Evolutionary Paths: Secular vs. Merger-Driven
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 03 / Article 08
Galactic evolution
Bars rearrange disks. Encounters redistribute stars and gas. A galaxy’s history emerges from the way these processes meet—and from the fuel available afterward.
A galaxy can change its rhythm many times
Imagine following a spiral galaxy for several billion years. Its bar draws some gas inward, stars change orbits, and a small companion leaves a stream across its halo. Later, fresh gas settles into its disk.
Which event defines the galaxy’s evolution? Each changes something different. The bar redistributes material already present; the companion contributes material of its own; new gas allows further stars to form.
After exploring irregular galaxies, we can now connect the processes behind many galactic forms. The useful questions are what changes, how quickly it changes, and what evidence remains.
What do secular and merger-driven mean?
Secular evolution is cumulative change that is slow compared with a galaxy’s local dynamical timescale—the time over which its stars and gas respond to gravity. In a disk, this often means changes building over many rotations.
This article focuses on internal secular evolution: bars, spiral structure, and other processes that gradually redistribute a galaxy’s own material. Such evolution commonly continues for billions of years, although the instability that first creates a bar can develop much faster.[1], [2]
A bar can keep reshaping its surroundings
Stars and gas respond differently to the same gravitational pattern, changing the disk’s structure over time.
A companion can alter the whole system
Tides disturb the galaxies; a merger can also add stars, gas, and dark matter to the final remnant.
These are illustrative systems, not successive stages. Neither panel specifies the galaxy’s future shape or star formation rate.
Merger-driven evolution involves changes associated with galaxies combining. The gravitational field can change rapidly during close passages and coalescence, even though the full encounter lasts much longer. A close interaction can also reshape galaxies that never merge.[8]
The distinction is useful, but incomplete. Smooth gas accretion and environmental gas removal also matter. An external encounter can even initiate a bar that later drives prolonged internal evolution.
How does a disk rearrange itself?
A bar is an elongated stellar pattern supported by orbiting stars. Its gravity exerts torques—turning forces—that exchange angular momentum, the quantity associated with orbital motion, among the bar, the outer disk, and the dark matter halo.[3]
Existing material moves
Stars can gain or lose angular momentum and change their orbital distribution. Rearranging old stars changes structure without requiring new stars to be born.
New central stars can form
Gas can shock, radiate energy, and move inward as it loses angular momentum. Some accumulates in nuclear rings or disks, where it may form stars.
Gas reaching the inner few thousand light-years still has a long way to travel before joining a black hole’s accretion flow. A bar can help supply the central region without guaranteeing an active nucleus.[1], [2]
Stars can migrate without strongly heating the disk
Near corotation, where a star’s average orbital angular speed matches a spiral pattern’s speed, an interaction can change the star’s guiding orbital radius with little increase in random motion. Stars then appear at radii different from where they formed, mixing populations with different ages and chemical compositions.[4]
Dynamical heating means increasing random orbital motion. It is a different effect from this form of migration; moving stars across a disk does not automatically make that disk much thicker.[2], [4]
Spiral patterns themselves can recur and fade, be associated with bars, or respond to companions. Disk evolution does not require every arm to remain one fixed density wave for billions of years.[5]
What does a bulge reveal about the past?
A bright central concentration is not a single kind of object. Separating its components helps explain how it formed.
Disky pseudobulge
A flattened central component with disk-like features and motions. Gas brought inward can build a nuclear disk and add new stars there.
Box or peanut structure
The vertically thick inner part of a bar, most recognizable in suitably inclined views. Its shape can arise by rearranging existing disk stars.
Classical bulge
A more spheroidal component with substantial random stellar motions. It can also rotate and can coexist with a bar or central disk.
Box- and peanut-shaped structures need not be young, and a disky pseudobulge need not still be actively forming stars. Conversely, a classical-looking bulge does not identify one unique merger event.[3], [6]
Observations find composite bulges: more than one central component within the same galaxy. Their overlapping light can make a simple label misleading.[7]
Can a single brightness-profile measurement settle the question?
No. A Sérsic profile describes how brightness changes with radius, but its fitted index is not a complete formation history. Color, profile shape, and apparent flattening are most useful when combined with stellar motions and population information.
Different components can also overlap in projection, and dust can change their apparent prominence. A convincing interpretation needs several consistent clues.[6], [7]
Why do mergers produce different remnants?
A major merger joins galaxies of broadly comparable mass; a minor merger involves a much smaller companion. The numerical dividing line varies between studies, and a stellar-mass ratio need not equal the ratio of their total masses.
During an encounter, tidal forces can draw out tails and bridges. Near coalescence, the changing collective gravitational field redistributes stellar orbital energies—a process called violent relaxation. This does not require stars to collide physically.
Gas behaves differently. Shocks and gravitational torques can concentrate some of it, sometimes producing an intense episode of star formation. How much gas reaches the center depends on the encounter and the galaxies involved.[8]
Growth through existing stars
A gas-poor, or “dry,” merger mainly adds and redistributes stellar populations. It can grow a spheroid or its outskirts without a large new starburst.
A disk can remain or grow again
Some gas-rich mergers preserve a disk component or allow gas to settle and form a new one. The remnant need not become a permanently gas-poor elliptical.
Simulations find a range of outcomes that depends on gas content, orbital geometry, galaxy mass, and halo properties. No single mass ratio determines the final morphology.[8], [9]
Smaller companions can deposit stars in streams and halos, disturb a disk, or deliver fresh gas. The effects depend on the satellite’s orbit and structure as well as its mass; they need not erase the host’s disk.[9]
For the mechanics of individual encounters, see Collisions and Mergers: Drivers of Galactic Growth.
What determines whether star formation continues?
A galaxy can keep forming stars if gas reaches sufficiently cold, dense conditions. Its future depends on the available reservoir, replenishment, cooling, and the energy supplied by stars and active black holes.
A merger does not automatically exhaust the gas. Observations of recently merged galaxies have found substantial atomic reservoirs, while molecular-gas surveys reveal rotating disks and rings in many remnants.[10], [15]
When were the stars born?
Stellar ages record earlier episodes of star formation, possibly in several different progenitor galaxies.
When was the galaxy assembled?
A later merger can bring those pre-existing stars together into one larger system.
When did star formation fade?
Quenching concerns a sustained decline in new star formation. It need not coincide with either stellar birth or assembly.
Quenching has more than one route
Merger samples do show an excess of post-starburst signatures: spectra indicating substantial recent star formation followed by a sharp decline. A 2024 study linked the strongest excess to the first several hundred million years after estimated coalescence. That supports a connection for some systems, rather than a compulsory stage after every merger.[11]
Elsewhere, motion through hot surrounding gas can strip a galaxy’s own gas, or its supply of fresh material can diminish. These environmental effects can reduce star formation without a galaxy merger. Removing gas also does not directly rearrange the existing stars into an elliptical system.[14]
In massive galaxies with hot atmospheres, jets from an active nucleus can offset cooling and limit the supply reaching cold, star-forming conditions. This regulation can continue through repeated episodes, rather than a single final blast.[22]
Does a merger have to switch on a quasar?
No. A 2025 analysis found enhanced AGN incidence soon after estimated merger coalescence, with the measured enhancement depending on the observational diagnostic and nuclear luminosity. Other work on a selected X-ray AGN sample found no significant excess of strongly disturbed hosts.
These studies ask related questions with different samples and detection methods. Together they support mergers as one way to promote black hole growth, while leaving room for substantial activity in galaxies without obvious recent major mergers.[12], [13]
What does a mixed evolutionary history look like?
A tidally initiated bar
A companion perturbs a disk and helps a bar develop. Afterward, the bar continues exchanging angular momentum and redistributing material.
Recent TNG50 simulations find bars with both internal and tidal origins; their later properties can overlap.[17]
A merger followed by disk growth
Gas that survives an encounter, returns from tidal material, or arrives later can settle into rotation and support further star formation.
A new disk may coexist with an older spheroid, retaining evidence of both phases.[8], [15]
A disk with less fuel
Environmental gas loss or reduced replenishment can suppress star formation while a recognizable stellar disk remains.
The later appearance also depends on fading stellar populations and any accompanying gravitational disturbances.[14]
NGC 7252: organized gas within a merger remnant
NGC 7252 carries extended evidence of a past merger, yet its center contains a rotating, star-forming gas “minispiral.” ESO observations mapped the gas motions directly. A remnant can therefore contain both disturbed outer structures and organized central gas; this small disk does not prove that a large spiral galaxy will eventually be rebuilt.[16]
The Milky Way: a disk with an accretion record
Our Galaxy’s disk and bar coexist with stellar debris from past mergers. Gaia measurements of stellar motions, combined with chemical information, identified the Gaia–Enceladus accretion event in its early history. A disk today can retain evidence of substantial external growth in other components.[18]
How do astronomers reconstruct a history?
There is no single image that reveals every past event. Astronomers combine light, motion, stellar populations, gas, and environment to test which histories fit the system.
| Observation | What it can reveal | What else is needed |
|---|---|---|
| Tails, shells, or stellar streams | Past gravitational encounters or the accretion of a companion. | Their visibility depends on age, image depth, viewing angle, and the kind of event. |
| Bars, rings, or central disks | Structures capable of redistributing gas and stars. | A bar’s presence alone does not identify whether it began internally or after an encounter. |
| Spatially resolved spectra | Rotation, random motions, chemistry, and varying stellar populations across a galaxy. | Overlapping components and the viewing direction must be modeled. |
| Atomic and molecular gas maps | The location and motion of fuel, including central disks and displaced reservoirs. | Gas on large scales need not already be dense enough to form stars. |
| Stellar ages and abundances | Different birth environments and episodes of star formation. | A star’s present location can differ from where it formed. |
Each clue constrains part of the history. The strongest interpretations explain several independent observations at once.[4], [6], [15], [17], [18], [19]
Integral-field spectroscopy records a spectrum at many positions across a galaxy, allowing astronomers to map motion and stellar properties rather than average everything into one measurement.
Merger statistics add another challenge: a visible merger fraction is not yet a merger rate. Converting one to the other requires estimating how long the selected features remain detectable. Close-pair searches and disturbed-shape searches can probe different stages and mass ratios.[19]
A smooth image may have lost its faint tidal signatures. A bright central population may conceal older stars. Reconstruction therefore remains an inference from incomplete evidence, with uncertainties that can be narrowed by better data.
How do these processes fit into cosmic history?
Galaxies at different epochs have different gas supplies, masses, and environments. The importance of mergers and internal evolution consequently varies across the population; it is not captured by one universal percentage.
Strong bursts also do not account for all star formation in the early universe. A study near redshift two found that most star formation in its analysis occurred in galaxies near the ordinary star-forming sequence—the relation between stellar mass and star formation rate—rather than in extreme bursts.
That is a statement about activity. It cannot be converted directly into the share caused by mergers, because an interacting galaxy can lie near the ordinary sequence and a burst need not uniquely identify a merger.[20]
What simulations can test
Simulations follow gravity, gas, star formation, and feedback together. They let researchers compare possible histories and investigate how changing a gas supply or encounter affects a remnant.
They also expose uncertainty. A comparison of EAGLE, Illustris, and IllustrisTNG found enhanced black hole accretion after mergers across the models, but different frequencies of rapid quenching. The way unresolved feedback is represented helps shape the outcome.[21]
Match several properties together
A useful model should account for a galaxy’s stellar structure and motions, gas distribution, star formation history, and surroundings. Simulated images can then be processed with the same limits in resolution and depth as the observations.
Agreement across these measurements makes a proposed history more convincing than a resemblance in shape alone.[9], [19], [21]
The goal is to understand how galaxies change through time while preserving records of earlier phases. Slow redistribution, fresh gas, minor accretion, major mergers, and environmental effects all contribute to that record.
A history written in stars, gas, and motion
A galaxy’s present form reflects what it inherited, what arrived, and how its material was rearranged. Its gas supply then influences whether the next chapter brings new stars, a quieter disk, or further central activity.
The next article follows that central activity into active galactic nuclei and quasars, where gas falling toward a supermassive black hole can power an extraordinary source of light.
Sources and further reading
Research papers, author reviews, and observatory material supporting the mechanisms and examples. Illustrations are conceptual comparisons, not measured galaxy maps or a prescribed evolutionary sequence.
- Kormendy & Kennicutt (2004) — Secular Evolution and the Formation of Pseudobulges in Disk GalaxiesSecular timescales, internal redistribution, gas inflow, and central growth.
- Sellwood (2014) — Secular Evolution in Disk GalaxiesThe distinct responses of stars and gas, disk heating, and angular-momentum exchange.
- Athanassoula (2013) — Bars and secular evolution in disk galaxies: Theoretical inputBar evolution, exchanges with the dark halo, and different central structures.
- Sellwood & Binney (2002) — Radial Mixing in Galactic DiscsChanges in stellar guiding radii near corotation without strong additional heating.
- Sellwood & Masters (2022) — Spirals in galaxiesSpiral mechanisms and the role of recurring patterns in disk evolution.
- Fisher & Drory (2016) — An Observational Guide to Identifying Pseudobulges and Classical Bulges in Disk GalaxiesUsing several structural, kinematic, and population diagnostics together.
- Erwin et al. (2015) — Composite Bulges: The Coexistence of Classical Bulges and Disky Pseudobulges in S0 and Spiral GalaxiesObserved galaxy centers containing more than one bulge-like component.
- Hopkins, Cox, Younger & Hernquist (2009) — How Do Disks Survive Mergers?How gas fraction, mass ratio, and orbital geometry affect disk survival and rebuilding.
- Rodriguez-Gomez et al. (2017) — The role of mergers and halo spin in shaping galaxy morphologySimulated morphological outcomes that depend on mass, gas, and assembly history.
- Ellison, Catinella & Cortese (2018) — Enhanced atomic gas fractions in recently merged galaxies: quenching is not a result of post-merger gas exhaustionEvidence that recently merged galaxies can retain substantial atomic gas reservoirs.
- Ellison et al. (2024) — Galaxy evolution in the post-merger regime. II — Post-merger quenching peaks within 500 Myr of coalescenceA statistical excess of post-starburst signatures after inferred merger coalescence.
- Ellison et al. (2025) — Galaxy evolution in the post-merger regime. III — The triggering of active galactic nuclei peaks immediately after coalescenceAGN enhancement as a function of estimated merger stage and observational diagnostic.
- Cisternas et al. (2011) — The bulk of the black hole growth since z ~ 1 occurs in a secular universe: No major merger–AGN connectionA selected X-ray AGN sample without a significant excess of strongly disturbed hosts.
- Boselli, Fossati & Sun (2022) — Ram Pressure Stripping in High-Density EnvironmentsEnvironmental gas removal and its distinction from direct changes to stellar orbits.
- Ueda et al. (2014) — Cold Molecular Gas in Merger Remnants: I. Formation of Molecular Gas DisksObserved rotating molecular disks and rings in nearby merger remnants.
- European Southern Observatory (2018) — Mapping a MergerThe rotating, star-forming central gas disk in the merger remnant NGC 7252.
- Frosst et al. (2026) — Origins and lifetimes of secular and tidal bars in simulated disc galaxiesTNG50 bars with internal and tidal origins whose later properties can overlap.
- Helmi et al. (2018) — The merger that led to the formation of the Milky Way’s inner stellar halo and thick diskStellar motions and chemistry identifying debris from the Gaia–Enceladus merger.
- Lotz et al. (2011) — The Major and Minor Galaxy Merger Rates at z < 1.5Why merger counts require visibility timescales and careful sample selection.
- Rodighiero et al. (2011) — The lesser role of starbursts for star formation at z = 2The minority contribution of strong starbursts in a study near the peak of cosmic star formation.
- Quai et al. (2023) — The interconnection between galaxy mergers, AGN activity and rapid quenching of star formation in simulated post-merger galaxiesDifferent quenching outcomes in EAGLE, Illustris, and IllustrisTNG and their sensitivity to model physics.
- McNamara & Nulsen (2012) — Mechanical Feedback from Active Galactic Nuclei in Galaxies, Groups, and ClustersObserved jet heating and the regulation of cooling in hot galactic atmospheres.
All articles in this chapter
- Dark Matter Halos: Galactic Foundations
- Hubble’s Galaxy Classification: Spiral, Elliptical, Irregular
- Collisions and Mergers: Drivers of Galactic Growth
- Galaxy Clusters and Superclusters
- Spiral Arms and Barred Galaxies
- Elliptical Galaxies: Formation and Features
- Irregular Galaxies: Chaos and Starbursts
- Evolutionary Paths: Secular vs. Merger-Driven — you are here
- Active Galactic Nuclei and Quasars
- Galactic Futures: Milkomeda and Beyond