Two paths to a galaxy’s heart
Key idea: The relationship between a galaxy’s central black hole and its stellar mass isn’t a single, universal rule. New work from researchers at Dartmouth College with NASA Goddard Space Flight Center shows that the growth history of a galaxy can push its black hole to different places on the MBH–M⋆ diagram, depending on whether the core or the outskirts grow first and how mergers spread mass outward. The lead author, Jonathan H. Cohn, and colleagues (Emmanuel Durodola, Quinn O. Casey, Erini Lambrides, and Ryan C. Hickox) frame their findings as an invitation to reimagine the dating profile of galaxies and their invisible engines. This study blends dusty, dynamic measurements in the nearby universe with the light from galaxies that existed when the universe was young, stitched together by a common question: who grows first, the black hole or the bulge?
For decades, astronomers have treated the MBH–M⋆ relation as a single, tidy fingerprint of coevolution. In practice, it’s a fingerprint that’s hard to read across cosmic time. At nearby distances, dynamical measurements of a black hole’s mass can be done with real-time tracking of stars or gas in the black hole’s gravitational grip. But push the era back to the early universe and dynamical dances become faint whispers. High-redshift black hole masses have often relied on single-epoch estimates anchored to local reverberation mapping calibrations—a method that can carry sizable systematic uncertainties when stretched to the distant past. The paper’s opening move is to acknowledge those uncertainties and then seek independent anchors.
Enter relics from the cosmic noon and a gravitationally lensed beacon from when the universe was only a few billion years old. The Dartmouth team uses six local compact galaxies that are quiescent remnants of the “red nuggets” that thrived around redshift two. These local relics preserve the core structures that likely housed the first robust black holes, and crucially, their black hole masses can be measured dynamically. In parallel, they include a rare, magnified quiescent galaxy at z ≈ 1.95 whose black hole mass was derived from stellar dynamics in the source plane thanks to a gravitational lens that magnified the object by about 29 times. Together, these objects offer a direct, less biased look at how black holes and their hosts relate when you’re not leaning on single-epoch glow measurements.
What they compared and how they compared
Key idea: The team assembled a cross-section of black holes spanning the local universe to the edge of the observable cosmos, and then tested them against competing scaling relations that themselves reflect different growth histories. On the local side, they reference MBH–M⋆ relations for bulges and for AGN hosts from Reines and Volonteri (2015). On the high-redshift side, they adopt a 4 < z < 7 relation from Pacucci and collaborators (2023) that is built to reflect a universe where black holes and their galaxies are rapidly building mass. The object set includes six local relics with dynamical MBH measurements (the most robust sort of mass estimate) and, at the high-z end, a suite of active galaxies selected as broad-line AGN or Little Red Dots (LRDs) that have MBH estimates from broad-line measurements or SED-based inferences. The aim is blunt: do high-z black holes simply lie on the same MBH–M⋆ line as nearby bulges, or do their growth histories pull them away?
Crucially, the authors do not rely on a single measure alone. They pull together dynamical masses from local relics, a lens-revealed z ≈ 2 galaxy, and high-z MBH estimates from multiple independent techniques, including Hα and Hβ broad lines and SED fitting for LRDs. They compare these MBH masses to two kinds of stellar mass: total M⋆ and bulge-dominated Mbul. In other words, the paper tests whether black holes track the core of a galaxy (the bulge) or the whole galaxy (including the outskirts) as galaxies grow through time. They also examine how well the MBH–σ⋆ relation—the connection between black hole mass and the central stellar velocity dispersion—holds across different epochs, a question that probes whether the central potential alone might be a more universal anchor.
What the samples reveal about growth histories
Key idea: The results flip a long-standing assumption on its head: the relics from the z ≈ 2 universe sit on both the local bulge relation and the high-z relation, while the population of high-z active galaxies mostly sits on the high-z relation and drifts away from the local bulge relation. In plain terms, if you only looked at active galaxies in the early universe, you might conclude that black holes are systematically overmassive for their hosts. But the relics, which have stopped growing their stars or black holes since z ≈ 2, tell a different story: their central engines are already in sync with the galaxies’ bulges, even as the galaxies’ outer envelopes continue to pile on mass through mergers. This duality suggests that there are at least two evolutionary pathways shaping the MBH–M⋆ landscape.
When the six local red nugget relics and the z ≈ 2 lensed galaxy are plotted alongside the high-z AGN samples, they align with the Pacucci et al. (2023) high-z relation. They also sit on the local bulge relation if you interpret M⋆ as bulge mass rather than total stellar mass. The implication is subtle but profound: the MBH–M⋆ relation that binds black holes to their hosts at z ≈ 2 appears to endure, at least for the dense cores, into the present day. The paper’s Monte Carlo exercise—drawing MBH and M⋆ within their uncertainties across thousands of iterations—shows that the median MBH for the combined sample sits broadly within the high-z relation’s scatter and is not wildly off the z ≈ 0 bulge relation for these core-dominated systems. The universe, it seems, kept a core-level script consistent across billions of years, even as outer mass moved to the galaxy’s fringes.
Yet the study also highlights a persistent tension: most high-z AGN, QSOs, and LRDs lie above the local MBH–M⋆ relation when total stellar mass is used. They are not randomly scattered; they systematically tilt toward higher MBH for a given M⋆, which matches the earlier hints from single-epoch studies that high-z black holes might be unusually massive for their hosts. The authors therefore emphasize that how you define M⋆ matters a lot. If you measure the core, bulge-dominated mass they call Mbul rather than the sprawling total mass, the high-z systems fall closer to the local bulge relation. In other words, the same galaxy could look overmassive or well-behaved depending on whether you focus on its heart or its entire body. That’s a powerful reminder that galactic growth is not one uniform track but a set of intertwined journeys.
The deep dive into the MBH–σ⋆ connection
Key idea: The MBH–σ⋆ relation, which ties black hole mass to the central stellar velocity dispersion, appears to be more robust against cosmic time than the MBH–M⋆ relation might be. The six relics and the z ≈ 2 lensed galaxy largely track the local MBH–σ⋆ relation within scatter, with a hint of a modest upward offset at high redshift. The one system that cleanly lands on the local relation is the lensed z ≈ 2 galaxy, a reminder that lensing can provide the kind of precision needed to pin down subtle shifts in these fundamental correlations. If MBH and σ⋆ both trace the central potential, it makes sense that their relationship might survive more faithfully across cosmic epochs, even as the galaxy’s total mass swells through mergers.
However, the authors caution that high-redshift measurements of σ⋆ are less direct than their local counterparts, often substituting proxies or relying on ionized gas kinematics. This introduces another layer of uncertainty and underscores the need for more direct dynamical measurements of σ⋆ in distant systems. The takeaway is not that the MBH–σ⋆ relation is inviolable, but that it may be a more faithful compass for tracking core growth through time. If real, a more fundamental link between a galaxy’s central potential and its black hole would help us untangle how much of the MBH growth is driven by the galaxy’s heart versus the galaxy’s outskirts.
Why this matters for cosmic history
Key idea: The study reframes a long-standing puzzle about how black holes grow in lockstep with galaxies. If the MBH–M⋆ relation is pathway dependent, then there isn’t a single destiny for every galaxy. Some galaxies may light up their central engines early and quickly, building a bulge and a black hole in tandem; others may accumulate mass in more extended, sprawling ways after the core has already formed. This difference in evolutionary routes could explain why high-z observations have seemed to show overmassive black holes for a given stellar mass when looking at total mass, yet show a more ordinary or even lagging growth when focusing on cores. The relics—the fossils of the early core-building phase—thus act as time capsules that help calibrate our models across the entire history of the universe.
In a broader sense, the authors argue that black hole scaling relations are not single equations etched in stone; they are snapshots of a galaxy’s growth script, which can vary with environment, merger history, gas supply, and timing. The same galaxy that becomes a quiescent relic after a brief, intense phase at z ≈ 2 may later accumulate mass in its outskirts through dry mergers, a growth that raises M⋆ without driving the core or the black hole much further. That decoupling is exactly the kind of nuance that makes galaxy formation models feel more like historical fiction than a straightforward ledger. The new analysis suggests that to understand the present-day MBH–M⋆ relation, we must reconstruct the branching pathways galaxies took to get there, not just measure their masses at a single moment in time.
What this means for future exploration
Key idea: The paper doesn’t declare a single universal MBH–M⋆ relation. Instead, it offers a roadmap for how to test the idea that different evolutionary paths leave different fingerprints on the central black hole’s growth. The relic galaxies and the lensed z ≈ 2 galaxy act as crucial benchmarks, anchoring the core-growth pathway with dynamical measurements that dodge many of the biases haunting high-z single-epoch masses. The high-z AGN, QSOs, and LRDs then illuminate the broader landscape where rapid early growth dominates, as opposed to the slower, merger-driven assembly that seems to govern the past and present-round bulges.
The researchers emphasize a practical takeaway for the field: to sharpen our understanding of the MBH–M⋆ relation across time, we need more dynamical black hole masses in relic galaxies and more dynamical MBH measurements in high-redshift lensed galaxies. Gravitational lensing, the natural telescope that it is, will continue to play a pivotal role, offering a way to peer into the cores of distant systems with an accuracy that would be impossible otherwise. And as JWST continues to deliver sharper spectra and better kinematic maps, the picture will become less blurred by systematics and more shaped by the galaxies’ true histories. If two galaxies can share a similar core growth while following divergent outer growth tracks, models of black hole seeding, feedback, and galaxy assembly must accommodate multiple highways rather than a single highway sign.
A snapshot of the players and the method
Institutional backbone: The work comes from the Department of Physics and Astronomy at Dartmouth College, with essential collaboration from the NASA Goddard Space Flight Center. The lead author is Jonathan H. Cohn, joined by Emmanuel Durodola, Quinn O. Casey, Erini Lambrides, and Ryan C. Hickox. This mix of universities and space agency researchers reflects a broader trend in modern astrophysics: solving big questions requires both ground-based, high-resolution dynamical work and space-based, high-redshift surveys. The narrative the paper builds—of growth routes etched into a galaxy’s inner and outer structure—rests on a careful synthesis of dynamical MBH measurements in local relics, a robust measurement in a lensed z ≈ 2 galaxy, and several high-z MBH estimates obtained through independent routes.
Techniques at play: The dynamical measurements in relics come from gas-dynamics and stellar-dynamics modeling that explicitly map motion inside the black hole’s sphere of influence. That makes these masses remarkably robust against the calibration uncertainties that plague single-epoch estimates at high z. The lensed z ≈ 2 galaxy, observed with JWST and Hubble, demonstrates how lensing magnification unlocks a direct dynamical MBH estimate at a time when galaxies were still building their cores. On the high-z side, the study relies on a mosaic of single-epoch masses and SED-based inferences, acknowledging their systematic caveats while using them to illuminate broad trends across the cosmic dawn. The results hinge on careful comparisons to established local relations, namely the bulge-focused MBH–M⋆ relation and the high-z Pacucci relation, and on a transparent Monte Carlo framework that propagates uncertainties across thousands of iterations.
Closing thought: a more nuanced map of growth
Takeaway: The universe does not hand us a single, tidy equation for how black holes grow in step with galaxies. Instead, it offers a tapestry of growth routes, with some galaxies building dense cores in a synchronized embrace with their black holes, while others accumulate mass in their outskirts through quiet, accreting mergers that don’t necessarily wake the central engine. This nuanced view matters because it changes how we interpret observations across time. If we pretend there is one scaling relation that governs all galaxies everywhere, we risk misreading the story of how black holes seeded themselves, how they fed, and how they shaped the galaxies around them.
The paper’s authors argue that studies of relic galaxies, along with lensed, distant systems, are essential stepping stones between z ≈ 2 and the present day. The takeaway is not that the high-z universe disproves the local relations, but that the path a galaxy takes—its history of core growth, bulge assembly, and outer mass accretion—decides where that galaxy sits in the MBH–M⋆ diagram at any given time. The work invites a more pluralistic view of black hole growth: multiple pathways, multiple anchors, and a more intricate map of how the cosmos built its most voracious engines. That’s a story worth following, as new data from JWST, ALMA, and future observatories continues to sharpen our sense of which path a galaxy chose on its long, eventful journey toward today.
