Tamsiosios materijos halai: galaktikų pamatas

Dark matter halos: the foundation of galaxies

How galaxies form within vast dark matter structures that determine their shapes and rotation curves

Modern astrophysics has revealed that the impressive spirals and glowing star clusters we see in galaxies are just the tip of the iceberg. Around every galaxy exists a huge, invisible dark matter concentration — about five times more massive than ordinary baryonic matter. These dark matter halos not only provide the gravitational "stage" for stars, gas, and dust but also govern galaxy rotation curves, large-scale structure, and long-term evolution.

In this article, we will discuss what dark matter halos are and their essential role in galaxy formation. We will examine how small density waves in the early stages of the Universe evolved into massive halos, how they attract gas for star formation, and what observational evidence — such as galaxy rotation speeds — proves the gravitational dominance of these unseen structures.

1. The invisible "spine" part of galaxies

1.1 What is a dark matter halo?

Dark matter halo is an approximately spherical or triaxial region composed of invisible (non-luminous) matter surrounding the visible components of a galaxy. Although dark matter interacts gravitationally, it interacts very weakly (or not at all) with electromagnetic radiation—hence it is not seen directly. However, its gravitational influence is proven by:

  • Galaxy rotation curves: Stars in the outer parts of spiral galaxies move faster than can be explained by the mass of visible matter alone.
  • Gravitational lensing: Galaxy clusters or individual galaxies can bend the light from background sources more than the visible mass alone would allow.
  • Cosmic structure formation: Simulations including dark matter realistically reproduce the large-scale "cosmic web" distribution of galaxies, matching observational data.

Halos can extend well beyond a galaxy's luminous edge—sometimes tens to hundreds of kiloparsecs from the center—and contain from ~1010 up to ~1013 Solar masses (depending on dwarf or giant galaxies). This mass strongly influences galaxy evolution over billions of years.

1.2 The mystery of dark matter

The exact nature of dark matter remains unclear. Leading candidates are WIMPs (weakly interacting massive particles) or other exotic models like axions. Whatever it is, dark matter neither absorbs nor emits light but clusters gravitationally. Observations show it is "cold" (slow-moving in the early Universe), thus allowing smaller density structures to "collapse" first (hierarchical formation). These first "mini-halos" merge and grow, eventually hosting luminous galaxies.

2. How halos form and evolve

2.1 Initial seeds

Shortly after the Big Bang, slight density inhomogeneities—possibly arising from enhanced quantum fluctuations during inflation—became the seeds of structures. As the Universe expanded, dark matter in denser regions began to collapse earlier and more efficiently than ordinary matter (which remained coupled to radiation for some time). Over time:

  1. Small halos appeared first, roughly the size of mini-halos.
  2. Mergers between halos gradually formed larger structures (galaxy-mass, group, or cluster halos).
  3. Hierarchical growth: This bottom-up model (ΛCDM) explains how galaxies can have substructures and satellite galaxies, observable even today.

2.2 Virialization and halo profile

As halos form, matter collapses and "virializes," reaching dynamic equilibrium where gravity is balanced by the velocities (velocity dispersion) of dark matter particles. A commonly used theoretical density distribution is the NFW (Navarro-Frenk-White) profile:

ρ(r) &propto 1 / [ (r / rs) (1 + r / rs)2 ],

where rs – scale radius. The density at the halo center can be very high, then decreases more steeply but extends to large distances. Real halos may show deviations (e.g., cored centers or substructures).

2.3 Subhalos and satellites

Large halos contain subhalos – smaller dark matter clumps formed earlier and not fully “merged” with the central part. Satellite galaxies can develop within them (like the Magellanic Clouds around the Milky Way). To connect ΛCDM predictions with observations (e.g., the number of dwarf satellites), it is important to study the role of subhalos. “Too big to fail” or “missing satellites” are tension examples appearing if simulations predict more or more massive subhalos than observed. New high-resolution data and improved feedback models help resolve these discrepancies.

3. Dark matter halos and galaxy formation

3.1 Baryonic accretion and the importance of cooling

When the dark matter halo collapses, surrounding baryonic matter (gas) from the intergalactic medium can fall into the gravitational potential, but only if it can radiate away energy and angular momentum. Key processes:

  • Radiative cooling: Hot gas loses energy (mostly through atomic radiation processes or, at higher temperatures, free charge radiation).
  • Shock heating and cooling flows: In massive halos, infalling gas is heated to the halo's characteristic virial temperature; if it cools, it settles into a rotating disk and fuels star formation.
  • Feedback: Stellar winds, supernovae, and active galactic nuclei (AGN) can blow out or heat gas, regulating whether baryons successfully accumulate in the disk.

Thus, the dark matter halo is the “frame” into which visible matter collapses, forming the observable galaxy. The halo's mass and structure determine whether the galaxy remains dwarf-sized, becomes a giant disk, or undergoes mergers transforming it into an elliptical system.

3.2 Determining galaxy shape

The halo sets the overall gravitational potential and influences the galaxy:

  1. Rotation curve: In the outer regions of spiral galaxies, star and gas velocities remain high even though luminous matter is sparse. This “flat” or gently declining curve indicates a massive dark matter halo extending beyond the optical disk boundaries.
  2. Disk vs. spheroidal shape: The halo's mass and angular momentum partly determine whether infalling gas forms a wide disk (if angular momentum is conserved) or undergoes major mergers (which can create elliptical structures).
  3. Stability: Dark matter can stabilize or, conversely, limit the formation of certain bars or spiral wave patterns. Meanwhile, bars transfer baryonic matter to the center, altering star formation.

3.3 Connection with galaxy mass

The ratio of stellar mass to halo mass can vary greatly: in dwarf galaxies, the halo can be enormous compared to the modest stellar content, while in large ellipticals a greater fraction of gas turns into stars. However, usually even massive galaxies do not use more than ~20–30% of baryonic matter because feedback and cosmic reionization limit efficiency. This interplay of halo mass, star formation efficiency, and feedback is fundamental in galaxy evolution models.

4. Rotation curves: the clearest sign

4.1 Discovery of the dark halo

One of the first proofs of dark matter's existence came from rotation velocity measurements in spiral galaxies. According to Newtonian dynamics, if most mass were only visible matter, the orbital velocity of stars v(r) should fall off as 1/&sqrt;r far beyond the stellar disk. Vera Rubin et al. found that the velocity remains nearly constant or declines only slightly:

vobserved(r) ≈ const at large r,

which means that the mass M(r) increases with radius. This revealed a huge, invisible matter halo.

4.2 Curve modeling

Astrophysicists model rotation curves by summing the gravitational contribution from:

  • Stellar disk
  • Bulge (nucleus)
  • Gas
  • Dark matter halo

Usually, to reproduce observations, it is necessary to assume an extended dark matter halo far exceeding the stellar mass. Galaxy formation models use such adjustments to calibrate halo properties — central densities, scale radii, total mass.

4.3 Dwarf galaxies

Even in faint dwarf galaxies, velocity dispersion observations show dark matter dominance. Some such dwarfs may have up to 99% of their mass invisible. These are particularly extreme examples that help deepen understanding of how small halos form and how feedback operates on these smallest scales.

5. Other observational evidence besides rotation curves

5.1 Gravitational lensing

General relativity theory states that mass distorts spacetime, bending light rays passing nearby. Galaxy-scale lensing can magnify and distort the images of background sources, while cluster-scale lensing can create arcs or multiple images. From these distortions, scientists determine the mass distribution — usually finding that most of the mass is dark matter. Such lensing data excellently complement rotation curve and velocity dispersion estimates.

5.2 X-ray emission from hot gas

In larger structures (galaxy groups and clusters), the gas temperature in halos can reach tens of millions of K, so they emit in the X-ray range. By analyzing the temperature and distribution of this gas (Chandra, XMM-Newton telescopes), we can determine the deep gravitational "well" of dark matter that holds this gas.

5.3 Satellite dynamics and stellar streams

In our Milky Way, measurements of satellite galaxy orbits (e.g., Magellanic Clouds) or tidal stellar streams (from disrupted dwarfs) also provide additional constraints on the total halo mass. Tangential velocities, radial velocities, and orbital history shape the halo's radial profile picture.

6. Halos over time

6.1 Galaxy formation at high redshift

Earlier (around z ∼ 2–6) galactic halos were smaller, but mergers occurred more frequently. Observations, e.g., from the James Webb Space Telescope (JWST) or ground-based spectrographs, show that young halos rapidly accreted gas, fueling star formation much more intense than today. The cosmic star formation rate density peaked around z ∼ 2–3, partly because many halos simultaneously reached sufficient masses for strong baryonic inflows.

6.2 Evolution of halo properties

As the universe expands, halo virial radii grow, and mergers and collisions create ever larger structures. Meanwhile, star formation may decrease if feedback or environmental effects (e.g., clusters) remove or heat gas. Over billions of years, the halo remains the main "frame" of the galaxy's structure, but the baryonic part can evolve from an active, star-filled disk into a gas-poor, "red and dead" elliptical system.

6.3 Galaxy clusters and superclusters

On the largest scale, halos merge into cluster halos, hosting several galactic halos within one gravitational well. Even larger structures are superclusters (not always fully virialized). This is the peak of hierarchical growth of dark matter, highlighting the densest nodes of the cosmic web.

7. Beyond the ΛCDM halo model

7.1 Alternative theories

Some other gravity theories, e.g. MOND or other modifications, suggest that dark matter can be replaced or supplemented by modified gravity laws in low acceleration regimes. However, the great success of ΛCDM (explaining CMB anisotropies, large-scale structure formation, lensing, halo sub-structures) still strongly supports the idea of dark matter halos. Nevertheless, small discrepancies (cuspy center vs. cored profile, missing satellites) encourage exploring "warm" dark matter or self-interacting dark matter.

7.2 Self-interacting or warm dark matter

  • Self-interacting DM: If dark matter particles interact with each other even slightly, halo centers could be less sharp (cuspy), potentially resolving some observational discrepancies.
  • Warm DM: Particles with significant velocity in the early Universe could smooth out small-scale structure formation, reducing the number of subhalos.

Such models can alter the internal structure of halos or the number of satellites but maintain the overall idea that massive halos act as the skeleton for galaxy formation.

8. Conclusions and future directions

Dark matter halos – invisible but essential frameworks that determine how galaxies form, rotate, and interact. From dwarf galaxies orbiting massive halos with almost no stars to huge cluster halos holding thousands of galaxies, these unseen structures dictate how matter is distributed in the Universe. Studies of rotation curves, lensing, satellite motions, and large-scale structures show that dark matter is not a side detail but a fundamental gravitational factor in the Universe's makeup.

Cosmologists and astronomers continue refining halo models using new data:

  1. High-resolution simulations: Projects like Illustris, FIRE, EAGLE, and others model star formation, feedback, and halo growth in detail, aiming to consistently connect all processes.
  2. Deeper observations: Telescopes like JWST or the Vera C. Rubin Observatory will detect faint dwarf satellites, assess halo shapes through lensing, and observe early halo collapse stages at high redshift.
  3. Partial particle physics searches: Both direct detection experiments and particle accelerators or astrophysical tests aim to determine what dark matter really is – to confirm or refute ΛCDM halo ideas.

Ultimately, dark matter halos are the fundamental building blocks of cosmic structure formation, linking the early seeds of microwave background anisotropies with the impressive galaxies we see in the present Universe. By studying the nature and dynamics of these halos, we approach fundamental questions about how gravity works, the distribution of matter, and the grand architecture of the cosmos.

Sources and literature

  • Navarro, J. F., Frenk, C. S., & White, S. D. M. (1996). “The Structure of Cold Dark Matter Halos.” The Astrophysical Journal, 462, 563–575.
    A classic article presenting the Navarro–Frenk–White (NFW) density profile and its significance for dark matter halos.
  • Navarro, J. F., Frenk, C. S., & White, S. D. M. (1997). “A Universal Density Profile from Hierarchical Clustering.” The Astrophysical Journal, 490, 493–508.
    A follow-up work refining the universal halo profile and demonstrating its application to various mass scales.
  • Rubin, V. C., & Ford, W. K. (1970). “Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions.” The Astrophysical Journal, 159, 379–403.
    One of the early seminal works measuring galaxy rotation curves and confirming the need for dark matter in the outer regions of galaxies.
  • Moore, B., Quinn, T., Governato, F., Stadel, J., & Lake, G. (1999). “Cold collapse and the core catastrophe.” Monthly Notices of the Royal Astronomical Society, 310, 1147–1152.
    Examines the “cusp-core” problem using high-resolution simulations, promoting alternative dark matter or feedback scenarios.
  • White, S. D. M., & Rees, M. J. (1978). “Core condensation in heavy halos – A two-stage theory for galaxy formation and the missing satellite problem.” Monthly Notices of the Royal Astronomical Society, 183, 341–358.
    A foundational paper outlining the theory of how baryons condense into dark matter potentials and discussing the hierarchical nature of galaxy formation.
  • Planck Collaboration. (2018). “Planck 2018 results. VI. Cosmological parameters.” Astronomy & Astrophysics, 641, A6.
    Provides precise cosmological parameters (e.g., matter density, Ωm), which affect the formation and growth rate of dark matter halos.
  • Vogelsberger, M., Genel, S., Springel, V., et al. (2014). “Introducing the Illustris Project: Simulating the coevolution of dark and visible matter in the Universe.” Monthly Notices of the Royal Astronomical Society, 444, 1518–1547.
    Presents a large-scale, high-resolution simulation describing the interplay of dark matter halos and baryonic processes in galaxy evolution.
  • Bullock, J. S., & Boylan-Kolchin, M. (2017). “Small-Scale Challenges to the ΛCDM Paradigm.” Annual Review of Astronomy and Astrophysics, 55, 343–387.
    Reviews discrepancies (e.g., missing satellites, “too big to fail”) between observations and ΛCDM model predictions, emphasizing halo substructure.
  • Bertone, G., & Hooper, D. (2018). “History of dark matter.” Reviews of Modern Physics, 90, 045002.
    Provides a detailed discussion of the concept and observational history of dark matter, including the role of halos in galaxies.

These works broadly cover theory and observations related to dark matter halos – from their fundamental role in galaxy formation theory to direct and indirect evidence (rotation curves, lensing, cosmic structure) of the invisible but significant influence on the evolution of the Universe.

Return to the blog