The galaxies before the merger (64 kb)
Text adapted from "Scientific and IMAX Technology Expertise Team in Galaxy Collision Project" by Mike Gannis, San Diego Supercomputing Center
Chris Mihos and Lars Hernquist of the University of California, Santa Cruz, are applying the capabilities of SDSC's Cray C90 to model complex physical processes involved in galactic encounters. These galactic simulations, the largest performed by anyone to date, require extremely high spatial and mass resolution to follow the dynamical evolution of merging disk galaxies and the consequent inflow of gas and onset of starburst activity. The combination of coupled, non-linear physical processes and a large size scale required the use of supercomputing technology to ensure accurate mathematical modeling. The simulated encounter evolved over several thousand time steps. Running on the Cray C90, the IMAX simulation required 750 CPU hours to complete and generated 65 GB of raw data.
In the simulations at SDSC, the galaxies were represented by swarms of self-gravitating particles; the IMAX project used over 250,000 particles to represent the galaxies. Mihos and Hernquist used an N-body simulation package to model the gravitational interplay of the particles and a technique called smoothed particle hydrodynamics to make local estimates of the pressure, density, and temperature of interstellar gas. The position of each particle was calculated at sequential time intervals according to the gravitational and hydrodynamic forces acting on it. The rate of star formation in the gas is related to the gas density and was modeled using a formula derived from observations of nearby galaxies.
The sequence follows the collision of two spiral galaxies consisting of stars and gas. The galaxies interpenetrate in the first phase of the encounter and then draw apart. Tidal forces and galactic rotation cause the galaxies to cast off stars and gas in the form of long, thin "tidal tails" similar to those observed in the collision of real galaxy pairs. Compression of interstellar gas causes enhanced star formation. As the galaxies pass by one another, their disks become distorted, forming prominent bars and spiral features that strongly alter the motions of gas clouds. At the same time, the mutual gravitational attraction of the galaxies swings them around and causes them to fall back and merge into a single object approximately 500 million years after the first collision.
As the galaxies coalesce, gravitational and hydrodynamic forces drive most of the interstellar gas into the core of the merged remnant, fueling an intense burst of star formation. This starburst quickly converts the dense gas at the core into stars and burns itself out in only 50 million years or so. After the burnout, the main body of the merger remnant resembles an elliptical galaxy, providing strong evidence that some elliptical galaxies may have formed through galactic mergers. Meanwhile, the gas in the extended tidal tails cools and collapses into self-gravitating, star-forming clumps, which may account for the formation of the young dwarf galaxies seen surrounding many larger galaxies (including the Milky Way). Many smaller condensations of gas also form clumps of stars, perhaps explaining how globular clusters arise.
The following images are preliminary renders of the galaxy merger sequences from the IMAX film. The colors are designed to allow the viewer to make distinctions between the two galaxies while giving a realistic feel. In the real universe, the two galaxies would look very similar.
Credits: Chris Mihos, Lars Hernquist (Santa Cruz), Donna Cox, Robert Patterson, Erik Wesselak, and Barry Sanders (NCSA). The rendering program was written by Loren Carpenter at PIXAR.
NOTE: There are subtle details in these images, especially in the gas. For best results, try changing the intensity curve of the color display to suit your monitor. For those familiar with the jargon, play with the gamma value. In the program "xv", use the "ColEdit" button.
The galaxies before the merger (64 kb)
The galaxies during the merger (81 kb)
The galaxies just after the merger (85 kb)