Active Galaxies and Quasars
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
- 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
- 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"
- 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
= 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
- 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
- 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 olemiss.edu>,
modified 29 sep 2012