BY:SpaceEyeNews.
Black hole growth may need far less galactic upheaval than many people imagine. Astronomers studying 2,435 disk galaxies identified 546 with little or no central stellar bulge. Yet their actively feeding black holes still follow a familiar relationship with their host galaxies.
The finding strengthens evidence that galaxies and their central black holes can develop together through gradual internal processes. Major mergers can encourage growth, but they do not appear essential for establishing this connection.
Black Hole Growth Without Galaxy Mergers.
The DESI Study Behind the Discovery
Sophie M. Jewell of the University of Oxford led the research, using data from the Dark Energy Spectroscopic Instrument, or DESI. The team selected disk galaxies containing active galactic nuclei, the bright central regions powered by accreting black holes.
Their study appeared in Monthly Notices of the Royal Astronomical Society on September 21, 2026. It expands the available sample of bulgeless galaxies hosting these active sources substantially.
Separating the Galaxy’s Light
Identifying a faint bulge beside a bright galactic center presents a challenge. To address it, the researchers used software called GALFITM.
The program modeled separate contributions from the central point source, stellar disk, and any bulge or bar. Researchers also inserted artificial bulges into real images. Those tests helped assess whether their analysis could recover structures that might otherwise escape detection.
Meanwhile, broad hydrogen-alpha emission lines provided estimates of black hole masses. Their widths and brightness offer clues about gas moving near the central object.
Why Bulgeless Galaxies Matter
A galaxy’s structure preserves clues about its past. Major mergers can rearrange stellar orbits and build prominent central bulges. Consequently, galaxies lacking substantial bulges offer useful places to investigate growth without that kind of restructuring.
However, their appearance cannot reveal every event in their history. A missing bulge suggests a relatively quiet past; it does not prove complete isolation.
This distinction matters because the study tests an explanation for linked galaxy and black hole development. Astronomers already know that several feeding mechanisms can operate. The question concerns whether internal processes can establish the mass relationships seen across galaxy populations.
The researchers therefore compared bulgeless systems with galaxies containing some bulge component. That comparison allowed them to examine which aspects of the host galaxy best matched its central black hole.
Black Hole Growth Tracks the Wider Galaxy
The strongest result emerged when the team compared black hole mass with total stellar mass. Bulgeless galaxies followed a relationship consistent with the broader disk galaxy sample.
In plain terms, these black holes did not need large stellar bulges to fit the overall pattern. Their masses still related to the amount of stellar material throughout their galaxies.
A Different Picture at the Center
The comparison changed when researchers considered bulge mass alone. Here, the bulgeless population differed clearly from galaxies containing measurable bulges.
Many central black holes appeared unusually massive relative to their tiny bulges or the limits placed on undetected bulges. Yet they remained consistent with the relationship involving total stellar mass.
That contrast supports a significant role for gradual internal evolution. It suggests that building a central bulge and growing a black hole need not proceed together.
Still, a population trend does not measure identical growth rates. Individual galaxies can have different feeding histories while contributing to a similar overall relationship. arxiv.org
How Bars and Spiral Arms Could Feed Black Holes
If a major merger is unnecessary, gas still needs a route toward the galactic center. Stellar bars and spiral arms offer plausible ways to help supply it.
Their gravitational influence can redistribute gas within a disk. Over time, inward movement can bring material closer to the central region, where further processes may deliver it to the black hole.
Gradual Feeding Can Add Up
The researchers found bars in roughly two-thirds of their sample, while spiral arms appeared in the vast majority. These common structures make internal feeding a credible explanation for the observed growth.
A sustained supply does not require a single dramatic event. Instead, repeated feeding episodes could contribute substantial mass over a galaxy’s lifetime.
Nevertheless, images showing bars and spiral arms do not trace gas all the way into a black hole. The proposed connection remains a physical interpretation of the evidence, rather than a direct observation of each object’s complete feeding history.
What the Findings Leave Unresolved
The sample includes actively feeding black holes, so it cannot represent every galaxy or every stage of growth. Bright central emission also complicates the search for small stellar bulges. The work examines later growth, rather than how the original black hole seeds formed.
Moreover, the findings do not rule out important contributions from mergers in other environments. They show that a quieter pathway deserves substantial attention.
Webb observations add wider interest to this question. Some early black holes appear unexpectedly massive compared with their hosts, while numerous early galaxies retain disk structures.
However, this DESI study does not directly explain those distant systems. Its results provide evidence about possible growth pathways, rather than a complete solution to early black hole formation.
Sharper Images Could Clarify Hidden Bulges
Better imaging could strengthen or revise the current classifications. Observations from space observatories such as Euclid and Roman should help researchers identify compact or faint central structures.
The crucial question concerns whether apparently bulgeless galaxies contain stellar components that current images cannot separate clearly.
Finding those structures would improve the mass comparisons. Confirming their absence would strengthen the evidence for growth in galaxies with very small bulges.
Black Hole Growth Has a Quieter Path
Black hole growth can apparently accompany galaxy development without requiring a major merger. The 546 bulgeless systems strengthen that conclusion while leaving individual histories open to investigation.
Understanding these central giants therefore requires attention to ordinary galactic structures. Bars, spiral arms, and gradual gas flows may help explain how galaxies nurture growing black holes over cosmic time.
Main sources
- Original research, Monthly Notices of the Royal Astronomical Society: https://doi.org/10.1093/mnras/stag1783
- Authors’ manuscript: https://arxiv.org/abs/2609.22434
- University of Oxford research listing: https://www.physics.ox.ac.uk/research/group/galaxy-formation-and-evolution/publications
- Universe Magazine