Lecture 25: Galactic Collisions and Evolution

Collisions

• The bigger an object is, the easier it is to collide with it.
• It is easier for a collision to occur if there is a large density of objects (in other words, lots of objects in a small volume).
• Consider two objects with the same radius R separated by a distance d.
• The probability of a collision in the time it take to fly over distance "d" is approximately (R/d)²

Collisions between Stars

• The distance between star systems is generally rather large compared to the size of the star.
• For instance: The Sun's radius is R_Sun = 7 x 10⁵ km.
• The distance to the nearest star system (Alpha Centauri) is about 1pc = 3 x 10¹³ km
• The distance between the Sun and our nearest neighbour is 4 x 10⁷ times larger than the radius of the Sun.
• The chance that the Sun will collide with another star is very tiny.
• In a Globular Cluster stars are much closer to each other. A typical separation is 0.1 pc. This is still 4 million times larger than the Sun.
• Some stars (Red Supergiants) are much larger than the Sun. It is possible for a Supergiant to be 1000 x larger than the Sun. This provides a larger target than the Sun, but is still much smaller than the typical interstellar distances.

Collisions between Galaxies

• The distances between galaxies in a cluster or a group are relatively small compared to the sizes of the galaxies.
• For instance: The Milky Way's visible disk has a diameter of 30 kpc.
• The distance to the Large Magellenic Cloud (LMC) is about 50 - 60 kpc, no more than 2x the diameter of the Milky Way. (The LMC is only 7 kpc in diameter.)
• The distance to the Andromeda Galaxy is 770 kpc, only about 20x the diameter of the Milky Way.
• Collisions between galaxies in our Local Group are very likely.
• Andromeda and the Milky Way are moving towards each other with a relative velocity of 120 km/s. We will probably collide in about 6 billion years.

Large Magellenic Cloud (LMC) The LMC has an irregular shape because the tidal force of the Milky Way is distorting it violently. The red ionized hydrogen regions and young, blue star clusters are evidence that star formation is occuring. The stars in the LMC have a very low metal content. This suggests that star formation in the LMC is a recent phenomenon, and is occuring because of the disruption caused by the Milky Way.

Galaxies in galaxy clusters and large scale Web

• Evolution of the structure in the Universe assembles small objects into the large ones.
• The motion under gravitational forces is not random, but leads to formation of galactic clusters with galaxies flowing into such concetration along filamentary web.
• Galactic interaction and mergers is not accident, but inevitable development.

The Virgo Cluster is the closest rich cluster of galaxies. (A rich cluster has more than 1000 galaxies in it.) The giant elliptical galaxy M87 lies at the centre of Virgo. Two other giant elliptical galaxies, M84 and M86 are in the upper-right-hand corner of the image. The diameter of the Virgo cluster is about 3 Mpc, so that the galaxies are closer to each other on average than in the Local Group. Rich clusters are composed of mainly elliptical galaxies and lenticular galaxies. This suggests that when galaxies collide, elliptical galaxies may result.

Click the image for movie

The Giant Elliptical Galaxy M87 At the centres of rich clusters lie giant elliptical galaxies. Giant elliptical galaxies are about 20 times larger than typical galaxies. The Giant elliptical galaxies may be the remains of many galaxy collisions occuring near the centre of a rich cluster. M87 in Virgo has a diameter which is approximately 700 kpc. Remember, the distance between the Milky Way and Andromeda is about 700 kpc. M87 is about the same size as the Local Group of galaxies!

Galaxy Collisions

When two galaxies collide, their dark matter, gas and dust clouds hit each other and can be flung out of the galaxies. This produces long tidal tails of matter This may produce large spiral arms. This can strip the galaxies of their gas. We see evidence of this when we view rich clusters and see evidence of hot intracluster gas. Intracluster gas is gas within the cluster (but not in a galaxy) which glows at X-ray wavelengths since the gas is over one million Kelvin.

Click the image for movie

Galaxy interaction: Starburst Galaxies

When galaxies come close to each other but miss, the tidal deformation can trigger a burst of star formation. In this case a star-burst galaxy is formed. An example is the galaxy M82 which seems to have narrowly missed colliding with the spiral galaxy M81. The M81 group of galaxies This image shows the 3 galaxies in the M81 group. The left image shows the visible light where the galaxies appear disconnected. The radio image (on the right) shows streams of hydrogen connecting the galaxies. M82 (the star-burst galaxy) is the upper-most galaxy.

Galaxy Mergers

• There are many photographs of galaxies caught in the act of merging.
• One of the most famous is called The Antennae.
• The Antennae was formed from the collision of two similar sized spiral galaxies.

• These images of the Antennae show the star formation which is occuring as a result of the collision.
• Dust has been funnelled into the central regions of the galaxies causing them to look red.
• The regions of star-formation are blue from the glow of the hot type O and B stars.
• The scale bars on the right-hand images are 1,500 light-years across.

The Antennae

Galactic Cannibalism

• When two galaxies merge, a larger galaxy results.
• Over time, very large galaxies can form inside of rich clusters.
• When a small galaxy merges with a much larger galaxy, the larger galaxy effectively "eats" the smaller one.
• During a merger of this sort, the larger galaxy is unaffected by the collision, while the smaller one is disrupted.
• This process is called galactic cannibalism and probably explains the formation of giant ellipticals at the centres of rich clusters.

Stellar Populations

• Surveys of stars in our galaxy (and others such as Andromeda) show that there are two main populations of stars.
• The two populations are called Population I and Population II.
• The populations are divided by chemical composition.
• Both populations are composed of mainly Hydrogen and Helium.
• The main difference is in the fraction of heavier elements, such as Carbon, Oxygen, Iron, etc.
• In Astronomy, all elements heavier than Helium are called metals.

Population I

• Population I stars are stars with a high abundance of metals.
• The Sun is a Population I star.
• Population I stars include young type O and B main sequence stars.
• Since O and B stars are bluish in colour and are the most luminous types of stars, their light tends to dominate any group of stars. This makes groups of Pop I stars to appear somewhat blue in colour.
• Poplulation I stars are mainly found in the disk of the galaxy.

Population II

• Population II stars are stars with a low abundance of metals.
• The heavy elements are only about 3% as abundant as in the Sun.
• Only older type K and M main sequence stars, as well as red giants are part of Pop II.
• These stars are all red in colour.
• Pop II stars are generally found in the central bulge of the galaxy, the globular clusters (in the halo) and in stray stars in the spherical halo.

Stellar Evolution and the Populations

• The two types of populations can be understood if the galaxy was initially composed of only Hydrogen and Helium.

• The Milky Way probably formed 10 - 15 billion years ago.

• The first stars which formed had almost no heavy elements. These will be the Pop II stars.

• Over the course of the first 5 billion years the high mass stars (Type O to type F) in the first generation will have evolved off of the main sequence.

• These stars will either evolve into planetary nebulae and white dwarfs or explode in supernovae.

• Either type of evolution will enrich the interstellar medium with higher mass elements.

• After 10 billion years, only the low mass main sequence star (K and M) will still be burning Hydrogen.

• Today, we would only expect to see type K and M main sequence stars as members of Pop II.

• The population I stars (like the Sun) form from the remains of the older stars and have more Carbon, Oxygen and Iron than the previous generation.

• Within the disk of Pop I stars, there is a gradient of heavy element abundance. The highest fraction of heavy elements are in the centre and the metals decrease outwards.

Galactic Evolution

• The location of Pop I and II stars provides some clues about the evolution of the Milky Way.

• The oldest stars (Pop II) occur mainly in the spherical halo and in the central nuclear bulge.

• The youngest stars (Pop I) are mainly seen in the galactic disk.

• In addition, the disk stars mainly move in circles around the galactic centre, and are confined to the disk.

• Stars in the spherical halo orbit the galactic centre, but on elliptical orbits which are not confined to the disk.

• Stars in the central bulge move in randomly oriented orbits within the bulge.

	>
Motion of Pop I stars	Motion of Pop II stars

A Possible History of the Milky Way Either one large spherical gas cloud, or a number of smaller gas clouds started to form stars. Star formation took place through-out the whole cloud(s). The original cloud is mainly H and He. If smaller clouds exist they merge with each other to form a larger cloud. The whole system slowly begins to collapse. Any small rotation of the original cloud(s) is amplified during the collapse due to angular momentum conservation. The slow gravitational collapse will tend to produce a region of higher stellar density near the centre. The angular momentum will flatten the distribution of gas. The old stars left behind in the halo will have orbits similar to their orbits before the collapse. In the disk of gas which rotates rapidly, a second generation of stars can form.