Ankstyvieji mini-halai ir protogalaktikos

Early mini-halos and protogalaxies

How the first galaxies were born in small dark matter "halos"

Long before the majestic spirals or gigantic elliptical galaxies, smaller and simpler structures existed at the dawn of early cosmic time. These primitive formations — mini-halos and protogalaxies — formed in gravitational wells created by dark matter. In this way, they prepared to become the foundation for the further evolution of all galaxies. In this article, we will examine how these early halos contracted, attracted gas, and became the birthplace of the first stars and the seeds of cosmic structure.

1. The Universe after recombination

1.1 Entering the Dark Ages

About 380,000 years after the Big Bang, the Universe cooled enough for free electrons and protons to combine into neutral hydrogen — this stage is called recombination. Photons, no longer scattered by free electrons, became free to travel, creating the cosmic microwave background (CMB) and leaving the young Universe essentially dark. Without formed stars, this epoch is called the Dark Ages.

1.2 Growth of density fluctuations

Despite the overall darkness, the Universe at this time carried small density fluctuations — a legacy of inflation in the form of dark and baryonic matter. Over time, gravity amplified these fluctuations, so denser regions attracted more mass. Eventually, small dark matter clumps became gravitationally bound, forming the first halos. Such structures, with masses around 105–106 M⊙, are often called mini-halos.

2. Dark matter as the main framework

2.1 Why is dark matter important?

In modern cosmology, dark matter outweighs ordinary baryonic matter by a factor of five in mass. It does not emit radiation and interacts mainly through gravity. Since dark matter does not experience radiation pressure like baryonic matter, it began to clump earlier, forming gravitational wells into which gas later fell.

2.2 From small to large (hierarchical growth)

The "bottom-up" structure forms according to the standard ΛCDM model:

  1. Small halos collapse first, later merging into larger structures.
  2. Mergers create increasingly larger and hotter halos capable of hosting a wider range of star formation.

Mini-halos are like the first step towards increasingly larger structures, including dwarf galaxies, larger galaxies, and clusters.

3. Gas cooling and collapse: mini-halo gases

3.1 The necessity of cooling

For gases (mostly hydrogen and helium at such an early phase) to condense and form stars, they must effectively cool down. If the gases are too hot, their pressure counteracts gravitational attraction. In the early Universe, without metals and with only trace amounts of lithium, cooling channels were limited. The main coolant was often molecular hydrogen (H2), which forms under certain conditions in the primitive gas environment.

3.2 Molecular hydrogen: the key to mini-halo collapse

  • Formation mechanisms: Remaining free electrons (after partial ionization) promoted H2 formation.
  • Low-temperature cooling: H2 rotational-vibrational transitions allowed the gas to radiate heat, lowering its temperature to a few hundred kelvins.
  • Fragmentation into dense cores: Cooled gas sank deeper into the gravitational wells of halos, forming dense centers — protostellar cores, where later Population III stars were born.

4. Birth of the first stars (Population III)

4.1 Primary star formation

In the absence of previous star populations, the gas in mini-halos was almost free of heavier elements (known in astronomy as “metallicity”). Under such conditions:

  • Large mass: Due to weaker cooling and less gas fragmentation, the first stars could be extremely massive (from several tens to a few hundred solar masses).
  • Intense UV radiation: Massive stars emitted a strong UV flux capable of ionizing surrounding hydrogen, thus affecting further star formation in that halo.

4.2 Feedback from massive stars

Massive Population III stars typically lived only a few million years before finally exploding as supernovae or even pair-instability supernovae (if their mass exceeded ~140 M⊙). The energy from these events had a dual effect:

  1. Gas disruption: Shock waves heated and sometimes blew gas out of the mini-halo, thereby suppressing additional star formation locally.
  2. Chemical enrichment: Heavier elements (C, O, Fe) ejected by supernovae enriched the environment. Even a small amount of these radically changed the course of later star formation, allowing gas to cool more effectively and form lower-mass stars.

5. Protogalaxies: merging and growth

5.1 Beyond the mini-halo boundaries

Over time, mini-halos merged or attracted additional mass, forming larger structures — protogalaxies. Their mass reached 107–108 M⊙ or more, the virial temperature was higher (~104 K), making atomic hydrogen cooling possible. Therefore, even more intense star formation occurred in protogalaxies:

  • More complex internal dynamics: As the halo mass increased, gas flow, rotation, and feedback became much more complex.
  • Possible early disk structures: In some cases, as the gas rotated, initial flat structures similar to the seeds of modern spirals could have formed.

5.2 Reionization and large-scale impact

Protogalaxies, enhanced by newly forming stars, emitted a significant portion of ionizing radiation, helping to convert neutral intergalactic hydrogen into ionized (reionization). This phase, covering redshifts roughly z ≈ 6–10 (or possibly even higher), is crucial as it shaped the large-scale environment in which later galaxies grew.

6. Observations of mini-halos and protogalaxies

6.1 Challenges of high redshift

These earliest structures formed at very high redshifts (z > 10), corresponding to just a few hundred million years after the Big Bang. Their light is:

  • Faint
  • Highly redshifted into the infrared or even longer wavelengths
  • Short-lived, as they rapidly change due to strong feedback

Therefore, direct observation of mini-halos remains challenging even for the latest generation instruments.

6.2 Indirect traces

  1. Local “fossils”: Especially faint dwarf galaxies in the Local Group may be remnants or have chemical signatures indicating the history of mini-halos.
  2. Metal-poor halo stars: Some Milky Way halo stars have very low metallicity with unique elemental ratios, possibly indicating enrichment by Population III supernovae in the mini-halo environment.
  3. 21 cm line observations: LOFAR, HERA, and the upcoming SKA aim to detect the distribution of neutral hydrogen via the 21 cm line, potentially revealing the small-scale structure network during the Dark Ages and cosmic dawn.

6.3 The role of JWST and future telescopes

James Webb Space Telescope (JWST) is designed to detect faint infrared sources at high redshifts, allowing closer examination of early galaxies, which are often just a step beyond mini-halos. Even if completely isolated mini-halos are difficult to observe, JWST data will reveal how somewhat larger halos and protogalaxies behave, helping to understand the transition from very small to more mature systems.

7. Advanced simulations

7.1 N-body and hydrodynamic methods

To understand the properties of mini-halos in detail, scientists combine N-body simulations (tracking the gravitational collapse of dark matter) with hydrodynamics (gas physics: cooling, star formation, feedback). Such simulations show:

  • The first halos collapse around z ~ 20–30, corresponding to the limits of CMB data.
  • Strong feedback loops begin to operate as soon as one or several massive stars form, affecting the star formation in nearby halos.

7.2 Key challenges

Despite the enormous growth in computational power, mini-halo simulations require extremely high resolution to properly capture molecular hydrogen dynamics, stellar feedback, and possible gas fragmentation. Small differences in modeling resolution or feedback parameters can significantly alter outcomes, such as star formation efficiency or enrichment levels.

8. The cosmic importance of mini-halos and protogalaxies

  1. Foundation of galaxy growth
    • These early “pioneers” initiated the first chemical enrichment and created conditions for more efficient star formation in later, more massive halos.
  2. Early sources of light
    • Massive Population III stars in mini-halos contributed to the flux of ionizing photons that helped reionize the Universe.
  3. Seeds of complexity
    • The interplay between the gravitational well of dark matter, gas cooling, and stellar feedback reflects a process later repeated on larger scales, forming galaxy clusters and superclusters.

9. Conclusion

Mini-halos and protogalaxies mark the first steps toward the majestic galaxies we observe in the modern cosmos. Formed shortly after recombination and sustained by molecular hydrogen cooling, these small halos grew the first stars (Population III), whose supernovae contributed to early chemical enrichment. Over time, halo mergers created protogalaxies, where more complex star formation occurred and the reionization of the Universe began.

Although these transient structures are difficult to detect directly, by combining high-resolution simulations, chemical abundance studies, and innovative telescopes such as JWST and the upcoming SKA, scientists are increasingly opening a window into this formative epoch of the Universe. Understanding the importance of mini-halos means grasping how the Universe became luminous and how the vast cosmic web in which we live was formed.

Links and further reading

  1. Bromm, V., & Yoshida, N. (2011). “The First Galaxies.” Annual Review of Astronomy and Astrophysics, 49, 373–407.
  2. Abel, T., Bryan, G. L., & Norman, M. L. (2002). “The Formation of the First Star in the Universe.” Science, 295, 93–98.
  3. Greif, T. H. (2015). “The Formation of the First Stars and Galaxies.” Computational Astrophysics and Cosmology, 2, 3.
  4. Yoshida, N., Omukai, K., Hernquist, L., & Abel, T. (2006). “Formation of Primordial Stars in a ΛCDM Universe.” The Astrophysical Journal, 652, 6–25.
  5. Chiaki, G., et al. (2019). “Formation of Extremely Metal-poor Stars Triggered by Supernova Shocks in Metal-free Environments.” Monthly Notices of the Royal Astronomical Society, 483, 3938–3955.
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