Seeing Galaxies of the Past

Because we see distant objects as they were in the past, all we have to do to learn how galaxies evolve is to look at more and more distant galaxies, which show us what they were like earlier and earlier in the history of the universe.  Galaxies 5 billion light years away must be at most 14 billion - 5 billion = 9 billion years old.  Galaxies 10 billion light years away must be at most 4 billion years old, and so on.  What is remarkable about galaxy formation is that we can see galaxies all the way to the limit of our present telescopes, about 12 billion light years away.  These galaxies are only 1 or 2 billion years old, which means that galaxies formed very early in the universe.  We see little evidence that galaxies are still forming -- apparently whatever caused their formation took place in only a short time.  Observationally, we have several problems with seeing galaxy formation by looking farther into space (and farther back in time):

• Very distant galaxies are very faint, and we cannot see them very well even with the largest of today's telescopes.  New, super-sized telescopes are planned, to allow us to see even farther into the past.
• Very distant galaxies have very large recession velocities -- they are moving away at a good fraction of the speed of light.  This causes extreme doppler redshifts, which means that distant galaxies are brightest in the infrared part of the spectrum.  To observe these, we need to put large infrared telescopes into space.
• Galaxies formed so quickly that it is not easy to see differences in galaxies until we reach close to the limit of our current capabilities.

We would like to tell you the complete story of galaxy formation, but much still remains hidden to us.  Instead, we can only outline the current ideas, and tell how it might have happened.  We will see that some of the brightest objects in the early universe are objects called quasars.  Luckily, quasars are so bright that we can see them nearly to the edge of the universe.  The light from quasars pass through many galaxies between us and the quasars, and this lets us study these intervening galaxies by looking at the spectral lines they absorb from the quasar light.  Quasars appear to be something that happened only in the early universe, and we see no nearby quasars today.

We can learn a lot about galaxy formation by studying the parts of our own galaxy.  Recall the schematic view of a spiral galaxy like our own: Here we see that there are objects (stars and globular clusters) in a more-or-less spherically symmetric "halo," which becomes thicker toward the center to make up the central bulge, but there are other objects (young stars, dust and gas, etc.) that are concentrated in a thin disk.  From this we envision that our galaxy formed from a more-or-less spherical cloud of hydrogen and helium that collapsed due to gravity, just as our solar system formed from a far smaller cloud.  In the early history of the collapsing cloud, the cloud would have remained cool due to radiating its heat away, so particularly dense regions would have formed the globular clusters and halo stars.

It is important to realize that once a star forms, it will not change its orbit to form a disk of stars.  It will forever orbit at whatever inclination angle it was when it formed.  By studying the orbits of globular clusters and halo stars, we can see that the gas of the original protogalaxy cloud was once more spherically distributed.  However, the gas that remained did continue to collapse toward a thin disk, because the gas molecules could interact and lose their random motions.  The law of conservation of momentum was at work to ensure that the remaining gas clouds orbited close to the plane of the disk.  Stars that formed later, when the gas was more and more nearly a disk, kept their inclinations just as the halo stars did, so stars in the thick disk should be older than stars in the thin disk.  In fact, stars in the halo and thick disk should be among the oldest stars in the galaxy, and because the original cloud was only hydrogen and helium (no heavier elements) the oldest stars should have no heavy elements (metals).  In fact, we do see that the halo and thick disk stars are older and have fewer metals, while disk stars are younger and have more metals.

Last time we learned about the different types of galaxies, especially spiral galaxies and elliptical galaxies, which are very different in their characteristics.  The cloud collapse idea we just covered can explain spiral galaxies, but how can we explain the ellipticals, which never form disks at all?

There are several different explanations to account for ellipticals, and perhaps some ellipticals formed in one way and others formed in another way.  One idea is that the clouds that formed ellipticals were somehow different from the clouds that formed spirals.  Since the clouds are all made of the same stuff (hydrogen and helium), something else would have to be different.  There are two scenarios:

• Lack of Rotation: Perhpas the initial cloud simply had very little angular momentum (very little rotation).  If this were the case, no disk would form and the stars would remain rather evenly distributed in the cloud.  However, this would suggest that ellipticals would have dust and gas from generations of stars, but the gas would be distributed throughout the elliptical.  In fact, ellipticals seem not to have any dust at all.
• Rate of Cooling: If the protocloud were cool enough, so many stars would form early on that there would have been little dust left to form a disk.  One way to make the cloud extra cool would be to make it have a high starting density. Evidence for this scenario comes from very distant ellipticals, which seem to be redder than their recessional redshift would give.  These galaxies must have no young blue or white stars, indicating that there is no new star formation going on, even though they are only a few billion years old.

There is another possibility that could explain ellipticals that does not rely on the initial cloud being different.  This scenario suggests that ellipticals form due to galactic collisions and mergers.

• Shaped by Collisions: We know that collisions among galaxies are quite common.  Recall that galaxies are very different from stars in this respect.  Stars are so far apart relative to their size that they essentially never collide.  Recall that if we scale things down so that the Sun were the size of an orange, Alpha Centauri (the next nearest star) would be another orange separated by 3000 miles.  If we then think about shrinking the galaxy to the size of an orange, then the Andromeda galaxy would be another orange only a few meters away!  So relative to their sizes, galaxies are much much closer together.  And recall that the universe is expanding.  In the distant past, when the universe was smaller, the distance between galaxies was far smaller than it is today.

What would a collision do to a pair of galaxies?  All of the stars in the two galaxies would pass completely by each other, without collisions, but the gas and dust would interact and be left behind.  The two stellar populations could eventually merge into a more-or-less spherical shape due to their mutual gravitational interactions, while the gas and dust might be left behind, or might sink to the center of the galaxy and form a huge region of new stars.  Such a huge region would eventually produce supernovas that could generate shocks and wind that could clear out the gas.

Let's look at a few galactic collisions in progress.

One thing that is clear, is that the giant ellipticals at the centers of clusters of galaxies show lots of evidence of mergers and collisions.  The interiors of some giant ellipticals show that stars are orbiting in opposite directions!  They also show multiple nuclei, as if several large galaxies had been gobbled up.  Such giant ellipticals are 10 or more times more massive than individual spiral galaxies. You can watch a video that describes the future of our own galaxy, when we collide with the Andromeda Galaxy.

See also M82 and starburst galaxies.

Some galaxies are called "active galaxies" because they have some sort of engine in their nucleus that is producing huge amounts of energy, sometimes 100s of times higher than a normal galaxy.   The very brightest and most energetic are called quasars, which is a shortening of the term quasi-stellar object.  As we mentioned earlier, quasars and other active galactic nuclei are a product of the early universe, and nearby (and hence present-day) galaxies do not seem to show such activity.  It may be that the engine is present in nearby galaxies (even our own), but is currently not active.

For many years the cause of the phenomena seen in active galactic nuclei was completely unknown, but now we believe that all can be explained by a central, super-massive black hole in the nucleus of some galaxies.  These giant black holes, of sometimes billions of solar masses, are active when new matter is being fed to them in accretion disks.  The accretion disks produce lots of X-rays and ultraviolet light.  The magnetic fields threading the disks create jets of particles that zoom far out into space to form radio galaxies.

Using quasars to probe the universe.