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Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Supermassive black holes can grow in disk galaxies without the kind of violent merger that builds a prominent central bulge, according to a 2026 study of DESI observations. Its key finding is that active black holes in bulgeless galaxies follow the same relationship between black-hole mass and total stellar mass as the broader disk-galaxy sample. That supports gradual, non-merger growth as an important part of galaxy–black-hole co-evolution—but it does not show that mergers never matter.
What the 2026 DESI study found
Sophie M. Jewell and colleagues examined 2,435 disk-dominated galaxies observed with the Dark Energy Spectroscopic Instrument (DESI). Each galaxy in their sample hosts an optical broad-line active galactic nucleus (AGN), meaning its central black hole is actively accreting matter and produces broad emission lines. The authors identified 546 of these hosts as bulgeless—at least five times as many as in earlier bulgeless-AGN samples, according to the paper’s abstract.
The study’s central comparison is between two ways of relating a galaxy’s black hole to its stars:
- Black-hole mass versus total stellar mass: The bulgeless hosts follow the same relation as the general disk-galaxy sample, with agreement within 3σ.
- Black-hole mass versus bulge stellar mass: The bulgeless hosts differ from galaxies with a bulge, with disagreement greater than 7σ.
In the DESI disk-galaxy sample, the reported logarithmic fit between black-hole mass and total stellar mass has a slope of 1.00 ± 0.03 and an intercept of −3.7 ± 0.3. Those values describe this study’s sample and parameterization; they are not a universal formula for every galaxy. The result suggests that a central bulge is not necessary for a galaxy’s black hole to participate in the observed relationship with the galaxy’s total stellar mass.
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The paper, “Galaxy-SMBH co-evolution in 2,435 DESI disk galaxies and merger-free BH growth in bulgeless disks,” appeared online on 21 September 2026 in Monthly Notices of the Royal Astronomical Society as an accepted manuscript. The journal describes accepted manuscripts as author-final versions published before copyediting and typesetting. Read the journal page and abstract.
How black holes can grow without a major merger
“Secular” growth means change driven by processes within a galaxy rather than by a merger with another galaxy. Gas can gradually move inward and feed a central black hole, allowing it to gain mass over time even when the host remains a disk without a large bulge.
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The DESI study supports a substantial role for these non-merger processes in the co-evolution of galaxies and their black holes. It does not isolate a particular feeding mechanism or measure how much growth comes from bars, spiral arms, or any other specific route. Those are possible examples of internal gas transport, not mechanisms this study directly tested.
Why “bulgeless” does not mean “never merged”
The authors use a galaxy’s lack of a bulge as an indicator of a relatively quiet merger history. It is a useful clue, not a complete record of the galaxy’s past. A galaxy’s present appearance cannot establish that it has never merged: simulations suggest some galaxies can experience mergers and later rebuild their disks.
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A previous Horizon-AGN analysis estimated that about one in four present-day bulgeless galaxies in its modeled population had undergone a major merger since redshift 2, and about one in three since redshift 3. That simulation also found a pattern broadly consistent with the DESI result: bulgeless galaxies aligned with the black-hole–total-stellar-mass relation while lying away from the black-hole–bulge relation. The estimates are simulation results, not merger-history measurements for the 546 DESI galaxies. See the Horizon-AGN analysis.
Where mergers still matter
The DESI findings do not rule out merger-triggered black-hole growth. They address a particular population—disk-dominated galaxies with optically visible broad-line AGN—and focus on how accumulated black-hole mass relates to host-galaxy stellar mass. Other kinds of evidence examine different galaxies and different stages of black-hole activity.
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A 2018 Nature study found late-stage nuclear mergers in 17.6% of a sample of obscured luminous black holes, compared with 1.1% of a matched inactive-galaxy sample. That result links mergers to a particular obscured, luminous population; it is not a counter-estimate of merger activity in the DESI sample. Read the Nature study.
Likewise, a Horizon-AGN simulation study attributed about 35% of black-hole mass in its modeled present-day massive-galaxy population directly to mergers—about 22% to major mergers and 13% to minor mergers—and about 65% to secular processes. These are model results under that study’s definitions, not direct measurements from DESI. Read the simulation study.
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Earlier observational work had also reported 13 AGN in massive bulgeless galaxies using Galaxy Zoo and Sloan Digital Sky Survey data, arguing that black holes could grow substantially without significant mergers. That small sample provides historical context rather than an independent replication of the larger DESI study. Read the earlier study.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the result does—and does not—establish
The strongest conclusion is that bulgeless disk galaxies with optical broad-line AGN can lie on the same black-hole–total-stellar-mass relation as other disk galaxies, despite differing sharply in the black-hole–bulge-mass comparison. This supports the idea that gradual internal processes contribute significantly to galaxy–black-hole co-evolution.
The study does not prove that every bulgeless galaxy has been merger-free, show that mergers are irrelevant, or identify exactly how gas reaches the black hole. Nor should black-hole mass be confused with present-day AGN luminosity: mass reflects accumulated growth, while luminosity indicates activity at the time it is observed. The authors also report tentative evidence that a control sample of early-type galaxies has higher AGN luminosity than the bulgeless sample, but find no statistical difference in black-hole mass.
The online paper is an accepted manuscript, so detailed claims about selection thresholds, methods, uncertainty treatment, or changes in the final typeset version should be checked against the full article. Its displayed abstract supports the main comparisons and conclusions summarized here.
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