How internal processes and external interactions shape a galaxy’s long-term evolution
Galaxies do not remain static over billions of years; instead, they evolve through a mix of internal (secular) processes and external (merger-driven) interactions. A galaxy’s morphology, star formation rate, and central black hole growth can be profoundly affected by either slow, steady changes within its disk or by rapid, sometimes cataclysmic encounters with neighbors. In this article, we will delve into how galaxies follow different “evolutionary paths”—secular and merger-driven— and how each route impacts their ultimate structure and stellar populations.
1. The Two Contrasting Modes of Evolution
1.1 Secular Evolution
Secular evolution refers to gradual, internal processes that redistribute a galaxy’s gas, stars, and angular momentum. These processes typically operate on timescales of hundreds of millions to billions of years, without relying on major external triggers:
- Bar Formation and Dissolution: Bars can drive gas inward, fuel central starbursts, and reshape bulges over long timescales.
- Spiral Density Waves: Slowly move through the disk, triggering star formation along spiral arms, steadily building stellar populations.
- Stellar Migration: Stars can drift radially through the disk due to resonances, changing local metallicity gradients and stellar population mixes [1].
1.2 Merger-Driven Evolution
Merger-driven processes occur when two or more galaxies collide or interact strongly, driving much faster, more dramatic changes:
- Major Mergers: Spirals of comparable mass can coalesce into a single elliptical, destroying disk structure and triggering starbursts.
- Minor Mergers: A smaller satellite merges with a larger host, potentially thickening the disk, building bulges, or fueling moderate star formation.
- Tidal Interactions: Even if a full merger does not occur, close gravitational encounters can distort disks, form bars or rings, and momentarily spike star formation rates [2].
2. Secular Evolution: Slow Internal Reshaping
2.1 Bar-Driven Gas Inflows
A central bar in a spiral galaxy can redistribute angular momentum and funnel gas from the outer disk toward the central kiloparsecs:
- Gas Piling Up: This inflow can accumulate in ring structures or directly in the bulge region, spurring star formation and potentially bulge growth.
- Bar Lifecycles: Bars may strengthen or weaken over cosmic time, affecting how gas cycles through the disk and fueling central supermassive black holes [3].
2.2 Pseudobulges vs. Classical Bulges
Secular evolution often leads to the formation of pseudobulges— bulges that retain disk-like characteristics (flattened shapes, younger stars) instead of the random orbital structure typical of classical bulges formed via mergers. Observations show:
- Pseudobulges typically have ongoing star formation, nuclear rings, or bars, suggesting slow internal assembly.
- Classical Bulges form rapidly in violent events (e.g., major mergers), with predominantly older stellar populations [4].
2.3 Spiral Waves and Disk Heating
Density wave theory proposes that spiral arms can persist as wave patterns, triggering continuous star formation in the disk. Additional processes like spiral arm migration or swing amplification can help maintain or amplify these patterns, slowly evolving the disk’s structure. Over time, stellar orbits might “heat” (increase velocity dispersion), thickening the disk slightly but not fully destroying it.
3. Merger-Driven Evolution: External Interactions and Transformations
3.1 Major Mergers: From Spirals to Ellipticals
One of the most transformative events in galaxy evolution is a major merger between two galaxies of similar mass:
- Violent Relaxation: Stellar orbits randomize due to the rapidly changing gravitational potential, often erasing disk structures.
- Starbursts: Gas flows to the center, fueling intense star formation.
- AGN Ignition: Central black holes can accrete large amounts of gas, turning the remnant temporarily into a quasar or active nucleus.
- Elliptical Remnant: The final product typically is a spheroidal system with an older stellar population and minimal cold gas [5].
3.2 Minor Mergers and Satellite Accretion
When the mass ratio is more uneven, the smaller galaxy is often tidally stripped or disrupted before fully merging with the larger host:
- Thickening Disk: Repeated minor mergers can deposit stars in the host’s halo or thicken its disk, possibly creating lenticular (S0) systems if gas is stripped.
- Incremental Growth: Over cosmic time, many small mergers can significantly contribute to the mass of bulges or halos, even if no single merger is catastrophic.
3.3 Tidal Interactions and Starbursts
Even without full coalescence, close passages can:
- Distort disks into peculiar shapes, forming tidal tails or bridges.
- Enhance star formation via gas compression in collisional “overlap” regions.
- Spawn ring galaxies or strongly barred galaxies if the geometry is just right (e.g., a perpendicular pass through the disk’s center).
4. Observational Evidence of Both Modes
4.1 Barred Spirals and Secular Bulges
Telescopes detect bars in over half of local spiral galaxies, many hosting ring-like structures and nuclear star-forming “pseudobulges.” Integral field spectroscopy reveals the slow inflow of gas along bar dust lanes and the presence of younger populations in the bulge region—hallmarks of secular processes [6].
4.2 Merging Systems: From Starburst to Elliptical
Examples like The Antennae (NGC 4038/4039) illustrate an ongoing major merger, with tidal tails, widespread starbursts, and luminous clusters. Other nearby examples, such as Arp 220, reveal dust-enshrouded star formation with possible AGN fueling. Meanwhile, NGC 7252 shows a post-merger “Atoms for Peace” galaxy on track to become a more relaxed elliptical [7].
4.3 Galaxy Surveys and Kinematic Signatures
Large surveys (e.g., SDSS, GAMA) find many galaxies exhibiting morphological or spectral signs of mergers (disturbed outer isophotes, double nuclei, tidal streams) or purely secular states (strong bars, stable disks). Kinematic studies (with MANGA, SAMI) highlight differences between rotation-dominated disks with bars vs. classical bulge systems formed by earlier merger events.
5. Hybrid Evolutionary Paths
5.1 Gas-Rich Mergers Followed by Secular Evolution
A galaxy can experience a major or minor merger, building a prominent bulge (or elliptical structure). If residual gas remains, or additional gas is accreted later, the system might re-form a disk or sustain ongoing star formation. Over time, secular processes can reshape the bulge, forming a “discy” bulge or reviving bar structures in what was once a merger remnant.
5.2 Secularly Evolving Disks that Eventually Merge
Spiral galaxies might evolve secularly for billions of years—forming pseudobulges, bars, or rings—until at some point they encounter a comparable mass galaxy. This external trigger can abruptly shift them onto a merger-driven track, culminating in an elliptical or lenticular product.
5.3 Environmental Cycling
A galaxy might drift from a low-density environment, focusing on internal, secular changes, into a cluster or group environment where close encounters or hot intracluster medium stripping become dominant. Conversely, post-merger remnants may fade in isolation, continuing slow internal changes if leftover gas or faint bars exist.
6. Implications for Galaxy Morphology and Star Formation
6.1 Early-Types vs. Late-Types
Mergers tend to quench star formation (especially major mergers that remove or heat much of the gas) and create older stellar populations—leading to elliptical or S0 morphologies (the early-type category). Meanwhile, purely secularly evolving disks can retain gas, fueling star formation over long durations, thus preserving late-type spiral or irregular morphologies [8].
6.2 AGN Activity and Feedback
- Secular Channel: Bars can slowly deliver gas to a central black hole, powering moderate AGN.
- Merger Channel: Rapid inflows during major collisions can spike AGN luminosities to quasar levels, often followed by feedback-driven quenching.
Either path shapes the galaxy’s gas content and future star formation trajectory.
6.3 Bulge Growth and Disk Maintenance
Secular evolution can build pseudobulges or preserve extended star-forming disks, while major mergers create classical bulges or elliptical remnants. Minor mergers straddle the line, potentially thickening disks or fueling moderate bulge growth without fully destroying disk structure.
7. Cosmological Context
7.1 Higher Merger Rates at Early Times
Observations suggest that at redshifts z ∼ 1–3, merger rates were higher—coinciding with a peak in cosmic star formation density. Large, gas-rich mergers likely played a major role in building massive ellipticals early on. Many galaxies that had stable, secularly evolving disks at later epochs probably endured an earlier violent assembly period [9].
7.2 Diversity of Galaxy Populations
Local galaxy populations reflect a blend of these paths: some large ellipticals formed via repeated mergers, some spirals grew steadily and remain gas-rich, while others show evidence of both. Detailed morphological and kinematic surveys reveal how no single channel alone can explain the diversity— both secular and merger-driven processes are crucial.
7.3 Predictions from Simulations
Cosmological simulations (e.g., IllustrisTNG, EAGLE) incorporate both major mergers and secular processes, generating populations of galaxies spanning Hubble types. They show that early massive galaxy assembly often involves mergers, but disk galaxies can form through gentle accretion and secular rearrangements, aligning with observational evidence of morphological transformations across cosmic time [10].
8. Future Prospects
8.1 Next-Generation Observations
Missions like the Nancy Grace Roman Space Telescope and extremely large ground-based telescopes will provide deeper, higher-resolution imaging and spectroscopy at earlier epochs, clarifying how galaxies shift from “merger-driven” to “secular” phases or combine both. Multi-wavelength data (radio, millimeter, infrared) will trace the gas flows fueling either path.
8.2 High-Resolution Numerical Models
Ever-improving computational power allows simulations to resolve smaller scales of galaxy disks, bars, and black hole accretion—capturing the synergy between secular disk instabilities and episodic merger events. These models can test how subtle bar instabilities compare to dramatic collisions in shaping morphological outcomes.
8.3 Connecting Barred Galaxies and Pseudobulges
Large surveys (e.g., with integral field spectroscopy) will systematically measure disk kinematics, bar strength, and bulge properties. Correlating these data with galaxy environment and halo mass might illuminate how frequently bars can mimic or overshadow minor mergers in building bulges, thus refining our evolutionary framework.
9. Conclusion
Galaxies trace two broad, interwoven evolutionary pathways:
- Secular Evolution: Slow, internal processes—bar-driven inflows, spiral density wave star formation, and stellar migration—reshape the disk and build up bulges over billions of years.
- Merger-Driven Evolution: Rapid, externally triggered events (major or minor mergers) can drastically alter morphology, quench star formation, and produce elliptical galaxies or thickened disks.
Real galaxies often experience hybrid paths, with periods of secular reformation punctuated by occasional collisions or minor mergers. This nuanced interplay produces the great morphological diversity we observe, from pure disks with bars and pseudobulges to the grand elliptical remnants of major collisions. By studying both routes—secular processes within stable disks and externally induced transformations via mergers—astronomers piece together the tapestry of galaxy evolution across cosmic time.
References and Further Reading
- Kormendy, J., & Kennicutt, R. C. (2004). “Secular Evolution and the Formation of Pseudobulges in Disk Galaxies.” Annual Review of Astronomy and Astrophysics, 42, 603–683.
- Barnes, J. E., & Hernquist, L. (1992). “Dynamics of Interacting Galaxies.” Annual Review of Astronomy and Astrophysics, 30, 705–742.
- Athanassoula, E. (2012). “Barred Galaxies and Secular Evolution.” IAU Symposium, 277, 141–150.
- Fisher, D. B., & Drory, N. (2008). “Bulges in Nearby Galaxies with Spitzer: Scaling Relations and Pseudobulges.” The Astronomical Journal, 136, 773–839.
- Hopkins, P. F., et al. (2008). “A Unified, Merger-driven Model of the Origin of Starbursts, Quasars, the Cosmic X-Ray Background, Supermassive Black Holes, and Galaxy Spheroids.” The Astrophysical Journal Supplement Series, 175, 356–389.
- Cheung, E., et al. (2013). “Bars in Disk Galaxies out to z = 1 from CANDELS: Do Bars Stall Secular Evolution?” The Astrophysical Journal, 779, 162.
- Hibbard, J. E., & van Gorkom, J. H. (1996). “HI, HII, and Star Formation in the Tidal Tails of NGC 4038/9.” The Astronomical Journal, 111, 655–665.
- Strateva, I., et al. (2001). “Color Separation of Galaxies into Red and Blue Sequences: SDSS.” The Astronomical Journal, 122, 1861–1874.
- Lotz, J. M., et al. (2011). “Major Galaxy Mergers at z < 1.5 in the COSMOS, GOODS-S, and AEGIS Fields.” The Astrophysical Journal, 742, 103.
- Nelson, D., et al. (2018). “First results from the IllustrisTNG simulations: The galaxy color bimodality.” Monthly Notices of the Royal Astronomical Society, 475, 624–647.