Interacting Galaxies on a Laptop

Ever wonder how a crotchety, old retired astronomer might fill some spare time? A few months ago I decided to try recreating one of the experiments on Chris Mihos' neat JavaLab site, now sadly offline. The experiment was titled GalaxyCrash - an interactive simulation of two disks in collision.

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The video was created using Python, running on my Win 10 laptop; it spans 5 Gyr of actual time.  Each disk contains several hundred stars.  Colours change at the same (arbitrary) potential energy per unit mass in both disks.  Motions in each disk are Keplerian at first.  Stars interact only with the massive centres of the two galaxies, shown in white.  Fully n-body sims are, ironically, easier to create but rather sluggish - surprise!  Stars near the centre of their disk are occasionally ejected, due I think to the size of the time step, and perhaps also to the softening length added to the Law of Gravitation.

For the future: Include DM halos? Explore more parameter space? Try to reproduce a few of Halton Arp's menagerie?

Despite largely achieving my goal I consider myself anything but a Python expert.  One unsolved problem: I can use the touchpad to change the viewing angle while the sim is running - a cool feature of Python 3D animations!  Annoyingly, though, I cannot (yet) preserve those changes in the saved video file.  Growl...

Gary Welch

Professor Emeritus, Department of Astronomy & Physics, Saint Mary's University, Halifax, Nova Scotia

There is life after professional astronomy.  But not after Astronomy!

Since retiring I've learned to play Bach, slowly, on our old piano, continued my passion for Challenge Square Dancing (oh yeah!), and with my wife have camped from Nova Scotia to Alaska and back twice in our Aliner trailer.   Most recently there's been the "Python Project", one of whose fruits I've posted.  I'm also an instructor in the Seniors' College Association of Nova Scotia, a group of willing "experts" from all fields who offer non-credit courses to the mostly over-50 crowd.  What a treat it's been to explore astronomy with interested, intelligent and questioning people!