Galaxy Dynamics and Cosmology on Mckenzie

John Dubinski

A new parallel supercomputer dedicated to computational astrophysics came on line earlier this year at the Canadian Institute for Theoretical Astrophysics (CITA).   The new computer called Mckenzie is a 512-CPU Beowulf cluster which has recently been benchmarked at 1.2 Tflops making it the fastest computer in Canada and number 39 in the world on the Top 500 list.

My research interests are in the formation, evolution and dynamics of galaxies.  N-body simulation is one of the most powerful tools for studying the complexity of the gravitational dynamics of galaxies.   Galaxies are modelled as a collection of gravitating particles that represent the stars and the mysterious dark matter. Particles are arranged in the same distribution and orbital configuration as the observed stars and presumed dark matter and their motion is calculated by determining the gravitational forces on each particle according to Newton's law of gravity and then solving the appropriate equations of motion for each particle.

The Collision and Merger of Spiral Galaxies

Spiral galaxies form in the early universe from the gravitational collapse of  clumps of dark matter and gas which grow out of primordial density fluctuations. Sometimes spirals form in bound pairs, or groups or even clusters and so even though they may have time to develop into a independent disks with spiral structure, they are destined to fall together and collide. The results are spectacular as gravity pulls out long tidal tails of stars during the first pass and leads to the ineluctable merger of the spirals into a single elliptical galaxy.  The merging process occurs very frequently in the early universe but less so today. Even so there are many nearby examples of merging galaxies.   Our own galaxy, the Milky Way, will most likely merge with its nearest neighbour Andromeda in 3 billion years.


Milky Way and Andromeda: Over the past several years, I have played around with a set of N-body models which describe one possible scenario of the future collision and merger of the Milky Way and Andromeda Galaxy.   The first models in the mid 90's incorporated about 20K particles  but that number grew to 107 million in 2000 in a demonstration simulation on Blue Horizon at the San Diego Supercomputer Centre.  

CITA's new computer Mckenzie has been used to do a new simulation using 153.6 million particles in each galaxy for a total of 307.2 million particles.  New rendering methods allow a true colour representation of the mixing of the old (red) bulge and young (blue) disk stellar populations as the galaxies merge. The unprecedented level of resolution also reveals extremely fine detail in the merger remnant.  This particular simulation was run using all 512 processors on Mckenzie over a period of 10 days.  The animation shows the evolution of the merger from 2 viewing directions with each sequence spanning about 2.5 billion years.  This simulation is being used as a test case to study the details of the merging process and its consequences for the fine structure in elliptical galaxies.


Watch a movie of the simulation of "The Mice" - a well-known interacting pair of galaxies. This simulation was done for National Geographic magazine and appeared in the Feb. 2003 issue. (File size is about 30 MB).

Galaxy Cluster

Galaxy Cluster

Spirals can also form in clusters of hundreds to thousands of galaxies.  As galaxies fall into a forming cluster, they interact strongly and merge to form a giant elliptical at the cluster centre and many other ellipticals surrounding the cluster.   Click on this image to fly through a galaxy cluster as it forms. Watch galaxies fall in and interact strongly, merge and disrupt as a giant elliptical galaxy grows in the centre(File size is about 50 MB)

Simulations of Cosmological Structure Formation

Cosmological Structure Formation:   All of the structure in the universe originates in the gravitational collapse of tiny density perturbations that are imprinted on the universe early in its history.  As the universe expands, these perturbations grow denser and collapse upon themselves to form galaxies and clusters of galaxies.  Cosmologists use N-body simulations to study this process.  Particles represent the dark matter distribution and fall into clumps that are commonly known as dark halos.  We can't observe these halos directly but we know of their presence through their gravitational influence on galaxies' rotational motions.

Click on this image to fly through the dark matter universe and watch the evolution of structure from the Big Bang to the present .  The small clumps are galaxy sized dark halos while the larger ones are clusters of galaxies. Look closely and you can see small halos orbiting within the larger ones. Time in years before the present ticks up on the left while the cosmological redshift ticks down on the right. (You may need DVD drivers to see this movie on a Windows/Apple machine - use xine or mplayer with Linux).

Click on the thumbnail images below in the sequence to see high resolution images from the Milky Way/Andromeda Merger sequence.

Sequence 1: Birds eye view - The Milkyway is viewed down the north galactic pole and comes in from the bottom to collide with the larger Andromeda Galaxy.

Sequence 2: Edge-on view - The Milkyway is viewed edge-on and comes in from the bottom again to collide with the larger Andromeda Galaxy.

These animations and images are copyright John Dubinski 2003