Chapter 15 Part 2

More About Galaxies

The Local Group

The Local Group is an association (cluster) of galaxies that includes the Milky Way. The two other large members are M31 (Andromeda) and M33 (Triangulum). The rest are smaller dwarf galaxies. The illustrations of page 428 show you how the Local Group looks. Here's another illustration.

Galaxy Clusters

Galaxies are found to be in clusters. The Milky Way Galaxy is part of a cluster called the Local Group; it consists of three large spirals and about 42 dwarf elliptical and irregular galaxies. This group is known as Hickson 44. The Coma Berenices Cluster is located in the Spring/Summer sky. This collection of galaxies is just a part of the Virgo Cluster.


Clusters of galaxies exist in superclusters, or clusters of clusters. We are located in the Local Supercluster, and you shouldn't be surprised to find out that we aren't in the center of that; we're sitting about 20 Mpc from the center of it.

Seeing VERY FAR Out

Before we had a fully functioning Hubble Space Telescope, the detection limit for galaxies and such was about magnitude 25 (m=25). Hubble extended that in the 1990's. The Director of the Hubble Space Telescope Science Institute has 5% of Hubble's observing to use as he wishes. He doesn't have to go through the process of applying for time on the telescope. This means that he can take some scientific risks, performing observations whose outcome is really unknown. In the mid 90's, Director Bob Williams had Hubble aimed at a spot just north of the Big Dipper. Over 100 hours of exposure were taken and combined into the Hubble Deep Field (see page 435, Figure 16.18). This was an area of sky where previous survey images showed nothing! That "nothing" turned out to be full of galaxies. Some of the nearest ones in the image are over 1,000 mpc away. The faintest ones are too far and too faint for spectroscopic study. The magnitude limit was pushed down to near 30 (m=30). The newer Hubble ultra Deep Field goes about one magnitude deeper.

Active Galaxies

Active galaxies have luminous nuclei. A Seyfert Galaxy nucleus can be 10,000 brighter than our galactic center. Its radiation covers a wider range than a normal galaxy like the Milky Way. The output can also vary over a period of a few months, indicating that the emitting region is very small. NGC 7742 is a good example.

Galaxy Collisions

Star collisions are rare; the distance between stars is millions of times their diameter. Galaxies, on the other hand, are in a different situation. Consider the Milky Way and its near neighbor M31 (Andromeda Spiral). They are about 2,500,000 ly apart and are about 100,000 ly in diameter. Their separation is only about 25 times their diameters. In such a case galaxies might actually collide (these two will in about a billion years!).

These are the Antennae Galaxies and are actually two spiral galaxies colliding. The gravitational interaction has thrown out the two long streamers of matter that make up the antennae.

A close-up view of the center reveals the two nuclei.

Hubble's Law

Using the 100-inch telescope at Mt. Wilson, Edwin Hubble took spectra of distant galaxies. When Hubble put the data together, he found that a galaxy's distance and recession velocity were linearly related.

The relation is written as v=Hd, where v is the recession velocity, H is Hubble's Constant and d is the galaxy's distance in megaparsecs.

Radio Galaxies

Radio galaxies are galaxies that are bright in radio wavelengths. The radio emission arises from non-thermal mechanisms. One of these is called synchrotron radiation, which is produced by electrons spiralling along a magnetic field at nearly the speed of light. This object is known as Centaurus A. It is a composite of optical and radio images.


Quasars started out as a modern astronomical mystery. Several hundred radio sources had been catalogued by radio astronomers, but optical astronomers were having no success at associating them with any optically visible objects. When this was finally done in 1963, the object looked like a faint star. It is known as 3C273. The only problem was that its spectrum was unrecognizable; it was a real modern example of the "What the heck is that?" phenomenon. The problem was solved when Maarten Schmidt (Palomar Observatory) tried interpreting the spectrum as if it were seriously redshifted. That worked. The unidentified lines were hydrogen lines redshifted about 16 percent.

The large redshift indicated large recession velocity, which in turn indicates very great distance. The bulk of them are farther than 1,000 mpc away! This distance opened up another mystery: their luminosity. Knowing the approximate distance (from the redshift) and the apparent magnitude (measured), the absolute magnitude (luminosity) could be calculated. It turned out to be almost unbelievably large. Really luminous quasars could be as luminous as 1,000 Milky Way Galaxies!

Interestingly, there are no nearby quasars; the nearest is 250 mpc away. Most are much farther. This tells us that conditions in the universe have changed; whatever process produces the prodigious energy output of quasars was active several billion years ago but is not operating now.

The Energy Source

The best explanation so far for the energy output of active galaxies and quasars is the central black hole mechanism. A VERY large black hole is swallowing material at a high rate, and the accretion disk is very luminous.

This also provides an explanation for the fact that we see quasars only at very great distances, which also means very far back in time; there are no quasars nearby. The idea is that the material which could fall into the hole has already done so and in the process left a clear region around the hole. The hole is still there, of course, but is quiescent because it is not getting fed. This black hole in NGC 4261 is unusual in that we can actually see the evidence for it using Hubble.

Here is a Hubble image of 6 quasars. Notice that Hubble can resolve the quasars as starlike spots in the center of very distant galaxies.