Lecture 18: Universe of Galaxies

Reading Assignment: Arny: Chapter 16

We have looked at the Milky Way as an example of a disk, or spiral galaxy. As late as the 1920s it was not proven that these "spiral nebulae" as they were known were outside of our Galaxy.

Shapley-Curtis debate about the nature of spiral nebulae

In 1920 there was a famous public debate between Harlow Shapley and Herber Curtis about the nature of the spiral nebulae.

It was Shapley's contention that the spiral nebulae were within the Milky Way Galaxy and that the Milky Way Was the extent of the entire Universe.

Shapley was basically correct about the size of the Milky Way Galaxy, but the data of spinning was later shown to be in error.

Curtis argued that the spiral nebulae were actually star systems comparable to the Milky Way Galaxy and at great distances. Island Universes as they were known at that time.

Curtis was right for the wrong reasons and Shapley was wrong for the right reasons.

Observation that the spiral nebulae were not seen at all in the galactic plane on the sky. This is sometimes called the Zone of Avoidance.

Use of cepheid variables and the period-luminosity relation to determine cosmic distances.

In 1924 Edwin Hubble made careful observations of cephied variables in these spiral nebulae. By comparing the observed periods of these stars with the period-luminosity relationship known at that time for cephieds he was able to get a luminosity for the stars. Once he had a luminosity and measured a brightness he could get the distance to the nebulae. He found that the Great Nebula in Andromeda was some 600 kpc distant, much greater than the size of the Milky Way Galaxy.

It was then proved that the spiral nebulae were actually galaxies in their own right, and the Universe suddenly became a very, very big place. The Andromeda Galaxy is our nearest spiral galaxy and its light takes 2.5 million years to reach us (the current value for the distance is 770 kpc). When we look up at night and see it we are seeing it as it was when humans were making their premier on Earth.

Distances to galaxies are difficult to determine once they are so far away that cepheid variables or any individual stars cannot be resolved. There have been some relations empirically discovered between the velocities in a galaxy and its luminosity.

It is found that L v4 for galaxies, where v is a velocity measurement. How v is measured differs for spiral and elliptical galaxies. So if the luminosity of an entire galaxy is inferred and a brighness measured, again we can find a distance.

There are indeed many other ways to determine distances, but most all use the idea of a "standard candle", i.e. something that you believe you know how intrinsically bright it is (how luminous it is) and you can determine its distance by measuring its brightness. Examples: Type I supernovae, Planetary Nebulae, Globular Clusters, and The Tip of the Red Giant Branch.

Types of Galaxies

Spiral Galaxies:
Barred Spiral Galaxies
Normal Galaxies

Hubble Classification

These types from Sa - Sc form a sequence of

Disks are where most of the star formation is taking place (as indicated by the O stars and the emission nebulae).

All of the regions between the spiral arms contain stars, not just the spiral arms themselves. Stars with high abundances of metals are found in the disk, indicating that they are at least second generation stars and likely third or more.

The bulges contain little gas (except often near the nucleus) and relatively old stars.

The halos contain few enough stars that they are not easily seen in images. The bulges and haloes contain stars with few metals which are likely first generation stars that are quite old.

Elliptical Galaxies:
These galaxies are classified E0 - E7 and their classification depends on their apparent axis ratio.

Elliptical galaxies compared to spirals

The biggest ellipticals have masses of about M = 1013M

Lenticular galaxies:
S0 galaxies - these are intermediate between ellipticals and spirals - they have featureless disks and bulges, but no spiral arms and little gas and dust, like ellipticals.

Irregular and Peculiar galaxies:
Irregular galaxies: e.g., Large Magellanic Cloud, lots of dust and gas

Peculiar galaxies: usually a spiral or elliptical with a weird shape.

Galaxies in the Universe

The Local Group

Contents of the Local Group

2 galaxies in the local group were discovered only within the last 10 years because they lay hidden behind the dust in the plane of the Milky Way. Are there others?

This suggests that dwarf galaxies may be the most common galaxies in the universe. They are too faint to be seen very far away.

Other nearby groups: M81 - M82 Group

Clustering of Galaxies

Galaxies tend to occur in clusters
Types of clusters

Galaxies and clusters define "bubbles" of size ~ 30 Mpc (100 million light years). Great voids with few galaxies surounded by "walls" of superclusters.

largest cluster structures (i.e. the super-clusters) can not have changed much since the formation of the Universe, so the clustering tells us something about the formation of the Universe itself.

Dark Matter

Just as is the case with the Milky Way Galaxy, rotation curves of other galaxies are also found to be flat and do not drop off at the extent of the visible matter. So all galaxies seem to have large quantities of dark matter in their haloes.

Clusters of galaxies also seem to have large quantities of dark matter. The clusters have the appearance of being gravitationally bound systems. However, the typical velocities of individual galaxies appear to be much larger than the velocity required to escape the system, if the required "escape velocity" (v2 = 2GM/R) is calculated assuming a cluster mass, M, traced only by the visible light from the galaxies.

Thus the only way the clusters can actually be bound is if there is much more mass in "dark" invisible matter. It is common to find that 90% of a cluster's mass is in the form of "dark matter".

Galaxy Interactions

Galaxies are often seen to be gravitaionally interacting with one another if not directly colliding and merging. Just recently a dwarf elliptal galaxy, called the Saggitarius Dwarf, has been discovered that is currently being absorbed by our own Milky Way Galaxy. It is likely that the SMC and LMC will some day be absorbed by the Milky Way. They show evidence that they have already made close passes to the Galaxy and have been tidally disrupted. Their odd shapes are a testament to their recent encounters with the Milky Way.

In about 5 billion years the Andromeda Galaxy and the Milky Way Galaxy will collide or at least make a close enough pass of one another to cause severe tidal disruptions in both galaxies. In other such collisions observed in the Universe we see large tails of material stripped out from colliding galaxies. Giant elliptical galaxies in the centers of clusters have likely gotten so large by assimilating their neighbors.

These collisions are not detructive at all. In fact, the distances between stars are so immense that direct stellar collisions almost never happen, except for perhaps in the much more dense galactic nuclei. The collisions of spiral galaxies with a lot of gas and dust often lead to increased star formation. The colliding gas shocks and collpases under gravity and in a very short time forms many, many more stars than normal. Such an event is called a starburst, and these galaxies are sometimes called starburst galaxies.

Galaxy Evolution

If we look to farther and farther distances we see further back in time. Thus we can look at how galaxies evolve in time.

Preliminary results using the Keck and Hubble telescopes:

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Last modified: Tue Jun 20 23:30:52 PDT 2000