The Universe of Galaxies
The study of extragalactic astronomy began in the 1920’s. Edwin Hubble did much of the pioneering work. He saw that the universe had many galaxies, but was dominated by only two types – ellipticals and spirals.
· Elliptical Galaxies have the elliptical shape and are classified by the eccentricity from E0 for spherical shapes to about E7 for ellipsoids of eccentricity 0.7. These galaxies typically have no interstellar gas and dust.
· Spiral Galaxies come in two forms – the normal spirals and the barred spirals. The classification is an estimate of the prominence of the nucleus as opposed to the spiral arms.
o Sa or Sba – very prominent nucleus with weak spiral arms.
o Sb or SBb – equally prominent spiral arms and nucleus.
o Sc or SBc – weak nucleus with prominent spiral arms.
Spiral galaxies contain large amounts of interstellar gas and dust.
A few galaxies defy classification and are called Irregular. Others fall into a category called Peculiar, that is, not normal for the apparent type of galaxy it represents. Among these are the AGN's (Active Galactic Nuclei), which include Seyfert Galaxies, BL Lacertae Objects, Blazars, and others. Another group are the radio active galaxies, that is, galaxies that are very bright in the radio region of the spectrum. In many cases these radio galaxies have a characteristic double-lobe structure that resembles the bi-polar jet model we have discussed before. In most of these cases the nuclear region of the galaxy is very active and many harbor suspected supermassive black holes. These are black holes that contain hundreds of millions of solar masses.
Clusters of galaxies exist consisting of a few up to hundreds of galaxies. The Milky Way is a member of the Local Group containing about 25 galaxies. Galactic clusters often show evidence of collisions. The energy released in these galactic encounters can give rise to star bursts (short periods of time during which the stellar formation rate is much higher than normal) or chain reaction supernovae.
On the large scale we see that galaxies seem to form on the boundaries of large void areas or bubbles. These voids are certainly related to the conditions present when the universe began.
Edwin Hubble made a major contribution to the understanding of the structure of the whole universe. He measured the spectra of distance galaxies. He used independent means to determine the distances to the galaxies. Two methods he used were to look at supergiant elliptical galaxies that tend to dominate clusters of galaxies. These galaxies, he found, were about the same brightness and size regardless of where they appeared in the universe. When they appear fainter or smaller, it was because they were farther away.
Upon taking the spectra of these galaxies, Hubble found that the each one was redshifted - the galaxies were moving away from us. Even more he found that the more distant galaxies were moving away faster. The information was presented in the form of a graph of recessional velocity vs. distance, which we call Hubble's Law. The conclusion we draw from Hubble's Law is that the universe is expanding. Using the graph we can get the distance of anything in the universe as follows:
The use of Hubble's Law does rely on the assumption that the redshifts are caused by the universal expansion. The relationship, however, would not be nearly so good if this conclusion were not the fact. Hubble's Law stands today as one of the pillars of Modern Cosmology. The question still remains, however, as to how the universe began.
Two cosmologies developed from Hubble's work:
Prior to the mid-1960's observations could not distinguish between these two cosmologies. Arno Penzias and Robert Wilson at Bell Labs changed all that with their discovery that the universe is emitting blackbody radiation indicative of a blackbody at 3 K. At the same time the Big Bang Cosmologists had determined that the temperature of the Big Bang was sufficient to make Hydrogen and Helium only and, therefore, was limited. Starting with that temperature and expanding the universe for 12 - 15 billion years at the observed rate yielded a universe that was, by now, much cooler. Recall that ideal gases cool when expanded. The present temperature of the Big Bang is 3 K, in exact agreement with the observed microwave background radiation discovered by Wilson and Penzias. Everyone now agrees that the universe began as a Big Bang. But many variations of the basic theory still exist to be sorted out. The 3 K background radiation is another of the pillars of cosmology today.
Quasi-stellar Objects (QSOs or quasars) were discovered in 1963. In rather short order it was found that these objects had enormous redshifts. Using Hubble's Law the large red shift implies large distance - in fact, QSOs are the most distant objects in the universe. The fact that we can see them at all implies that they are tremendously luminous. Normal galaxies fade from view after a certain distance. Even more perplexing is that many QSOs are variable over time periods of weeks to about a month. The Light Time Argument limits the size of the QSOs to the distance light can travel in this period of time - roughly the size of the solar system. To recap QSOs, because of their great distance, must produce the light of 1000 Milky Way galaxies in the volume of a typical solar system. If we are to accept the Cosmological Hypothesis (QSOs are at the distances implied by Hubble's Law), we have difficulty with the energy source. Even fusion is incapable of producing the required energy.
Astronomers who were unhappy with the exotic energy requirements of the cosmological hypothesis created the Local Hypothesis. It is not necessary to have exotic energy sources if the QSOs are not that far away. This group would like the QSOs to be about as far away as normal galaxies. The problem with this hypothesis is that the extreme redshifts are very difficult to explain. Several attempts have been made to produce redshifts this great that are not associated wi the expansion of the universe.
The vast majority of astronomers today prefer the Cosmological Hypothesis. Several other observational facts have come to light in recent years. If the QSOs are very distant, then we are seeing them as they were a very long time ago. The present thinking is that QSOs are very young galaxies and have nuclei that are extremely active. The leading candidate for the energy source is a supermassive black hole. When the galaxy formed the very massive black hole formed at its center. The size of the event horizon is about the size of a solar system. Recall that many QSOs are variable and have implied sizes of a solar system. An enormous accretion disk forms around the black hole. As in the bi-polar jet model, material spirals around the black hole and heats up. The gas in the disk can release vast quantities of energy just before plunging into the event horizon of the black hole. Over time the accretion disk thins as material gets used up. The QSO becomes less active and finally inactive. This explains why we do not observe nearby QSOs. In fact intermediate objects exist between the QSO and normal galaxies. Both BL Lac objects and Seyfert galaxies share much in common with the QSOs but without the huge luminosities.
As material in the accretion disk gets thinner and thinner, the following scenario plays out:
If this scenario is correct, then many nearby galaxies ought to harbor supermassive black holes in their nuclei. Evidence is growing that this is precisely what we have. Even the Milky Way is suspected of having a large black hole at its center. We can observe these cases to get the mass at the nucleus. We assume that the mass is in the form of stars and thus, we can relate the brightness of the center of the galaxy to its mass. We can also get at the mass by observing the motions of the stars near the center. The two techniques do not agree by large factors, leading to the conclusion that some galaxies harbor large amounts of mass in very small regions at the center.
Additional evidence for the cosmological hypothesis is gravitational lensing. This is an effect predicted by Einstein in which a distant object would have its shape distorted if the light from the distant source passes close to a massive galaxy on the way to the Earth. We employ here the predicted bending of starlight predicted by general relativity. Several very convincing examples of gravitational lensing have now been found. Clearly, if we observe gravitational lenses, then the QSOs being lensed are much farther away than the lensing galaxy. In many cases the lensing galaxy cannot be observed.