Galaxy Collisions Top left is an image of the Cartwheel galaxy. The ring of young stars was likely created as a smaller galaxy passed through the disk of the larger one. Bottom left is an image of two interacting galaxies, which will eventually merge into one (a billion years from now). No human will ever witness an entire galaxy collision as the process takes many millions of years.

Galaxy Collisions Computers can simulate a galaxy collision in a matter of hours. Galaxies in clusters collide quite often. In small groups, the galaxies move slowly and tend to stick together and merge. In large groups, the galaxies move more quickly and pass through each other without merging. In a few billion years, Andromeda (our nearest large neighbor) will collide with the Milky Way (it’s moving 120 km/s towards us).

Galaxy Collisions While a collision will wreak havoc on the large scale structure of a galaxy, the individual stars remain unaffected. The stars in a galaxy are so small compared to the distances in between them, that the stars in two colliding galaxies generally never come into actual contact. The stars may be pulled / pushed to new locations due to the gravitational interactions, but they do not (in general) run into each other.

Galaxy Formation and Evolution We know of no simple evolutionary connections among the various categories in Hubble’s classification scheme. The theory of galaxy formation is still very much in its infancy. Complicating the picture are the numerous collisions and mergers a galaxy can experience during its lifetime.

Galaxy Interactions Left alone, a galaxy will evolve as predicted by stellar evolution. However, few galaxies remain in isolation their entire lifetimes. Interactions can rearrange a galaxy’s stars, as well as trigger a sudden burst of new star formation. See above for examples of starburst galaxies, where the interaction induced star formation is extremely intense. Encounters may also “divert fuel” to a central black hole, powering violent activity in some galactic nuclei.

Galaxy Interactions Computer simulations have shown that dark matter halos surrounding galaxies are crucial to galaxy interactions. Galactic cannibalism - when a less massive galaxy interacts with a more massive galaxy, the more massive galaxy takes matter from the less massive one. Galaxy interactions between similarly (but not exactly the same) sized disk galaxies can produce the appearance of spiral arms. The entire event takes several hundred million years.

Making the Hubble Sequence While collisions are random events, and not a true evolutionary process, they can form one type of galaxy from another. If two comparable disk galaxies collide, the resulting galaxy (after a merger) will be an elliptical. If a large galaxy merges with a smaller one, the large galaxy maintains its original form. Spiral galaxies are more common in less dense regions of the universe - their disks are easily destroyed by collisions and mergers, which are much more common in the centers of galaxy clusters (regions of high density).

Black Holes and Active Galaxies Since quasars are much more common at greater distances from us, they must have been more prevalent in the past than they are today. Quasars represent an early stage of galaxy evolution denoted by large activity, after which galaxies calm down.

The Quasar Epoch Where did the supermassive black holes which form quasars come from? We’re not yet sure. Since quasars “eat” about a thousand solar masses of material per year, they could not maintain their luminosities for very long. Quasars likely run out of fuel after a few million years. So, most quasars were relatively brief events that occurred a long time in the past. To make a quasar, we need a black hole and enough fuel to power it. Fuel (gas and stars) was in abundance in the early universe. But, how do you get a supermassive black hole? One possibility - smaller (more normal sized) black holes merged in galaxy centers. Another likely event - when galaxies collide and merge, so do their central black holes. Earliest known quasars formed 13 billion years ago. The height of the quasar epoch occurred 11 billion years ago.

The Quasar Epoch In 2002, Chandra (X-ray observatory) discovered a binary black hole. The two massive black holes are falling in toward each other, and are predicted to merge in about 400 million years. See top left. This is the first direct evidence that such events do actually happen. Since distant galaxies are much fainter than their quasar centers, it’s difficult to discern any galactic structure in quasar images. See top right for Hubble images of a quasar and its parent galaxy (first time a host galaxy was actually seen). Active galaxies tend to be in clusters. Quasar activity appears to be intimately linked to interactions and collisions in galaxy clusters.

Active and Normal Galaxies Rapid decline in number of quasars marks the end of the quasar epoch - roughly 10 billion years ago. Today, the number of quasars has dropped to virtually zero (recall the closest one to us is hundreds of megaparsecs away). The black holes don’t vanish - so galaxies should still have central black holes after the quasars have run out of fuel. The black holes become active again when given new sources of fuel - this is what happens for active galaxies. Matter gets near the black hole, and a previously normal galaxy becomes active once again.

Active Galaxy Formation The largest black holes tend to be found in the most massive galaxies. The most massive black holes power the brightest active galactic nuclei. These galaxies formed from major mergers of large galaxies, which would result in ellipticals. So radio galaxies should be associated with ellipticals. Smaller mergers would have formed spirals and the less active Seyferts.