Merging and Hierarchical Growth
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 02 / Article 07
Merging & hierarchical growth
How smaller systems become part of larger ones—and why a galaxy’s history includes both inherited stars and new beginnings.
A galaxy can inherit stars from several earlier homes.
Some stars in the Milky Way began their lives in other galaxies. Their motions and chemistry still carry traces of those earlier homes.
Gravity brought those systems together. Encounters stretched their outer material, changed stellar orbits, and eventually scattered some of their stars through our Galaxy.[10]
That history is part of hierarchical growth: smaller structures contribute to larger ones over cosmic time. To understand it, we need to follow dark matter halos, the galaxies they contain, and the gas that keeps forming new stars.
What does “hierarchical” describe?
In the standard ΛCDM model, cold dark matter supplies much of the gravity that builds cosmic structure. “Cold” means its early random motions were small enough for density variations to survive on small scales. Λ represents a cosmological constant associated with accelerated expansion.
Gravity amplifies those variations. Bound concentrations called halos form, acquire more matter, and join larger systems. This statistical tendency toward assembly from smaller progenitors is the basis of hierarchical growth.[1]
A halo can grow through recognizable mergers and through matter arriving outside identified smaller halos. The boundary between smooth accretion and unresolved small mergers depends partly on how a simulation identifies and resolves structures.[2]
Galaxies develop within this evolving environment. Their stars do not trace dark matter growth in a simple one-to-one way: gas must cool, survive feedback, and become dense enough to form stars.
What actually happens when galaxies meet?
A galactic encounter often unfolds over hundreds of millions of years or longer. The galaxies can pass one another more than once before their centers coalesce. A close passage can also remain a flyby.[3], [4]
Tides reshape the outskirts
The companion’s gravitational pull differs from one side of a galaxy to the other. These tidal forces can stretch stars and gas into tails, bridges, and streams.[4]
Orbits change
Individual stars are so widely separated that direct stellar collisions remain rare. Their paths can nevertheless change substantially as the galaxies’ combined gravitational field evolves.[4]
Why do some galaxies spiral inward?
A massive satellite moving through its host disturbs the surrounding matter, creating a gravitational wake. The wake pulls back on it and transfers energy and angular momentum from its orbit to the environment. This is dynamical friction.
The inward journey depends on the satellite’s mass, orbit, and loss of material. Some satellites merge; others are tidally disrupted or continue orbiting for a very long time.[3]
What do major, minor, wet, and dry mean?
A merger’s description usually combines the relative sizes of its participants with the amount of cold gas they contain. These labels help compare events, but do not uniquely predict the outcome.
| Term | A common meaning | What it tells us |
|---|---|---|
| Major merger | The smaller galaxy has at least roughly one-quarter of the larger galaxy’s stellar mass. | Both participants can strongly affect the encounter’s dynamics. |
| Minor merger | A smaller companion joins a substantially more massive galaxy, below the chosen major-merger threshold. | The larger galaxy may preserve much of its structure while acquiring stars and gas. |
| Gas-rich or “wet” | Substantial cold gas is available. | Inflow, cooling, and new star formation may influence the remnant. |
| Gas-poor or “dry” | Little cold gas is available. | The event can mainly rearrange existing stars and enlarge the stellar system. |
The 1:4 boundary is a common convention. Some studies choose different thresholds or compare total halo mass or baryonic mass instead of stellar mass. Those ratios are not interchangeable; the time at which masses are measured also matters.[5], [7]
For example, a 1:4 stellar mass ratio means the smaller galaxy contains one-quarter as much mass in stars. It does not mean its dark matter halo must also be one-quarter as massive.
How do we read a merger tree?
A merger tree connects a later object to its earlier progenitors. Researchers construct these trees in simulations by following matter and identifying which earlier systems contribute to later descendants.
Reading backward reveals a branching ancestry. Reading forward shows separate systems joining a common history.[1], [2]
Where did the stars themselves form?
New stars from available gas
A galaxy forms stars from gas that has reached it and cooled. That gas may have arrived gradually, returned after an outflow, or come in with another galaxy.
Stars inherited from another galaxy
These stars were already born before being acquired through merging or tidal stripping. Their ages and chemistry can preserve clues to a different origin.
This distinction tracks a star’s birthplace. Gas delivered by a merger can later form in-situ stars in the receiving galaxy. Hierarchical assembly therefore does not require most of a galaxy’s stars to have arrived fully formed.[5]
Can a recently assembled galaxy contain mostly old stars?
Yes. Its stars may have formed early in several progenitor galaxies, while the larger system assembled much later. The ages of its stars and the date of its final assembly describe different parts of its history.
A merger also redistributes existing chemical elements. New heavy elements are produced through stellar evolution and related nuclear processes, including the explosions described in Primordial Supernovae: Element Synthesis.
Does every merger make an elliptical galaxy?
Mergers can rearrange orderly stellar rotation into a wider distribution of orbits, build central bulges, and spread stars into extended envelopes. A gas-poor encounter may leave a remnant supported largely by stars moving on varied orbits.
Gas changes the possibilities. Some disks can survive an encounter, while surviving or newly supplied gas can settle into a rotating disk afterward. Preserving a disk and rebuilding one are different routes to a similar later appearance.[7]
A burst of star formation
Tidal disturbances and inflow can concentrate gas, producing a temporary rise in the star-formation rate. Some nearby merging systems show spectacular starbursts.[4]
The outcome depends on the available gas and its response to the encounter.
A smaller change in activity
A galaxy may already be forming stars rapidly. Models of some gas-rich high-redshift mergers find only a modest additional enhancement.
A high star-formation rate alone does not establish that the merger caused a large burst.[6]
Radiation, explosions, and black-hole activity can then alter the fuel supply. A temporary burst or interruption need not determine the galaxy’s fate forever; later inflow can change the situation again. The surrounding gas cycle is explored in Feedback Effects: Radiation and Winds.
How do we know galaxies have grown this way?
We cannot watch a complete galactic merger unfold within a human lifetime. Instead, astronomers combine encounters seen at different stages, the debris left by earlier events, and statistical measurements across cosmic time.
Stellar motions and chemistry
Streams, unusual orbits, and shared chemical patterns can identify stars that once belonged together. Gaia measurements make this reconstruction possible across large parts of the Milky Way.
In 2018, Helmi and colleagues linked a major component of the inner stellar halo to the ancient Gaia–Enceladus accretion event, about ten billion years ago. Its debris provides a record of earlier assembly.[10]
Close pairs and tidal structures
Images reveal tidal tails, bridges, and galaxies with nearby companions. Spectra help establish whether apparent neighbors are also close in distance.
Turning a pair count into a merger rate requires a model for how long a pair remains observable and how likely it is to merge. Projection, sample selection, and uncertain masses affect the result.[4], [11]
Frequent mergers can coexist with growth dominated by new stars
A 2025 JADES study examined major-merger pairs at redshifts around 3–9. Its revised analysis estimated that major mergers supplied about 3–13% of the stellar mass growth over the interval studied, with in-situ star formation providing most of the growth.
This estimate depends on the sample and the conversion from pair counts to merger rates. It concerns directly acquired stellar mass; mergers may also bring gas or stimulate star formation. A galaxy can participate in hierarchical assembly while building most of its stars locally.[11]
Mapping the setting for those encounters
Redshift surveys map galaxy positions and clustering over enormous volumes. DESI completed the observations for its original five-year survey in April 2026, having measured more than 47 million galaxies and quasars.
Such maps allow researchers to compare the distribution of galaxies with models of cosmic structure. They reveal populations and environments across time, rather than directly tracking any single galaxy through its whole ancestry.[12]
How does assembly continue into groups and clusters?
Galaxy groups and clusters grow as halos acquire galaxies, gas, and dark matter. Smaller groups can become part of a cluster, and whole clusters can merge.
A cluster is a common gravitationally bound system containing many distinct galaxies. Much of its ordinary matter lies in hot, diffuse gas between them, while dark matter supplies most of the total mass.[13]
The individual galaxies inside a rich cluster often move past one another at high speeds. Encounters can be frequent without efficiently producing mergers. Slower encounters in groups can be more favorable for coalescence.[8]
Meanwhile, interaction with the cluster’s hot gas can strip material from a galaxy, changing its ability to form stars. Environmental change therefore includes much more than merging.
There is no compulsory sequence in which every galaxy becomes part of a group, then a cluster, then a single enormous remnant. Location, surrounding density, and cosmic expansion influence which structures can assemble.
The next article explores Galaxy Clusters and the Cosmic Web in more detail.
What can simulations test—and what remains uncertain?
Cosmological simulations evolve matter under gravity. Hydrodynamic calculations also follow gas, with models for cooling, star formation, and feedback. Researchers then compare predicted populations and structures with observations.
The numerical particles in a dark matter simulation each represent a large amount of matter. They are not individual microscopic dark matter particles. A calculation that follows a vast cosmic volume may not resolve the tiny halos relevant to the first stars.[15]
The original Millennium project, for example, combined a dark matter calculation with separate models for galaxy evolution. Different methods and resolutions answer different questions.[1]
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How much growth comes from each route?
Mergers, diffuse accretion, and new star formation contribute differently across galaxy masses and epochs.
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How long do satellites survive?
Orbits, dynamical friction, and tidal mass loss affect both merger rates and the debris left behind.
-
How does gas change the remnant?
Cooling and feedback influence whether disks survive, stars form, or activity becomes suppressed.
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Which cosmological picture fits?
Cluster abundance, masses, and clustering test how structure grows within an expanding universe.[13]
What could gravitational waves add to this story?
A galaxy merger can bring two central black holes into the same system. Their eventual coalescence is a further process with its own dynamics and delays.
The planned LISA space observatory aims to detect gravitational waves from massive black-hole mergers and other sources. Such signals would probe part of black-hole assembly that is difficult to reconstruct from light alone. They would complement galaxy observations; a black-hole merger and a galaxy merger are different stages of the wider history.[14]
Larger systems keep traces of their smaller beginnings.
Some traces survive as satellites, stellar streams, or old stars with a shared chemical history. Others are reshaped as gas cools, new stars form, and orbits change. A galaxy’s present appearance is the result of all those processes acting together.
On larger scales, these histories meet within groups, clusters, and connecting filaments. We follow that expanding view next, into the cosmic web.
Sources and further reading
Observations and mission descriptions reviewed in September 2026. Merger classifications and inferred rates depend on the definitions, samples, and models used.
- Springel et al. (2005) — Simulations of the formation, evolution and clustering of galaxies and quasarsThe Millennium simulation: dark matter assembly and models of the galaxies within it.
- Genel et al. (2010) — The growth of dark matter halos: evidence for significant smooth accretionHalo growth through mergers and matter arriving outside identified bound progenitors.
- Boylan-Kolchin, Ma & Quataert (2008) — Dynamical Friction and Galaxy Merging TimescalesWhy halo entry and galaxy coalescence occur on different timescales.
- ESA/Hubble (2008) — Galaxies gone wild!Observed interacting galaxies, tidal structures, and the different responses of stars and gas.
- Rodriguez-Gomez et al. (2016) — The stellar mass assembly of galaxies in the Illustris simulation: growth by mergers and the spatial distribution of accreted starsStars formed within a galaxy, stars acquired from elsewhere, and merger mass-ratio definitions.
- Fensch et al. (2017) — High-redshift major mergers weakly enhance star formationSimulations showing why an encounter need not cause a strong additional burst of star formation.
- Hopkins et al. (2009) — How Do Disks Survive Mergers?The role of gas in the survival and rebuilding of galactic disks.
- Makino & Hut (1997) — Merger Rate of Equal-Mass Spherical GalaxiesHow relative velocity affects merger rates in different environments.
- Chon, Böhringer & Zaroubi (2015) — On the definition of superclustersThe distinction between a large association and a region destined to collapse together.
- Helmi et al. (2018) — The merger that led to the formation of the Milky Way’s inner stellar halo and thick diskKinematic and chemical evidence for the ancient Gaia–Enceladus accretion event.
- Puskás et al. (2025) — Constraining the major merger history of z ~ 3–9 galaxies using JADES: dominant in-situ star formationJWST pair counts and inferred stellar growth; the revised May 2025 analysis is used here.
- DESI Collaboration (2026) — DESI Reaches Mapping Milestone, Surpassing ExpectationsCompletion of the original five-year spectroscopic survey and its map of galaxies and quasars.
- Kravtsov & Borgani (2012) — Formation of Galaxy ClustersCluster assembly, hot gas, cosmological tests, and the limits of current modeling.
- European Space Agency — LISA factsheetThe planned space observatory and the massive black-hole mergers it aims to study.
- Vogelsberger et al. (2014) — Introducing the Illustris Project: Simulating the coevolution of dark and visible matter in the UniverseHydrodynamic galaxy formation, numerical resolution, and the treatment of gas and feedback.
All articles in this chapter
- Gravitational Clumping and Density Fluctuations
- Population III Stars: The Universe’s First Generation
- Early Mini-Halos and Protogalaxies
- Supermassive Black Hole Seeds
- Primordial Supernovae: Element Synthesis
- Feedback Effects: Radiation and Winds
- Merging and Hierarchical Growth — you are here
- Galaxy Clusters and the Cosmic Web
- Active Galactic Nuclei in the Young Universe
- Observing the First Billion Years