On this CDROM you will find a collection of computer animations of various simulations of galaxy formation and dynamics. The files are at different resolutions typically at 320x240 to 640x480. Here is a brief description of the contents of these files. Dark Matter simulations ----------------------- There are 4 movies which show the process of gravitational instability on density fluctuations in dark matter. The blue particles in these animations represent the "cold dark matter" which is believed to make up most of the mass of the universe. Galaxies themselves are not shown. In the standard picture, at the time of the Big Bang the universe is nearly perfectly uniform with tiny fluctuations in the density. As the universe cools and expands, these fluctuations become unstable because of gravity and collapse upon themselves. Bright objects are seen to grow in these simulations. These are the so-called "dark halos" which are believed to surround galaxies. Although they are quite massive, the matter is invisible to us. Simulations like these show us how gravity acts on a collection of density fluctuations to form dark halos and by extension galaxies under different scenarios of the Big Bang. 1. file cdm_630Mpc_1G.mpg This first simulation is an example of a cosmological simulation of the formation large-scale structure. In this case, 1 billion particles are placed in a cube representing a volume of the universe 630 megaparsecs on a side (about 2 billion light years). This representative volume would normally be expanding with the Big Bang but in these kind of simulations we subtract off the uniform expansion so that we may see the collapse of dark halos more clearly. The simulation shows the transformation from a featureless uniform distribution of mass into a system of interconnecting filaments made out of dark halos (the bright spots). These are the places where galaxies form. 2. cdm_80Mpc_16M.mpg This is another example of a cosmological simulation similar to the one above but on a smaller scale (80 megaparsecs on a side or about 250 million light years) and using only 16 million particles. We are zooming in here and we see the formation of large dark halos which correspond to clusters of galaxies. Smaller dark halos fall into these large structures and form clusters of galaxies. 3. cdmcluster.mpg Yet another close up only 40 megaparsecs on a side. We can see the violence of the collapse of density fluctuations and their relaxation into dark halos. 4. cdmcruise.mpg This animation is a cruise on a random trajectory through simulation #2 above. A virtual spaceship flys through a single snapshot of this simulation representing a time about 5 billion years after the Big Bang. The dark halos are clearly apparent as blobs of particles and the filamentary structure which develops as a result of gravitational instability is also obvious in this ride. Galaxy Interaction Movies ------------------------- 5. mwa2001.mpg Galaxies actually form within the centers of dark halos and as you could see above the process which drives structure formation involves strong interactions and mergers of galaxies. The galaxies that we see are made out of stars and gas (plus a surrounding invisible dark halo) and generally are found as spiral galaxies. Spiral galaxies are great collections of stars moving on nearly circular orbits with a disk. Gravitational effects cause them to develop the spirals see. When two spiral galaxies interact, the two disks are dramatically altered by the mutual gravitational tidal fields of the two galaxies. First they form beautiful open spiral arms some of which continue to grow and become "tidal" tails. However, the process of interaction takes orbital energy away from the two galaxies and causes them to merge and form a single, featureless galaxy with an ellipsoidal shape. About 10% of galaxies look this way and are commonly known as ellipticals. It is widely believed that their origin probably is in the merger of two or more spiral galaxies. The animations portray the collision of two galaxies as viewed from three different perspectives and then repeated again but as viewed from a larger distance. The two galaxies in this animation are models of our own galaxy the Milky Way and it's nearest neighbour the Andromeda galaxy which currently lies at a distance of 2 million light years. According to some calculations with reasonable assumptions, over the next 3 billion years the Andromeda and Milky Way galaxies will get closer and closer and eventually collide as depicted here and then merge to form an elliptical galaxy. The smaller galaxy in these animations is the Milky Way. 6. cluster.mpg As seen in the dark matter simulations, dark halos fall into clusters but retain their integrity and don't dissolve away remaining in orbit for a long time. The visible parts of galaxies (the stars and gas) lie at the center of these halos and we see many examples in the nearby universe of clusters of galaxies - the Virgo and Coma clusters are the best-known examples. In these objects, hundreds to thousands of galaxies are in orbit about a common center defined by the cluster. But since galaxies are extended objects they can suffer interactions and maybe even mergers as they fall in and settle into an orbit within a cluster. This simulation is an illustration of what happens to a population of 100 spiral galaxies that are set on orbits taken from a dark matter simulation. The galaxies are given masses and sizes corresponding to real galaxies and so the outcome should be a realistic rendition of what happens gravitationally over the history of the universe. This simulation spans 10 billions most of the age of the universe. We see groups of galaxies falling and interacting with themselves to form ellipticals which settle into orbit in the cluster. We see the formation of a large giant elliptical galaxy at the heart of the cluster. These giant ellipticals are seen almost without exception in all clusters of galaxies so the scenario portrayed here is probably how they form in general. We also see many examples of galaxies which suffer strong gravitational interactions that throw off tidal tails and therefore pollute the intergalactic space in clusters with stars. 7. fly1600.mpg Finally, I have one animation which is in the same spirit as the dark matter cruise but here we show the stars in the spiral galaxies at the heart of the dark halos and keep the dark matter invisible. The animation is again a flight on a virtual space ship that intersects 20 galaxies in a region which is collapsing to eventually form a cluster. The spaceship is flying through a static snapshot at an early time just as the core of the cluster is forming as well as various galaxies are beginning to fall in and interact with each other in groups. There are many examples of ongoing mergers and tidal tails in these simulations. The main point here is we expect at early times in the history universe to see many more interacting galaxies than we see today in the nearby universe. This is simply a reflection of the more vigorous growth of structure from density fluctuations at earlier times than today.