Tailored Galaxy Simulations

Galaxy simulations are some of the coolest computational projects in astrophysics.  Cosmological simulations are incredibly informative and produce some of the most beautiful movies in science.  They show us how the Universe formed over time and have been used to figure out why the various large scale structures that we see (galaxies, clusters, super-clusters, filaments, voids, the cosmic web, etc.) came to be.  But, as great as they are, there are certain tasks and problems that need a different approach.  For instance, it can be difficult to model a specific individual galaxy, or to compare the effect of a something like the bulge/disk ratio while keeping the halo constant.

That’s where tailored simulations come in.  They allow a user to build a galaxy with a specified density profile.  Once built, the galaxy can be evolved either alone or in some pair in order to investigate interesting problems.

The evolved gas disk of a galaxy built by GalactICS

A lot of my recent work has been with Prof. Widrow and Prof. Carignan to rebuild and utilize a particular code for making initial conditions called GalactICS.  This code builds galaxies with up to five components; a bulge, two stellar disks, a gas disk, and a dark matter halo.

The results of a Milky Way simulation.  The panels show the surface density (X-Y) and cross-sectional density (X-Z) for the disk components and the surface density of the bulge in both panels.

I’ve used this code for a number of applications.  One of the more fun and interesting applications is building a Milky Way model.  For this exercise I adapted the best model found in McMillan (2017) and evolved it for 2 Gyr.  After one Gyr, the galaxy developed a bar and  spiral arms as seen in the actual galaxy.  The bar itself actually has the right strength, although the bending angles aren’t quite right.  One of the most interesting things is that the bar and spiral structure actually differs across the various components!

The mock image (left) and velocity profile (right) of the gas disk of a single galaxy in an interacting galaxy pair.  The gif shows how the galaxy would look as an observer moves around the thing.  It only shows the gas disk of one galaxy, but the cloud of particles at the end of one of the streams have actually been captured by the second galaxy (which I’ll emphasize is not being shown here).

Another fun project that I’ve used GalactICS for is studying interacting galaxies.  By making pairs of galaxies and throwing them at each other, I’ve been able to study how the orbital parameters affect the details of the merger.  In particular, I’ve been looking at ways of quantifying how non-symmetric the galaxy is and how the way we view the galaxy affects the measurement of this asymmetry.  It turns out that the viewing angle is vitally important and can, in certain circumstances, hide the disruption caused by the interaction!

Of course, these are not the only projects that I have worked on.  I’ve been involved with modeling real galaxies, figuring out if some features seen in dwarf galaxies are from a particular interaction and more.  Recently, I’ve even worked with Prof. Jarrett and the visualization team in IDIA to load some of the simulations into VR!  While we don’t have the full results yet, it has been exciting to be able to walk around inside two galaxies crashing together and look at them easily from all perspectives.

Even though tailored simulations might seem like worse versions of cosmological simulations, nothing could be further from the truth.  They are powerful tools that are complementary to those amazing cosmological and zoom-in simulations.  Those simulations are certainly much more realistic, but tailored simulations allow for more controlled experiments where the user knows exactly what the individual galaxies start from.  The key, as with most things, is using the right tool for the job at hand.

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