How early starburst regions and black holes regulated subsequent star formation
During the early cosmic dawn, the first stars and seed black holes were not just passive inhabitants of the Universe. They played an active role, injecting large amounts of energy and radiation into their surroundings. These processes, collectively called feedback, strongly influenced the star formation cycle — either suppressing or promoting further gas collapse in different regions. This article examines how radiation, winds, and outflows from early starburst regions and forming black holes shaped the evolution of galaxies.
1. Initial background: the first light sources
1.1 From the Dark Ages to the Enlightenment
After the Cosmic Dark Ages (epochs after recombination, when there were no bright light sources), Population III stars appeared in mini-halos containing dark matter and primordial gas. These stars were often very massive and extremely hot, intensely emitting ultraviolet light. Around the same time, or shortly after, seeds of supermassive black holes (SMBH) could have begun to form — possibly through direct collapse or from the remnants of massive Population III stars.
1.2 Why is feedback important?
In the expanding Universe, star formation occurs when gas can cool and gravitationally collapse. However, if local energy sources — stars or black holes — disrupt the integrity of gas clouds or raise their temperature, future star formation can be suppressed or delayed. On the other hand, under certain conditions, shock waves and outflows can compress gas regions, promoting new star formation. Understanding these positive and negative feedbacks is crucial for creating a realistic picture of early galaxy formation.
2. Radiation feedback
2.1 Ionizing photons from massive stars
Massive, metal-free Population III stars generated strong Lyman continuum photons capable of ionizing neutral hydrogen. Thus, they created H II regions — ionized bubbles around them:
- Heating and pressure: Ionized gas reaches ~104 K, exhibiting high thermodynamic pressure.
- Photoevaporation: Surrounding neutral gas clouds can be “ablated” as ionizing photons strip electrons from hydrogen atoms, heating and dispersing them.
- Suppression or promotion: On small scales, photoionization can suppress fragmentation by increasing the local Jeans mass, but on larger scales ionization fronts can promote compression of neighboring neutral clouds, thus triggering star formation.
2.2 Lyman–Werner radiation
In the early Universe, Lyman–Werner (LW) photons with energies of 11.2–13.6 eV were important for dissociating molecular hydrogen (H2), which was the main coolant in low-metallicity environments. If an early stellar region or a nascent black hole emitted LW photons:
- H2 destruction: If H2 is broken down, gas finds it difficult to cool.
- Star formation delay: Losing H2 can suppress gas collapse in nearby mini-halos, delaying new star formation.
- "Inter-halo" effect: LW photons can travel long distances, so a single bright source can affect star formation in neighboring halos.
2.3 Reionization and large-scale heating
Around z ≈ 6–10, the combined radiation from early stars and quasars reionized the intergalactic medium (IGM). During this process:
- IGM heating: Singly ionized hydrogen reaches ~104 K, increasing the minimum halo mass threshold required to gravitationally retain gas.
- Galaxy growth suppression: Low-mass halos may no longer retain enough gas to form stars, so star formation shifts to more massive structures.
Thus reionization acts as a large-scale feedback, transforming the Universe from a neutral, cool space into an ionized, hotter medium and changing future star formation conditions.
3. Stellar winds and supernovae
3.1 Massive star winds
Even before stars explode as supernovae, they can emit powerful stellar winds. Massive metal-free (Population III) stars may have had somewhat different wind properties than modern metal-rich stars, but even low metallicity can allow strong winds, especially in very massive or rotating stars. These winds can:
- Expel gas from mini-halos: If the halo's gravitational potential is shallow, winds can blow out a significant portion of the gas.
- Create “bubbles”: Stellar wind “bubbles” carve cavities in the interstellar medium, altering star formation rates.
3.2 Supernova explosions
As massive stars end their lives, core-collapse or pair-instability supernovae release enormous amounts of kinetic energy (~1051 erg for typical core-collapse, possibly more for pair-instability cases). Thus:
- Shock waves: They travel outward, heating and possibly halting further gas collapse.
- Chemical enrichment: Newly synthesized heavier elements are expelled, significantly altering ISM chemistry. Metals improve cooling, thus encouraging lower-mass star formation in the future.
- Galactic outflows: In larger halos or forming galaxies, repeated supernovae can create broader outflows, ejecting material far into intergalactic space.
3.3 Positive vs. negative feedback
While supernova shock waves can disperse gas (negative feedback), they can also compress surrounding clouds, promoting gravitational collapse (positive feedback). The specific outcome depends on local conditions — gas density, halo mass, shock wave geometry, etc.
4. Early black hole feedback
4.1 Accretion luminosity and winds
Besides stellar feedback, accreting black holes (especially evolving into quasars or AGN) cause strong feedback through radiation pressure and winds:
- Radiation pressure: Rapid mass accretion onto a black hole efficiently converts mass into energy, emitting intense X-rays and UV waves. This can ionize or heat the surrounding gases.
- AGN outflows: Quasar winds and jets can “sweep out” gas over several kiloparsecs, controlling star formation in the main galaxy.
4.2 Quasar and proto-AGN seeds
In the first stage, black hole seeds (e.g., remnants of Population III stars or direct collapse black holes) may not have been bright enough to dominate feedback beyond the mini-halo boundary. However, as they grow via accretion or mergers, some can become bright enough to strongly affect the IGM. Early quasar-type sources:
- Promotes reionization: Harder radiation from accreting black holes can ionize helium and hydrogen more strongly at greater distances.
- Suppresses or promotes star formation: Powerful outflows or jets can blow out or compress gas in surrounding star-forming clouds.
5. The broad impact of early feedback
5.1 Regulation of galaxy growth
The combined feedback from stellar populations and black holes defines a galaxy’s “baryon cycle” — i.e., how much gas remains, how long it cools, and when it is blown out:
- Suppression of gas inflow: If outflows or radiative heating prevent gas retention, star formation remains low.
- The path to larger halos: Over time, more massive halos form with deeper gravitational potential, able to retain gas even with feedback.
5.2 Enrichment of the cosmic web
Supernova- and AGN-driven winds can carry metals into the cosmic web, spreading them on filament and void scales. This ensures that later-forming galaxies find somewhat enriched gas.
5.3 Determining the pace and structure of reionization
Observations show that reionization likely occurred patchily, with ionized “bubbles” expanding around early star halos and AGN centers. Feedback — especially from bright sources — significantly influences how quickly and uniformly the IGM becomes ionized.
6. Observational evidence and data
6.1 Metal-poor galaxies and dwarfs
Modern astronomers study local analogs — for example, metal-poor dwarf galaxies — to understand how feedback affects low-mass systems. Intense star "bursts" are often observed to blow out a large portion of the interstellar medium. This resembles a possible scenario in early mini-halos when supernova feedback began.
6.2 Observations of quasars and gamma-ray bursts (GRB)
Gamma-ray bursts arising from massive star collapses at high redshift can help study the content of environmental gases and ionization levels. Meanwhile, quasar absorption lines at different redshifts reveal the metal content and temperature of the IGM, allowing assessment of how much star-driven outflows have affected surrounding spaces.
6.3 Emission line marks
Spectral features (e.g., Lyman–alpha emission, metal lines like [O III], C IV) help reveal the presence of winds or superbubbles in high-redshift galaxies. The James Webb Space Telescope (JWST) is capable of detecting these signatures much more clearly even in faint early galaxies.
7. Simulations: from mini-halos to cosmic scales
7.1 Hydrodynamics + radiation transport
Next-generation cosmological simulations (e.g., FIRE, IllustrisTNG, CROC) combine hydrodynamics, star formation, and radiation transport to model feedback self-consistently. This enables scientists to:
- Determine how ionizing radiation from massive stars and AGN interacts with gas on various scales.
- Capture the emergence of outflows, their propagation, and impact on subsequent gas accretion.
7.2 Sensitivity to model assumptions
Results vary significantly depending on:
- Initial mass function (IMF) of stars: The mass distribution (slope, limits) determines how many massive stars form, how much energy is radiated, or how many supernovae occur.
- AGN feedback recipes: Different methods of accretion energy interaction with gas determine varying outflow intensities.
- Metal mixing: The rate at which metals spread determines the local cooling time, strongly affecting subsequent star formation.
8. Why feedback determines early cosmic evolution
8.1 Directionality of the first galaxy formation
Feedback is not just a side effect; it is a key factor explaining how small halos merge and grow into recognizable galaxies. Outflows from a single massive star cluster or nascent black hole can cause significant local changes in star formation efficiency.
8.2 Controlling the pace of reionization
Since feedback controls the number of stars in small halos (and thus the amount of ionizing photons), it is closely related to the Universe's reionization process. With strong feedback, low-mass galaxies may form fewer stars, slowing reionization; if feedback is weaker, many small systems can contribute to faster reionization.
8.3 Determining conditions for planetary and biological evolution
On even larger cosmic scales, feedback determines the distribution of metals, and metals are essential for planet formation and possibly life. Thus, early feedback episodes shaped the Universe not only energetically but also chemically, creating conditions for the development of increasingly complex astrophysical structures.
9. Future prospects
9. Next-generation observatories
- JWST: By studying the epoch of reionization, JWST's infrared instruments will reveal dust-obscured regions, show winds driven by stellar bursts, and AGN feedback in the first billion years.
- Extremely Large Telescopes (ELT): High-resolution spectroscopy will allow even more detailed analysis of wind and outflow signatures (metal lines) at high redshift.
- SKA (Square Kilometre Array): 21 cm tomography may capture how ionized regions expanded under the influence of stellar and AGN feedback.
9.2 Improved Simulations and Theory
Higher-resolution simulations with improved physics (e.g., better treatment of dust, turbulence, magnetic fields) will allow a deeper look into the complexity of feedback. The synergy of theory and observations promises to answer pressing questions — for example, what scale of winds could black holes have driven in early dwarf galaxies, or how short-lived stellar "bursts" altered the cosmic web.
10. Conclusion
Early feedback — through radiation, winds, and supernova/AGN outflows — acted like cosmic "gatekeepers," setting the pace of star formation and the development of large-scale structures. Photoionization, suppressing the collapse of neighboring halos, and powerful outflows, blowing out or compressing gas, created a complex mosaic of positive and negative feedback loops. While these phenomena are important on local scales, they also echoed through the evolving cosmic web, affecting reionization, chemical enrichment, and the hierarchical growth of galaxies.
Building on theoretical models, high-resolution simulations, and advanced telescope discoveries, astronomers are increasingly probing how these early feedback processes led the Universe into the epoch of bright galaxies, creating conditions for even more complex astrophysical structures, including the chemistry needed for planets and possibly life.
Links and further reading
- Ciardi, B., & Ferrara, A. (2005). “The First Cosmic Structures and Their Effects.” Space Science Reviews, 116, 625–705.
- Bromm, V., & Yoshida, N. (2011). “The First Galaxies.” Annual Review of Astronomy and Astrophysics, 49, 373–407.
- Muratov, A. L., et al. (2015). “Gusty, gaseous flows in the FIRE simulations: galactic winds driven by stellar feedback.” Monthly Notices of the Royal Astronomical Society, 454, 2691–2713.
- Dayal, P., & Ferrara, A. (2018). “Early galaxy formation and its large-scale effects.” Physics Reports, 780–782, 1–64.
- Hopkins, P. F., et al. (2018). “FIRE-2 Simulations: Physics, Numerics, and Methods.” Monthly Notices of the Royal Astronomical Society, 480, 800–863.