2.4 Feedback processes

Feedback processes play a crucial role in shaping galaxies and the universe. From star formation to galaxy evolution, these mechanisms can either amplify or suppress various cosmic phenomena, influencing the structure and composition of celestial bodies.

Positive feedback enhances processes, leading to runaway growth, while negative feedback promotes stability. Understanding these dynamics is essential for grasping how stars form, galaxies evolve, and large-scale structures develop in the cosmos.

Positive vs negative feedback

• Positive feedback amplifies or enhances a process, leading to runaway growth or instability in a system
• Negative feedback dampens or suppresses a process, promoting stability and self-regulation in a system
• In the context of galaxies and the universe, feedback processes can have both positive and negative effects on star formation, galaxy evolution, and structure formation

Feedback in star formation

Molecular cloud collapse

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  ESA Science & Technology: Herschel's view of the Taurus molecular cloud - annotated View original

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  ESA Science & Technology: Herschel's view of the Taurus molecular cloud - annotated View original

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  ESA Science & Technology: Herschel's view of the Taurus molecular cloud - annotated View original

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  ESA Science & Technology: Herschel's view of the Taurus molecular cloud - annotated View original

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• Gravitational instability in cold, dense molecular clouds triggers the collapse of gas and dust, initiating the star formation process
• As the cloud collapses, it fragments into smaller clumps, each potentially forming a star or multiple star system
• The increasing density and temperature in the collapsing core eventually leads to the ignition of nuclear fusion, marking the birth of a new star

Stellar winds and radiation pressure

• Newly formed massive stars emit intense radiation and generate powerful stellar winds
• The radiation pressure and stellar winds can disperse the surrounding gas, potentially halting further star formation in the immediate vicinity
• However, the compression of gas at the edges of the expanding bubble can trigger new rounds of star formation

Supernova explosions

• Massive stars (> 8 solar masses) end their lives in spectacular supernova explosions
• The explosive energy release can sweep away the surrounding gas, shutting down star formation locally
• Supernova shock waves can also compress nearby molecular clouds, triggering the collapse of gas and initiating new star formation episodes

Feedback in galaxy evolution

Supernova-driven galactic winds

• Collective energy and momentum input from numerous supernovae can drive large-scale galactic winds
• These winds can expel significant amounts of gas and metals from the galaxy, regulating its chemical evolution and gas content
• Galactic winds can also enrich the surrounding intergalactic medium with heavy elements

Active galactic nuclei feedback

• Supermassive black holes at the centers of galaxies can accrete matter, releasing enormous amounts of energy (active galactic nuclei or AGN)
• AGN feedback can take the form of jets, winds, or radiation pressure
• The energy output from AGN can heat and expel gas from the galaxy, suppressing star formation and regulating galaxy growth

Starburst-driven outflows

• Galaxies undergoing intense episodes of star formation (starbursts) can generate powerful outflows
• The collective effect of stellar winds and supernovae in starburst regions can drive gas outflows
• These outflows can remove gas from the galaxy, limiting the available fuel for future star formation

Impact of feedback on structure formation

Regulation of star formation rates

• Feedback processes play a crucial role in regulating the star formation rates in galaxies
• Negative feedback (e.g., supernova explosions, AGN) can suppress star formation by heating and expelling gas
• Positive feedback (e.g., supernova-triggered star formation) can enhance star formation in certain regions

Influence on galaxy morphology

• Feedback mechanisms can shape the morphology and structure of galaxies
• Galactic winds and outflows can redistribute gas and stars, affecting the galaxy's appearance
• AGN feedback can prevent the excessive growth of galaxy bulges, maintaining the observed galaxy scaling relations

Role in shaping the interstellar medium

• Feedback processes significantly impact the properties and structure of the interstellar medium (ISM)
• Supernovae and stellar winds can create cavities, bubbles, and shells in the ISM
• Feedback-driven turbulence can regulate the ISM's density and temperature distribution

Observational evidence of feedback

Galactic superwinds

• Observations of starburst galaxies reveal the presence of large-scale galactic superwinds
• These winds are detected through the presence of extended X-ray, Hα, and radio emission
• Superwinds are observed to transport gas and metals out of the galaxy and into the intergalactic medium

Superbubbles and chimneys

• Observations of nearby galaxies show the existence of giant cavities (superbubbles) and vertical structures (chimneys) in the ISM
• These features are created by the collective impact of multiple supernovae and stellar winds
• Superbubbles and chimneys facilitate the transport of energy and metals from the disk to the galaxy's halo

Feedback-driven turbulence

• Observations of the ISM reveal the presence of turbulent motions on various scales
• Feedback processes, such as supernovae and stellar winds, are thought to be the primary drivers of ISM turbulence
• Turbulence plays a crucial role in regulating star formation and shaping the ISM's structure

Feedback in cosmological simulations

Subgrid feedback models

• Cosmological simulations cannot resolve the detailed physics of feedback processes due to limited resolution
• Subgrid models are employed to approximate the effects of feedback on scales smaller than the simulation resolution
• These models aim to capture the essential impact of feedback on galaxy formation and evolution

Challenges in implementing feedback

• Accurately modeling feedback processes in simulations is challenging due to the complex and multi-scale nature of the physics involved
• Simulations need to balance the computational cost and the fidelity of feedback implementations
• Different feedback models can lead to varying results, highlighting the uncertainties in our understanding of feedback

Effects on galaxy population properties

• Feedback processes implemented in cosmological simulations significantly impact the predicted properties of galaxy populations
• Simulations that include feedback can better reproduce the observed galaxy stellar mass function, star formation histories, and gas content
• Feedback helps to reconcile the discrepancies between the predicted and observed galaxy properties in a $\Lambda$CDM universe