A. Catagories of galaxies
Most galaxies fit into one of two catagories: spirals and ellipticals.
Spirals have disks with young stars, spiral arms, gas and dust. They also has central
bulges and halos of older stars.
The light from spiral galaxies is dominated by the O and B main sequence stars in the
disk and bulge.
Spirals are catagorized by how tightly wound the spiral is, and how big and what shape
the central bulge is. For spirals with a circular bulge, the catagories run from Sa (tight
arms, large bulge) to Sc (loose arms, small bulge). Other spirals with an eccentric or
``barred'' central bulge are listed from SBa to SBc.
S0 galaxies have a disk and a large central bulge, but no signs of spiral arms.
Spiral galaxies range in mass from 109Mo to 1012.
Ellipticals typically have little or no gas and dust (less than 1% of the mass). There
is no disk structure. The stars mainly orbit radially.
The lack of gas means that no star formation occurs. All stars are old (often mainly Pop
II), so most of the light comes from red giant stars.
Ellipticals are catagorized by shape, from E0 (circular) to E7 (most elliptical).
25% of all galaxies are irregular. Irregulars are gas rich but often metal poor. Star
formation has been slow and inefficient.
Irregular masses range from 108Mo to 1011Mo.
  
  
 
B. Distribution of Galaxies
The Milky Way is part of a small cluster of galaxies called the local group.
The Local Group consists of the Milky Way, the Andromeda galaxy, M33, and their
satellite galaxies (including the Large and Small Magellenic clouds). All of these
galaxies are within 1 Mpc (106 pc) of each other.
There are other small to moderate sized clusters nearby.
The nearest big cluster is in Virgo. It has several thousand galaxies, including the
central dominant galaxy (Cd) M87. It is roughly 20Mpc away, and 2Mpc across.
As we study larger scales, we see larger structures, including superclusters (clusters
of clusters), walls, voids, and filaments on scales of hundreds of Mpc's.
C. The Cosmic
Distance Ladder
Cepheid variables have been observed out to 5Mpc (with recent work pushing that to
20Mpc).
Beyond this we need some other ``standard candle''. Some that are used are the brightest
globular clusters and supernovae. For spiral galaxies, the rotation rate is correlated
with the luminosity (the Tully-Fisher relation). This method is useful out to 200 Mpc.
One consequence of general relativity is that the universe as a whole has to be either
expanding or contracting.
In 1912 it was noticed that most galaxies appear to be moving away from us (they're
redshifted). In the 1920's, Edwin Hubble noticed that more distant galaxies move away more
rapidly.
Hubble's observations showed that the universe is expanding, implying that at one time
everything was in one place.
Also, since distance is correlated with velocity away from us, redshifts determine
distance. This ``Hubble Law'' is v = Ho x D, where v is the velocity (in km/s),
D is the distance (in Mpc), and Ho is the Hubble constant = 50 to 100 km/s/Mpc.
Redshift distances only work beyond 100Mpc. At shorter distances, local `peculiar'
velocities dominate.
Note that the Hubble Law doesn't imply that we are in a special location in the
universe. An observer in any galaxy would see the same thing: everything expanding away
from them.
D. Peculiar and Active Galaxies
Many galaxies don't fit into the standard catagories listed above, either because of
their shape or because their cores are unusual. D.1 Interacting Galaxies
Galaxies are close together relative to their sizes, so collisions and mergers are
relatively common.
In such collisions, the stars don't collide, but gas clouds will hit each other and the
mutual gravity of the two systems can greatly perturb each other.
Interaction between galaxies can produce long tails, rings, and even the complete
disruption of a galaxy.
The interactions also produce shock waves in the galaxies gas clouds. This results in a
burst of star formation. These ``starburst'' galaxies are often very bright in the
infrared due to the abundance of heated dust.
Starbursts probably use up most of the galaxy's gas in a short time. This, combined with
the randomization in orbits, can turn the galaxy into an elliptical.
Collisions may explain why the dense central parts of clusters have very few spiral
galaxies, and often 1 or 2 giant Cd galaxies.
A spiral can probably absorb much smaller galaxies without changing much. The Milky Way
will probably tidally tear apart the Magellenic Clouds and absorb them. The Andromeda
Galaxy may have already done the same thing. D.2 Active Galactic Nuclei
Many galaxies have unusual activity in their cores.
Seyfert galaxies are spirals with very strong emission line sources in their cores. The
brightest cores can be 10 times the luminosity of the Milky Way.
Seyfert luminosities often vary on periods of a fraction of a year. This implies that
the energy source is very compact (< 1/3 pc).
Some galaxies have very strong radio sources, either right in their cores, or in vast
lobes and jets coming from the core. The lobes can be much larger than the galaxy itself.
Seyferts and radio galaxies have energies ranging from 1043 -> 1045
erg/s. D.3 Quasars
In the 1960's, certain star-like (quasi-stellar) objects seemed to have very strong,
unknown emission lines. Many were also noticed to be strong radio sources.
Eventually, it was realized that the lines were ultraviolet lines of hydrogen,
redshifted into the visible. The lines were redshifted by tens of percent, implying
velocities of a large fraction of the speed of light. Converting this into distances put
them at 1000's of Mpc.
Given these distances and the fact that the sources were relatively bright, these
quasi-stellar objects (or quasars) must have luminosities of 1038 -> 1042Joules/s.
Quasars also vary in brightness on timescales of days, implying that they are solar
system sized objects. D.4 The Power Source for AGN's and Quasars
Quasars and active galactic nuclei may be closely related objects. New images of quasars
show that they are at the centers of galaxies.
Nuclear energy can't produce the energies observed in these compact objects. The only
source with enough power is gravitational infall.
In order to produce such luminosities, the most efficient process is for material to
spiral in a disk into a black hole. In this manner, the energy radiated can be up to
(1/2)mv2 with v = (1/2)c. If the luminosity is 1045erg/s, the black
hole has to consume m = 2E/v2 = 8E/c2 = 1025erg/s = 1Mo7
years.
To stuff this much material into a black hole, the black hole has to be very large: 107
-> 109Mo.
Jets would be produced by forcing ionized gas out along the disk spin axes (possibly by
radiation pressure).
The black hole may have formed through coallescence of stellar mass black holes, or
through direct collapse of a central gas cloud.
A quasar is only as powerful as its fuel source. Gas has to continue feeding into the
black hole, or the quasar shuts off. Galaxy mergers may provide the material.
With less fuel, a quasar may look like an ordinary galaxy. There are signs of massive
compact objects in several nearby galaxies, including a 106Mo object
in the center of our own galaxy.
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