Galaxy Evolution:
Active Galaxies and Quasars

  Galaxy Formation and Early Evolution

  • General situation: We know much less than for stars, because we can't see the universe before galaxies formed - but can see young ones because of lookback time - and evolution is slower, more complex - but we can compare cases.
  • Basic idea: (i) Galactic building blocks form in areas of denser H & He gas in the early universe, less than 1 Gyr old - these probably include the first supermassive stars, and the black holes they turn into act as seeds for galactic black holes; (ii) Protogalactic clouds merge (bottom-up scenario, with galactic black hole probably forming first, as opposed to top-down fragmentation, as for stars) with a high rate of star formation, at 2 Gyr; (iii) Early galaxies were very active, while mature-looking galaxies appeared after 4-5 Gyr; (iv) Galaxies keep evolving, and assimilate smaller ones.
  • Factors: Rotation of the original cloud (leads to spirals after first stars have formed); Crowding of region (ellipticals in dense clusters); Interactions, collisions, mergers between galaxies (in a sense, galaxy formation and development is not over yet).
  • Galaxy clusters: The time at which clusters form is not well known yet, but a proto-cluster of "infant galaxies" is known that existed more than 12 billion years ago

Evidence from Normal Galaxies

  • From the Milky Way: Ages of stars in different parts of the galaxy; Populations of stars in the halo that seem to come from different fragments; Gravitational interaction with satellites and M31 (they may all eventually merge into an elliptical galaxy).
  • Interactions: They are common; Can lead to shape changes (ellipticals to spirals, or spirals to ellipticals), and can be seen as examples of tidal forces; Spectacular examples are the Antennae and Stephan's Quintet; We can date some collisions with star clusters (M82/M81).
  • Mergers: Can lead to cannibalism (as in cluster Abell 2199); Some galaxies seem to have two cores (like M83).
  • More evidence: Computer models, other galaxies including punch-throughs; galaxies with cores and spirals rotating differently, like NGC 4672 and 4698.

  • Normal vs active galaxies: Normal ones are basically made of stars emitting visible light; Active ones are much brighter and have a different, "nonstellar" spectrum.

  Active Galaxies

  • What do we see? Many distant galaxies, and even some nearby ones, like Centaurus A, look like ordinary galaxies, but emit much more in radio waves and IR than the MW, from a central "active" region.
  • Seyfert galaxies: Spiral-like, but they have a tiny, 1-ly across active rotating nucleus, of variable brightness.
  • Radio galaxies: Elliptical-looking, like M87 or Centaurus A (the nearest one), but they have a more extended region of ejected matter and a radio emitting halo or lobes.
  • Where does the energy come from? According to the current theory, supermassive black holes at the center (millions or billions of solar masses), convert part of the infalling matter into energy (E = mc2) and may have a big influence on the whole galaxy; Particles spiraling in magnetic fields produce the radio waves we see.
  • Starburst Galaxies: 100 times the MW's star formation rate; Strong IR radiation, surrounded by X-ray emitting galactic winds; The nearest example is M82, the irregular Cigar galaxy 10 Mly away, that collided with M81 about 300 Myr ago.


  • What do we see? Objects that look like stars, but their light is extremely redshifted, several times the regular wavelength! More than 200 Mpc away, usually more than 1000 Mpc, and the farthest one at 14.5 Gly.
  • Why are they so special? They are the brightest objects in the universe (1000 times as bright as the MW), but their size may not be much larger than the solar system!
  • What are they? Probably cores of distant galaxies, containing very massive black holes. We don't see them nearby because they probably became normal (elliptical?) galaxies later in life; most were active 1-3 billion years after the big bang.
  • Twin quasars? Most multiple images are produced by gravitational lenses and used to find dark matter, others are true binary quasars.

  The Most Distant Objects

  • Observation: The Sloan Digital Sky Survey in New Mexico has cataloged more than 500 quasars, including some at a redshift z > 6; will chart 1/4 of the sky and reconstruct the birth and evolution of galaxies.
  • Protogalaxies? We have not yet seen the first stars, but with lensing, we may have started seeing galaxy "building blocks"; Some may be showing up as "Heros", Hyper Extremely Red Objects.


  • Status: It is generally believed that star formation started about 200 Myr after the Big Bang, peaking around 2Gyr, and that early galaxies, very active and looking very different from today's, formed when stars started bunching together.
  • Puzzles: Some early galaxies look very similar to normal ones, are not that active, and don't confirm this general scenario...

page by luca bombelli <bombelli at>, modified 29 sep 2012