Black holes can grow to normal sizes without galaxy mergers, study finds

By the Everything Space news desk · 02 Oct 2026 · written from the sources below

Image: Phys.org

New analysis of thousands of disk galaxies suggests gas funneled by bars and spiral arms, not violent collisions, can drive black hole growth.

A study of thousands of galaxies with actively feeding black holes finds that galaxies lacking a central stellar bulge grow their black holes just as effectively as those with bulges, challenging the idea that major mergers are necessary for normal black hole growth.

Researchers using imaging and spectroscopic data from the Dark Energy Spectroscopic Instrument examined 2,435 disk galaxies with actively growing black holes, isolating 546 galaxies with little or no central bulge. Because bulges are often considered relics of past major mergers, bulgeless galaxies offer a way to test whether black holes can grow without such violent events.

The team used an automated imaging technique to separate each galaxy's light into components such as a disk, a possible bulge or bar, and a central point source, verifying their method by inserting artificial bulges into images to check detection accuracy. Black hole masses were then estimated from the properties of hydrogen emission lines and compared against total stellar mass and bulge mass.

The bulgeless galaxies followed the same overall relationship between black hole mass and total stellar mass as bulge-hosting galaxies, suggesting they can achieve comparable black hole growth without merger-driven bulge formation. However, when compared specifically to bulge mass, the two galaxy types diverged, with bulgeless galaxies hosting black holes that were overmassive relative to their minimal bulges.

Modeling indicated that these black holes could have reached their observed sizes within a reasonable time without major mergers, pointing instead to stellar bars and spiral arms as mechanisms for funneling gas toward the galactic center. Most galaxies in the sample contained bars or visible spiral structure, both known to channel gas inward.

When tested against a major cosmological simulation, the simulation struggled to reproduce the abundance of low-mass black holes seen in the observed sample, hinting that current simulations may underrepresent this population.

The authors suggest that upcoming space telescopes with sharper imaging will help determine how truly bulgeless these galaxies are, offering a clearer test of whether gradual, merger-free processes are a major pathway for black hole growth across cosmic history.

Written in Everything Space’s own words by our news-desk model from the sources above, and checked against them in code: every number in this report appears in its sources, and no sentence is copied from them.