Life After the Quasar: Overmassive Black Holes and Remnant Ionised Bubbles in and Around Two z~6.6 Galaxies

Romain A. Meyer, Pascal A. Oesch, Callum Witten, Richard S. Elllis, Sarah E. I. Bosman, Fred Davies, Alyssa B. Drake, Nicolas Laporte, Jorryt Matthee, Fabian Walter

First listed 2026-05-01 | Last updated 2026-05-01

Abstract

Supermassive black holes (SMBH, $M_{\rm{BH}} > 10^8 M_\odot$) powering luminous quasars already exist one billion years after the Big Bang, yet their connection to their star-forming host galaxies, their relation to the general galaxy population and their contribution to Reionisation remains deeply enigmatic. JWST is finding numerous Active Galactic Nuclei (AGN) in high-redshift galaxies with black hole masses that appear to be over-massive compared to their host's stellar mass, but rarely as massive as those found in luminous quasars. Here we report JWST/NIRSpec observations revealing overmassive SMBH in two ultra-luminous Lyman-$α$ emitters at $z\sim6.6$ that exhibit rare double-peaked Lyman-alpha profiles. The broad Balmer lines indicate black hole masses $M_{\rm{BH}}\simeq 2\times10^8 M_\odot$, matching that found in faint $z\sim 6-7$ quasars, and very high BH-to-stellar-mass ratio ($\sim 0.1-0.2$) that exceed the local relation by a factor $\sim$400-800. Stellar population modelling favours young ages ($<50$ Myr), inconsistent with the sustained average Eddington-rate accretion required to reach the observed BH masses by $z=6.6$. The double-peak Lyman-$α$ profiles require a large ionised bubble and high photoionisation rate that is consistent with the ionising output of quasars powered by black holes of similar mass, thus constraining the cessation of the last quasar episode to $<1$ Myr. We interpret both systems as post-quasar galaxies in which AGN feedback has delayed stellar mass assembly, and propose that episodic quasar activity partially explains the unexpected prevalence of large ionised bubbles deep into the Epoch of Reionisation.

Short digest

JWST/NIRSpec IFU spectra of two ultra-luminous Lyα emitters at z≈6.6 (COLA1, NEPLA4; not LRDs by color selection) reveal broad Balmer lines implying ~2×10^8 M⊙ black holes and extreme BH-to-stellar mass ratios of ~0.1–0.2, ≈400–800× the local relation. Stellar-population fits favor very young hosts (<50 Myr), inconsistent with the prolonged near-Eddington growth needed to reach these masses, pointing to earlier quasar episodes. The rare double-peaked Lyα profiles demand large, highly ionised bubbles and a locally boosted photoionisation rate, consistent with a recent quasar phase that shut down within <1 Myr. The systems are interpreted as post-quasar galaxies where AGN feedback delayed stellar assembly, offering an explanation for unexpectedly large ionised bubbles deep in reionisation.

Key figures to inspect

• Fig. 1: NIRSpec IFU Hβ+[O III] and Hα+[N II] complexes—inspect the broad Balmer components used for single-epoch virial MBH, the fitted narrow and outflowing components, and residuals that establish robust AGN line decompositions.
• Fig. 2 (left): Black hole growth tracks versus assumed Eddington ratios—verify that matching the z≈6.6 MBH requires sustained near-Eddington growth and/or earlier episodes, underscoring an episodic duty cycle.
• Fig. 2 (middle): BAGPIPES-derived stellar mass assembly—check that most of the stellar mass forms within the last <50 Myr, highlighting the recent starburst contrasted with earlier BH growth.
• Fig. 2 (right): MBH/M* versus JWST AGN and z~6–7 quasars—see COLA1 and NEPLA4 at ~0.1–0.2 (≈400–800× local), bridging the gap between AGN hosts and faint quasars.
• Lyα double-peaked profiles and IGM transmission modeling—use the blue peak and trough placement to infer a large ionised bubble and to read off the <1 Myr constraint on quasar shutdown.