What you see
The two galaxies begin apart. As they approach, the first close pass stretches their disks. Stars and gas are drawn out between the bright centers, and those centers swing past each other before falling back together.
By the end there is one galaxy: a bright center, with an irregular spread of light around it. The pale glow is starlight from the disks. Darker streaks across that light are dust in front of it. The film plays the encounter from the approach through the merger, then repeats.
How the simulation works
Gravity and gas are computed while the encounter runs. The stellar disks, bulges, and halos are particles under direct softened gravity, moved with a velocity Verlet integrator. The gas is smoothed-particle hydrodynamics. It cools, forms stars where the density is high enough, and returns thermal feedback. Gravitational mass is conserved. New stars are tracked as cohorts, and they remain on the gas particle that formed them.
The spiral arms begin as seeded density perturbations. After that, only gravity and the gas move them. No extra force is left in place to hold the arms.
The picture is a separate rendering of those same particles. It adds stellar light, foreground dust, blue light from young populations, and illustrative H-alpha and [O III] emission. Exposure and bloom change the image only. Emission color, brightness, and fading time are adjustable stand-ins, not a photoionization or stellar-population calculation. Blue light and oxygen emission are controlled separately.
The run uses code units. It illustrates this model. It is not a calibrated prediction for a particular pair of galaxies. The particle count and the softening length limit the small-scale structure that can appear.
Source
The viewer, scenes, and notes are in the Galaxy Merger folder of the Scientific Visualizations repository.
Galaxy Merger on GitHub →