Red Dot samples on the M_BH-M_star and M_BH-sigma_* planes. The relics and the z~2 quiescent system land on both the local bulge M_BH-M_star relation and the 4<z<7 Pacucci et al. relation, while luminous QSOs and broad-line AGN follow Pacucci et al. more closely and LRDs scatter across all three; on M_BH-sigma_* most sources sit above the local median except the lensed system. The authors argue the bulge relation was already in place at high z and that differing evolutionary pathways—compact-core assembly by z~2 followed by dry-merger stellar-mass growth versus ongoing BH and star formation in AGN—govern positions on scaling planes. Given large systematics in single‑epoch high‑z masses, they emphasize dynamical detections in relics and strongly lensed galaxies as a complementary, less biased probe of early scaling relations.

Key figures to inspect

• Figure 1 (M_BH vs M_star): Verify that cyan/magenta diamond relics and the z~2 lensed quiescent point sit on both the Reines & Volonteri bulge line and the Pacucci et al. (4<z<7) relation; contrast with squares/Xs (QSOs/AGN) that hug Pacucci et al. and note the broad LRD scatter, plus the placement of GN‑z11.
• Figure 2 (M_BH vs sigma_*): Check that most AGN and relics lie above the median Saglia et al. relation, with the lensed quiescent galaxy near the median; note that only the Maiolino et al. sample has sigma_* estimates, highlighting how central potential may evolve less than total mass.
• Figure 3 (pathways schematic): Read vectors showing dry mergers moving systems rightward in M_star at near-constant M_BH (relic/ETG track) versus AGN accretion driving upward movement in M_BH (AGN/LRD track), clarifying why bulge-based scaling appears stable while total-mass scaling depends on assembly history.