Normal Galaxies and Hubble’s Law

Lecture 21

Classification of Galaxies

Astronomers divide galaxies up into three different major types: spiral galaxies, elliptical galaxies, and irregular galaxies.

Spiral galaxies are designated with the letter “S.” These are divided into three subcategories, Sa, Sb, and Sc. Sa galaxies have the largest bulges and the least open spiral arms. Sc galaxies have the smallest bulges, the most open spiral arms, and the most knotty spiral arms. Some spiral galaxies are classified as “barred spirals,” when they have an especially elongated central bulge, and are designated with the letters “SB.” These barred spirals are further subdivided into three subcategories, SBa, SBb, SBc. The difference between these three is the same as for non-barred spiral galaxies.

Elliptical galaxies are designated with the letter “E.” These are divided into eight subcategories, E0, E1, E2, E3, E4, E5, E6, and E7. These subcategories indicate how elongated the elliptical galaxies appears, E0 for the least elongated and E7 for the most elongated. Keep in mind that this elongation is only a lower limit for the true elongation of the galaxies because it is based on the galaxies appearance from Earth. A very elongated galaxy seen end-on would appear quite round. Elliptical galaxies span a large range of sizes, from dwarf ellipticals (as little as 1 kpc across) to giant ellipticals (as large as a few Mpc across).

Irregular galaxies defy classification into one of the two groups listed above. The Large and Small Magellanic Clouds are examples of irregular galaxies.

Summary of galaxies properties:

	SPIRAL	ELLIPTICAL	IRREGULAR
SHAPE	Flattened disk, spiral arms	No disk, smooth ellipsoidal distribution	No obvious structure
STELLAR CONTENT	Young and old stars	Old stars only	Young and old stars
GAS AND DUST	Large amounts	Hot X-ray emitting gas only	Very abundant
STARS FORMING?	Yes, ongoing	None recently	Yes, and vigorously
STELLAR MOTION	Circular orbits in disk	Random 3D orbits	Very irregular orbits

Three-dimension organization of material in the Universe

When astronomers study the sky, they must measure the distances to objects in order to determine their three-dimensional distribution. When looking through a telescope in a particular direction, objects are seen at a wide range of distances from the Earth.

The first method of distance determination that is useful for extra-galactic (i.e. outside the galaxy) distances is that of Cepheid variable stars, as discussed in the lecture on the Milky Way. With Cepheid variables, distances can be determined for objects within 25 million parsecs of the Sun. This range encompassed a large number of other galaxies. Cepheid variables are considered “standard candles.” This phrase refers to astronomical objects whose luminosities are easily determined based on the type of object. For example, all Cepheid variables with 60-day periods have the same intrinsic luminosity. By measuring the period, the luminosity is therefore known. With the luminosity in hand, a simple measure of the object’s apparent brightness is all that is needed to determine its distance.

Type I supernovae are another standard candle. Recall that these are the explosions that result after a white dwarf in a binary acquires sufficient mass from its companion. White dwarf stars are supported by electron degeneracy pressure. Add too much mass, though, and gravity overcomes this pressure. At this point, the star becomes unstable and explodes. Because these explosions always begin with an object that has just exceeded that limit, they are quite consistent from star to star in terms of their brightness, duration, etc, just what we need for a standard candle. In contrast, type II supernovae are NOT standard candles. These supernovae result when massive stars exhaust their nuclear fuel and implode/explode. These massive stars can have a variety of initial masses; any star with more than about 15 times the Sun’s mass will end this way. The appearance of the resulting explosion depends greatly on the initial mass of the star.

Type I supernovae can be used to measure distances out to at least 1 billion parsecs and quite possible farther.

A third method that is used to determine distances to other galaxies is something called the Tully-Fisher Relation. It turns out that the rotational speed of a galaxy is well correlated with its luminosity. The rotation speed can be measured by observing the Doppler shift of material within the disk. One side of the disk shows a red-shift, because the material is rotating away from us, the other side shows a blue-shift, because the material is rotating towards us. The amount of the shift tells us the speed. From this speed, we may infer the luminosity.

Using these (and other) methods of determining distances, astronomers have mapped out the three-dimensional distribution of galaxies within the Universe. The Milky Way is contained within a small cluster of galaxies known as the “Local Group.” The major galaxies within this group are the Milky Way, Andromeda, and M33. The Milky Way and Andromeda are about 2.5 million light-years apart and M33 is quite near Andromeda. These 3 spiral galaxies are accompanied by a few tens of dwarf elliptical galaxies and small irregular galaxies.

Most Galaxies, however, are found in much denser habitats, clusters containing hundreds or thousands of galaxies. The Virgo Cluster is the nearest of these at 60 million light-years.

Just as we can use the motion of objects within galaxies to determine that they must contain 3-10x more material than we can see (in the form of dark matter), we can use the motions of galaxies within galaxy clusters to determine that the clusters contain even more dark matter. The motions of galaxies within clusters suggest that over 90% of the universe consists of dark matter!!!

Evolution of Galaxies

Galaxies are seen to evolve over time. By looking at very distant galaxies, billions of light-years away, astronomers can determine what the overall properties of galaxies were in the past. It appears that galaxies now are both larger and fewer in number than in the past. This change is no doubt a result of galaxies interactions, collisions, and mergers. Unlike stars, galaxies are quite close together compared to their sizes. Their separations are, on average, roughly 20x their size. Within dense clusters of galaxies, they are even closer. This proximity means that galaxies are quite likely to interact and/or collide.

Computer simulations indicate that the merger of two spiral galaxies may result in a single elliptical galaxy. The merger stimulates star formation because the molecular clouds from the two galaxies collide and compress. The supernovae that occur after the most massive of these stars evolve could easily eject all remaining gas and dust from the new galaxy.

Similarly, small galaxies can easily be incorporated into large galaxies with little disruption for the large galaxies. Many giant elliptical galaxies are seen to contain multiple cores – the central regions of the smaller galaxies they have “eaten.”

Hubble’s Law

Around 1912, Vesto Slipher measured the Doppler shifts of many “spiral nebulae” and made the surprising discovery that the majority of these objects are moving away from us. Because objects moving away from us have their light shifted towards the red end of the spectrum, we refer to these are “redshifts” and say that most of these “spiral nebulae” are redshifted.

In the 1920s, Edwin Hubble measured the distances to many of these objects using Cepheid variables. When he plotted their velocities against their distances, he found that these were related! The farther away a galaxy is, the faster it is moving away from us.

We will discuss the meaning of this observation during the cosmology segment of the course; for now, it is enough that this is an empirical law, repeatedly supported by measurements made since then. The farther away a galaxy is from us, the faster is moves away from us.

This fact can be quantified by Hubble’s Law:

Recession velocity = (Hubble’s Constant) x Distance

Astronomers are still arguing over the value of Hubble’s Constant, but 70 km/s/Mpc is probably not too far off.

Strange units, but the meaning is fairly straightforward. A galaxy that is 1 Mpc (1 million parsecs) away moves away from us at 70 km/s. A galaxies that is 2 Mpc away moves away from us at 140 km/s. A galaxies that is 10 Mpc away moves away from us at 700 km/s. And so on. There is some scatter to the relation; galaxies do not move at exactly the velocities that Hubble’s Law would predict, but on average, they do.

This empirical fact provides astronomers with an excellent method for determining the overall distribution of thousands upon thousands of galaxies. Determining distances using Cepheid variables can be quite painstaking and is limited to about 25 Mpc. Determining distances using supernovae requires waiting for a supernova, which is fine if you are observing many galaxies and don’t care which exact ones you get distances to, but not so fine if you want to know the distances to all of those galaxies. Using Hubble’s Law, though, all we need to estimate the distance to a galaxies is a measurement of the redshift of the galaxy (which tells us its velocity). Determining the redshift requires a single spectrum.

Using this law, astronomers have mapped out the Universe.

Galaxies are not homogeneously distributed on small scales. As discussed above, most galaxies are found within dense clusters. Huge voids, 10s of Mpc on a side, separate these clusters. On very large scales, though, a few 100 Mpc on a side, the overall distribution of material smoothes out, however. On these scales, the distribution of galaxies looks rather frothy, like steamed milk on a hot beverage.