Active galaxies and galaxy evolution
Some galaxies have extraordinarily bright centres powered by supermassive black holes. Observing distant galaxies reveals how galaxies have changed over cosmic time.
Active galactic nuclei
Active galactic nuclei (AGN) are compact galactic centres that emit enormous energy, sometimes outshining their whole galaxy. They are powered by gas falling into supermassive black holes, forming a hot accretion disc that releases energy far more efficiently than fusion.
Quasars
Quasars are the most luminous AGN, appearing star-like but with huge redshifts, meaning they are very distant. In 1963, Maarten Schmidt recognised that quasar 3C 273 was about 2 billion light-years away, implying it emitted the energy of hundreds of galaxies from a region smaller than our solar system.
Jets and other AGN
Many AGN launch jets of particles moving close to the speed of light, extending far beyond their galaxies, as in the giant elliptical galaxy M87. Different types of AGN, such as Seyfert galaxies, quasars, and blazars, may be similar objects seen from different angles and at different activity levels.
Black holes and galaxies
The mass of a galaxy's central black hole is closely correlated with the mass of its central bulge, suggesting black holes and galaxies grow together. Energy from AGN can heat or expel gas, shutting down star formation in a process called feedback.
Looking back in time
Because light takes time to travel, distant galaxies are seen as they were long ago. Observing galaxies at different distances is like looking at photographs of galaxies at different ages. Deep surveys show that young galaxies were smaller, more irregular, and forming stars much faster; cosmic star formation peaked about 10 billion years ago.
JWST and the early universe
The James Webb Space Telescope has found surprisingly bright, massive galaxies just a few hundred million years after the Big Bang, and confirmed galaxies with redshifts above 14. These discoveries are prompting astronomers to refine models of how quickly the first galaxies formed.
Galaxy evolution
Galaxies evolve through gas accretion, star formation, mergers, and feedback. Over billions of years, many galaxies have used up or lost their gas and become 'red and dead' ellipticals, while spirals like the Milky Way continue forming stars at a modest rate, about a few solar masses per year.
Worked example
Light from a quasar with a redshift of 6 left it when the universe was less than a billion years old. To shine so brightly, its black hole must already have grown to about a billion solar masses. Explaining how black holes became so massive so quickly is an active research question.
Key takeaways
Active galactic nuclei are powered by gas falling into supermassive black holes. Quasars are the most luminous, identified as distant by Schmidt in 1963. AGN can launch near-light-speed jets, as in M87; types differ by angle and activity. Black hole mass correlates with bulge mass, and feedback regulates star formation. Distant galaxies show the past: star formation peaked about 10 billion years ago. JWST is finding early, bright galaxies. Galaxies evolve through accretion, mergers, and feedback.
Flashcard
Bright centre powered by a black hole
Check your understanding
What powers a quasar?