Black holes may grow quietly alongside galaxies even without violent mergers, according to new research that challenges a long-held assumption about how these massive objects form. The finding comes from an analysis of 2,435 galaxies hosting actively feeding black holes, using data from the Dark Energy Spectroscopic Instrument (DESI). Among those galaxies, researchers identified 546 “bulgeless” systems — galaxies with little or no central stellar bulge. The paper, published in Monthly Notices of the Royal Astronomical Society, suggests that these galaxies grow black holes just as effectively as systems that have a central bulge. That is a surprising sign that black holes do not need galaxy mergers to grow normally.
What DESI Data Showed
The DESI instrument gathers light from distant objects across the sky, allowing astronomers to measure the properties of galaxies and the matter around them. For this study, the team focused on galaxies with active black holes at their centers, which are consuming gas and emitting radiation. The sample size is large enough to draw meaningful conclusions. With 2,435 galaxies in the full set and 546 bulgeless galaxies singled out, the comparison between bulged and bulgeless systems can be made with confidence.
Bulgeless Galaxies Host Growing Black Holes
The key result is that bulgeless galaxies host growing black holes as effectively as those with central bulges. The expectation had been that galaxy mergers drive black hole growth, with collisions between galaxies triggering the accretion of gas onto black holes. Instead, the data suggest that black holes can grow through quieter processes. The paper’s authors describe the finding as a sign that black holes do not need galaxy mergers to grow normally.
Why This Matters
The result pushes back against a common picture of galaxy evolution. In that picture, galaxy mergers are the engine of growth for both stars and black holes. A merger brings fresh gas into the center of a galaxy, feeding the black hole while also building up the central bulge. If black holes can grow in bulgeless galaxies, that model needs revision. The finding suggests that black hole growth can happen independently of the larger structure of the galaxy around it.
The Paper’s Claim
The paper presents the finding as a surprise. The authors themselves note that the discovery contradicts expectations that galaxy mergers drive black hole growth. The reaction is honest: the result was not anticipated. The data simply showed what the data showed.
What We Know So Far
The research rests on a specific dataset and a specific measurement. Here is what the paper establishes:
- 2,435 galaxies with actively feeding black holes were studied
- 546 of those galaxies were classified as bulgeless
- Bulgeless galaxies host growing black holes as effectively as galaxies with central bulges
The paper does not claim that galaxy mergers never feed black holes. It claims that black holes can grow without them. That is a subtle but important distinction.
The Case for Quiet Growth
The evidence for quiet growth is statistical, not individual. The researchers looked at the population as a whole and found no difference in black hole growth rates between bulged and bulgeless galaxies. That pattern suggests that the physics governing black hole growth operates at a smaller scale than previously assumed. Instead of waiting for a merger to deliver fresh gas, black holes may find other ways to feed. The finding also raises questions about the history of galaxies. If black holes can grow quietly, then some of the biggest black holes in the universe may have formed through steady accretion rather than dramatic collisions.
What Comes Next
The paper opens a door rather than closing one. It shows that black holes can grow without mergers, but it does not explain how. Future work will likely explore the mechanisms that allow bulgeless galaxies to sustain black hole growth. That could include studying the gas flows that bring matter to the galactic center, or the conditions that trigger accretion. For now, the takeaway is simple: black holes do not need galaxy mergers to grow. They can do it on their own. The paper’s authors describe the finding as a sign that black holes do not need galaxy mergers to grow normally. That is the honest reaction to a surprising result.
Source material: “Black holes may grow quietly alongside galaxies even without violent mergers,” Phys.org.
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