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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Black hole accretion is the process of matter moving inward toward a black hole; a galaxy merger is a gravitational interaction in which two galaxies combine. They are not the same event: a merger can funnel gas toward a galaxy’s central black hole and increase its accretion, but accretion can also happen without a galaxy merger.
How accretion differs from a galaxy merger
| Question | Black hole accretion | Galaxy merger |
|---|---|---|
| What is happening? | Gas or other matter moves inward toward a black hole. | Two galaxies interact and combine under gravity. |
| What is the scale? | The black hole’s surrounding disk and nearby environment. | The galaxies and their contents, including stars, gas, dust and dark matter. |
| What can cause it? | A supply of matter and a way for it to lose energy and move inward. | The galaxies’ changing gravitational influence can redistribute gas and drive some inward. |
| What might astronomers observe? | Radiation from hot disk material, including X-rays, and clues in spectra. | Interacting or disturbed galaxies; sometimes an active nucleus or a close pair of central black holes. |
| How are they related? | Accretion can occur without a galaxy merger. | A merger can trigger or increase accretion, but it is not accretion itself. |
Accretion is not a black hole “sucking in” everything nearby. Matter can orbit for a long time; it must lose energy and angular momentum to move inward. The bright radiation associated with accretion usually comes from heated matter outside the event horizon, not from the black hole itself. Once light crosses the event horizon, it cannot escape. NASA’s black hole overview explains how black holes grow by drawing in material.
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How a galaxy merger can feed a black hole
When galaxies interact, their combined gravitational effects can disturb gas and dust that would otherwise orbit within them. Some of that material may lose energy, move toward a galactic center and feed a supermassive black hole. If the central region becomes bright because of activity around that black hole, it is called an active galactic nucleus, or AGN. An AGN is not another name for a merger.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsThis is a possible chain of events, not a guarantee: a merger can promote accretion, but not every merger produces a bright AGN. Nor does accretion require a merger; black holes can gain material from other sources, including nearby stars. NASA’s Jet Propulsion Laboratory account of NuSTAR observations describes how merger-driven disturbances can channel gas toward black holes.
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What astronomers look for
Light from hot accreting gas
As gas in an accretion disk moves rapidly and collides, it heats to millions of degrees and can emit detectable X-rays. Astronomers also examine spectra: the light’s pattern can help distinguish hot, fast-moving gas around a black hole from cooler, slower gas associated with star formation. X-rays indicate energetic activity, but on their own they do not prove that the host galaxy is merging. NASA’s Imagine the Universe! explanation of black holes describes accretion-disk emission.
Dust-obscured activity in merging galaxies
Dust and gas can hide an active nucleus, especially in later stages of a merger. In a 2017 account, NASA/JPL reported that researchers used NuSTAR to observe 52 galaxies, about half of them in later merger stages, alongside observations from Swift, Chandra and ESA’s XMM-Newton. High-energy X-rays can penetrate obscuring material that blocks lower-energy X-rays, helping reveal a hidden AGN. The study’s lead author, Claudio Ricci, said, “The further along the merger is, the more enshrouded the AGN will be.” NASA/JPL’s report describes the sample and observing approach.
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Close pairs of supermassive black holes
A galaxy merger can bring the galaxies’ central black holes into proximity, but the black holes do not necessarily merge at the same time as their host galaxies. NASA reported in 2024 that two supermassive black holes in the merging galaxy MCG-03-34-064 were approximately 300 light-years apart, based on X-ray evidence alongside optical and radio observations. That distance applies to this particular system as reported; it is not a standard separation for galaxy mergers. The pair may continue spiraling inward before eventually coalescing. NASA’s report on MCG-03-34-064 describes the observed pair.
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When black holes merge, they produce gravitational waves. LIGO has detected waves from mergers of stellar-mass black holes, while the longer wavelengths expected from supermassive black hole mergers are beyond LIGO’s capability. NASA describes LISA as a planned space mission intended to observe those longer wavelengths; mission schedules can change. NASA’s black hole overview discusses black hole mergers and gravitational waves.
Simulations have also predicted that gas around close supermassive black hole binaries may glow mainly in ultraviolet light, with some high-energy X-rays. That is a model prediction for a particular stage, not a universal signature of every galaxy merger. NASA’s account of the simulations explains the predicted emissions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What is established about black hole and galaxy growth
Supermassive black holes can grow through accretion and through mergers, and their host galaxies appear to co-evolve with them. NASA notes that the quantitative relationship between black hole growth and galaxy evolution remains incomplete; the available evidence does not establish a general percentage of growth attributable to accretion versus mergers. As NASA puts it, “Whether supermassive black holes grow through mergers or accretion, their host galaxies appear to have co-evolved with them.” NASA’s overview of black holes and galaxy evolution discusses the connection and remaining uncertainty.
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