Early Mini-Halos and Protogalaxies
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 02 / Article 03
Early minihalos and protogalaxies
Before grand galaxies, small concentrations of dark matter gathered gas. Cooling, starlight, and fresh inflows helped decide which became lasting homes for stars.
A halo could gather gas. Could it keep making stars?
The first stars needed places to form. Those places were small, fragile systems shaped by a much larger concentration of invisible matter.
Within early dark matter halos, hydrogen and helium collected, cooled, and sometimes collapsed into stars. But the stars could heat or expel the very gas that had made them. A bright beginning did not guarantee continuing star formation.
This article follows the passage from minihalos to assembling protogalaxies: how they acquired gas, how they responded to their first stars, and how larger systems began to emerge.[1], [4]
What exactly is a minihalo?
A halo is a gravitationally bound concentration of dark matter. It can contain ordinary matter, including gas and stars, but its name does not imply that it shines.
Dark matter halo
A structure dominated by dark matter, whose gravity influences how gas collects and moves. Halos occur across a wide range of masses.
Minihalo
In first-galaxy research, usually a halo below the temperature threshold for efficient atomic hydrogen cooling. Molecular cooling can allow some of these halos to form stars.
Protogalaxy
A young galactic system still assembling its gas, stars, and structure. It describes a stage of development, without one universally agreed mass boundary.
Star-forming minihalos in many early-universe models have total masses around 100,000 to a million solar masses. Most of that mass is dark matter; only a small part may become stars. These values illustrate a common regime, rather than define every minihalo.[1], [3], [12]
How did the first hosts assemble?
The process began with small differences in the universe’s density. Gravity amplified these differences: regions containing slightly more matter attracted additional material. Dark matter concentrations were already developing before the first stars appeared.
After recombination, the gas was mostly neutral. During the Dark Ages, it increasingly collected in growing dark matter wells. These wells assembled through both the arrival of surrounding matter and mergers with other halos.[5], [7]
Gas did not exactly copy the dark matter distribution. Pressure resisted compression, and gas moving relative to the dark matter could be harder to capture. Once stars appeared nearby, their radiation could also change the conditions for cooling.
Consequently, two halos of similar mass could have different histories. One might develop a cold, dense core; another might remain without stars until it grew larger or its surroundings changed.[3]
For the growth of the underlying density variations, see Gravitational Clumping and Density Fluctuations.
Why was molecular hydrogen so important?
Falling gas is compressed and heated. That heat raises its pressure, which opposes further contraction. To reach the densities needed for star formation, the gas must find ways to radiate energy away.
Pristine gas contained almost no elements made by earlier stars and initially lacked their dust. In minihalos, small amounts of molecular hydrogen, H2, provided an important cooling route. Collisions could transfer thermal energy into the molecules, which then emitted photons that escaped from the cloud.[2], [12]
Under typical primordial minihalo conditions, this cooling can bring gas down to a few hundred kelvins. A dense central region can then continue toward collapse and the formation of protostars. The final outcome also depends on accretion, fragmentation, and radiation from the growing stars.[6], [12]
How could hydrogen molecules form without dust?
A small population of free electrons survived recombination. One important pathway used those electrons as helpers:
H + e− → H− + photon
H− + H → H2 + e−
An electron first attaches to a hydrogen atom, creating the negative hydrogen ion H−. That ion can react with another hydrogen atom to form H2, releasing the electron again. The electron acts as a catalyst; dust is not required. Other chemical pathways also contribute under appropriate conditions.[2]
What happened when the first stars switched on?
The earliest stars formed from essentially pristine gas and are called Population III stars. Their effect on a small host could be disproportionate to the fraction of its mass locked into stars.
Ionizing radiation
Energetic photons remove electrons from atoms and heat the surrounding gas. It can expand or escape from a shallow well, reducing the supply available for another generation of stars.[4], [6]
Disrupting H2
Ultraviolet light in the Lyman–Werner bands can break apart molecular hydrogen. This can weaken cooling in nearby clouds, although shielding and local conditions affect the result.[3]
Gas loss can interrupt the story without ending it
Expelling gas does not automatically destroy the dark matter halo. Over time, material can cool, return, or arrive from outside, while the halo itself continues growing. A deeper potential well may then hold gas more effectively.
There is no universal recovery time. Simulations find that it depends on the host’s mass and environment, the progenitor star, and the strength of its radiation and explosion.[4]
These responses are examples of stellar feedback: stars alter the conditions that control future star formation.
What changes in an atomic-cooling halo?
As a halo grows, its gravitational well deepens. A useful measure is its virial temperature: a characteristic temperature associated with the depth of that well. It is not the actual temperature of every parcel of gas.
At roughly 10,000 K, atomic hydrogen cooling becomes effective. A halo reaching this regime can cool gas through atomic processes even when molecular cooling is limited. The corresponding mass depends on cosmic time; illustrative early-universe values are around ten million to a hundred million solar masses.[1], [5]
| Property | Star-forming minihalo | Atomic-cooling halo |
|---|---|---|
| Illustrative total mass | 105–106 solar masses | 107–108 solar masses |
| Characteristic virial temperature | Below the atomic-cooling threshold | Around 10,000 K or above |
| Important cooling route | Molecular hydrogen in pristine gas | Atomic hydrogen can contribute; molecules and later enrichment can enable further cooling |
| Response to feedback | A shallow well can lose much of its gas | A deeper well generally improves retention, although gas loss can still occur |
The mass ranges are examples, not fixed dividing lines. Redshift, composition, gas motion, and radiation affect the conditions for collapse.[1], [3], [5]
Atomic cooling broadens the conditions in which gas can contract. Whether the host develops into a continuing star-forming system still depends on its gas supply, chemical history, and feedback.
How does a protogalaxy take shape?
A young galaxy grows through several processes at once. Gas can flow inward from its surroundings while smaller halos merge into the host. The arriving material brings different motions, temperatures, and chemical histories.[5]
Fresh inflow and mergers
Accretion adds new material, while mergers combine existing structures. Both can stir the gas and create shocks. Their relative importance varies from one system to another.[5]
Episodes of star formation
Gas can accumulate, form stars, and then be heated or dispersed. Recovery permits another episode. Early growth can therefore be bursty rather than steady.[4], [7]
Enrichment and mixing
Explosions carry heavier elements into surrounding gas, but mixing takes time. Enriched material and less-enriched pockets can coexist, giving later clouds different cooling possibilities.[11]
As young galaxies accumulated stars, the ionizing photons that escaped them contributed to the wider reionization of intergalactic hydrogen. How much radiation escaped depended on the structure and changing gas content of the hosts.[1], [6]
The larger story continues in Merging and Hierarchical Growth, where early systems become parts of more massive galaxies.
How can astronomers study such small, ancient systems?
JWST sees the luminous contents
At great distances, expanding space shifts early galaxies’ light into the infrared. JWST can study their stars and glowing gas, sometimes with extra magnification supplied by gravitational lensing.
This does not directly reveal an entire dark matter minihalo. A halo without stars has no stellar light to detect, and estimating a luminous system’s host mass requires models. Even resolved star clusters should not be mistaken for resolved dark matter halos.[1], [8]
Ancient stars in faint galaxies
Some very faint dwarf galaxies contain ancient, metal-poor stellar populations. Their ages and elemental patterns help reconstruct early star formation and enrichment.
They are historical records that need interpretation: later star formation, gas loss, and gravitational interactions may have changed the systems we observe today.[9]
The 21-centimeter signal
Neutral hydrogen’s redshifted radio signal can constrain how gas was heated and ionized as early sources developed. Published HERA analyses, for example, have used upper limits to test models of this history.
This approach probes large-scale gas conditions statistically. It does not provide a catalog of individually resolved minihalos.[10]
What are simulations still trying to resolve?
Simulating a first galaxy requires following dark matter gravity alongside gas flow, chemistry, cooling, radiation, and stellar feedback. The calculation must connect a halo’s surroundings to much smaller star-forming regions.
That enormous range of scales creates practical limits. Some processes can be followed directly; others need simplified prescriptions. Researchers test how outcomes change with numerical resolution, physical assumptions, and the choice of environment.[7]
A successful model must explain more than whether a single cloud makes a star. It must account for the variety of young systems, their gas supply, and the chemical evidence preserved in later populations.
Galaxies began with a changing relationship between gas and gravity.
Dark matter gathered the material. Cooling allowed dense clouds to form. The first stars changed those clouds, while fresh inflow and mergers gave growing systems further opportunities to build stellar populations.
Some of these early environments also hosted black holes. The next article examines supermassive black hole seeds and the proposed starting points of the enormous black holes found in young galaxies.
Sources and further reading
- Bromm & Yoshida (2011) — The First GalaxiesResearch review of early halos, the first galactic systems, and the meaning of a first galaxy.
- Galli & Palla (1998) — The Chemistry of the Early UniversePrimordial chemistry, including the reactions that form molecular hydrogen without dust.
- Kulkarni, Visbal & Bryan (2021) — The critical dark matter halo mass for Population III star formationHow radiation, gas motion, and cosmic time affect the threshold for star formation.
- Jeon et al. (2014) — Recovery from Population III supernova explosions and the onset of second generation star formationSimulations showing why gas recovery after an explosion depends on the star and its host.
- Greif et al. (2008) — The First Galaxies: Assembly, Cooling and the Onset of TurbulenceGas accretion, mergers, shocks, and turbulence during early galactic assembly.
- Klessen & Glover (2023) — The first stars: formation, properties, and impactThe cooling of pristine gas, the first stellar populations, and their feedback.
- Greif (2015) — The numerical frontier of the high-redshift UniverseThe physical processes and numerical challenges involved in simulating early structure.
- ESA/Webb (2024) — Webb captures star clusters in Cosmic Gems arcA reported example of gravitational lensing and JWST revealing compact young star clusters.
- Simon (2019) — The Faintest Dwarf GalaxiesAncient stellar populations, dark matter, and the archaeological value of faint dwarf galaxies.
- HERA Collaboration (2022) — HERA Phase I Limits on the Cosmic 21-cm Signal: Constraints on Astrophysics and Cosmology During the Epoch of ReionizationAn early published example of using radio limits to constrain heating and ionization models.
- Greif et al. (2010) — The First Galaxies: Chemical Enrichment, Mixing, and Star FormationA simulation study of the transport and mixing of elements made by the first stars.
- Abel, Bryan & Norman (2002) — The Formation of the First Star in the UniverseA foundational calculation of molecular cooling and gas collapse in an early minihalo.
All articles in this chapter
- Gravitational Clumping and Density Fluctuations
- Population III Stars: The Universe’s First Generation
- Early Mini-Halos and Protogalaxies — you are here
- Supermassive Black Hole Seeds
- Primordial Supernovae: Element Synthesis
- Feedback Effects: Radiation and Winds
- Merging and Hierarchical Growth
- Galaxy Clusters and the Cosmic Web
- Active Galactic Nuclei in the Young Universe
- Observing the First Billion Years