A Side of Science: Modeling Galaxy Collisions

In my latest video, I talk about ring galaxies and how astronomers have found that they originate in collisions of just the right size galaxies coming together at just the right angles.

Lynds and Toomre, ApJ 209:382 (1976)

While I was researching that video, I found a paper that showed the results of a computer simulation of their formation from the 1970s – I was excited by these simulations because my Mom had gone into computer science in the 1970s, which felt like a very new career at the time. I was ready to write this Side of Science about those early computer models, but as I was searching for more information, I stumbled upon something even more remarkable.

I found a few references to a scientist named Erik Holmberg who modeled galaxy collisions in the 1940s (!) using an analog computer. So I looked up his paper, and thought I’d share some of what he did here.

  • Holmberg set out to examine whether or not it was possible for two galaxies passing close to each other to merge (or “result in a capture”).
  • To do this, he set up a laboratory experiment where he replaced gravity with light. He modeled two galaxies using 37 light bulbs in each. He then varied the voltages to those bulbs to model the distribution of mass in a galaxy.The central bulbs were tuned to a higher candle-power than the ones on the outer edges of the light bulb galaxies to mimic a higher density of mass at the center and lower mass at the edges.

    Holmberg’s galaxy analog – each circle represents a light bulb. (From Holmberg’s paper)
  • Each galaxy model had a diameter of 80 cm, and the total space used for the simulation was 300 x 400 cm (3 x 4 meters).
  • He moved the lamps to simulate rotating galaxies as they were also moving toward each other. And he did several trials varying the directions of the rotation of the galaxies, the initial velocities of the galaxies, and the closest approach of the galaxies.
  • To figure out the orbital elements of the lamps, he used a photocell – since light was being used as a stand-in for gravity, he could measure the light at each point with a photocell to estimate the acceleration each lamp would experience based on the amount of light in each direction (+x, -x, +y, -y).
  • The resulting orbits and lamp positions were drawn out on large sheets of graph paper.

    The beginning and end of one of Holmberg’s trials (from Holmberg’s paper).
  • Ultimately he concludes that enough energy can be lost in the interactions of these two “galaxies” that captures can occur. In other words, one the two galaxies can collide to become one.

Of course, these days we take it for granted that galaxies will collide and merge. But it took about 30 years after his work for computers to catch up and confirm his results.

I was particularly struck by how similar the end-point of one of his trials looks like some photos of interacting galaxies that I’ve seen.

Hubble image of M51, the Whirlpool galaxy, which is interacting with a neighbor galaxy.
End-point of one of Holmberg’s experiments (from Holmberg’s paper).







I often wonder how astronomers modeled systems before the advent of computers. What an ingenious solution!

And, in case you missed my ring galaxy video, be sure to check it out.


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