Star Formation and Feedback in Galaxies
Galaxies convert cold gas into stars at rates set by their gas content, and the energy returned by those stars regulates and sometimes halts further star formation.
Definition
Star formation in galaxies is the conversion of cold interstellar gas into stars, while feedback is the return of energy, momentum, and enriched material from stars and accreting black holes to the surrounding gas, which together set how fast and for how long a galaxy forms stars.
Scope
This topic covers the empirical star-formation law relating gas density to star-formation rate, the inefficiency of converting gas into stars, the energy and momentum injected by massive stars and supernovae, the role of active galactic nuclei in feedback, and how feedback shapes the masses and gas content of galaxies.
Core questions
- What relation connects a galaxy's gas content to its star-formation rate?
- Why is star formation so inefficient on galactic scales?
- How do supernovae and massive stars feed energy back into the interstellar medium?
- How does feedback regulate the growth and quenching of galaxies?
Key theories
- The Kennicutt-Schmidt law
- The star-formation rate per unit area of a galaxy scales as a power of its gas surface density, an empirical relation that summarizes how gas is turned into stars across galaxies.
- Stellar feedback
- Radiation, winds, and supernova explosions from massive stars heat and disperse gas, limiting star-formation efficiency and driving galactic outflows.
- Feedback-regulated galaxy growth
- Combined stellar and black hole feedback is required for models to reproduce the observed galaxy mass function, preventing too many stars from forming in both small and massive galaxies.
Clinical relevance
Star formation and feedback set the visible properties of galaxies, the chemical enrichment of their gas, and the masses at which galaxies stop growing; capturing feedback correctly is the central challenge in simulating realistic galaxies.
History
Schmidt's mid-twentieth-century proposal of a law linking gas density to star formation was placed on a firm observational footing by Kennicutt in 1998. As simulations matured, the recognition that feedback from supernovae and active galactic nuclei is essential to reproduce real galaxies became a defining theme of the field.
Key figures
- Robert Kennicutt
- Maarten Schmidt
- Thorsten Naab
- Jeremiah Ostriker
Related topics
Seminal works
- kennicutt1998
- kennicutt2012
- naab2017
Frequently asked questions
- Why don't galaxies turn all their gas into stars?
- Star formation is self-limiting. The energy and momentum from young massive stars and supernovae heat and blow away surrounding gas, so only a small fraction of a galaxy's gas is converted into stars per orbital time.
- What is feedback in galaxy formation?
- Feedback is the return of energy and material from stars and accreting black holes to the surrounding gas. It regulates star formation, drives outflows, and is essential for explaining why galaxies are not far more massive than observed.