A Stellar Framework for Galactic Disk Evolution
A trio of scholars from the School of Natural Sciences, John N. Bahcall Fellow Chris Hamilton (2021–26), visiting graduate student Shaunak Modak (2023–26), and Professor Emeritus Scott Tremaine, have developed a new theoretical framework for understanding galactic disks.
In recent years, scientists’ capability to observe galactic disks has significantly improved, both within our own Milky Way galaxy through the Gaia space telescope, and in external galaxies using the James Webb Space Telescope. These amazingly precise observations demand an equally sophisticated theory if they are to be understood.
“Such theories do exist, but they are so complicated that connecting them with the observations is difficult,” explains Hamilton. To make these theories easier to use, he and his colleagues have developed an approximation scheme which drastically simplifies the mathematical formulae without compromising their accuracy. “Galactic disks evolve due to gravitational perturbations that occur on many different length scales,” continues Hamilton. “Previous theories have tended to treat all of these perturbations using one very complicated formula. The key advance of our work is to split these perturbations up into different wavelength categories (long, intermediate, and short), and then to treat each category separately.”
The insight for this paper came from an analog with plasma physics of the kind that Hamilton described in “Dynamics in Translation,” an article published in the Fall 2023 issue of The Institute Letter. “In a magnetized plasma, electrons revolve around the magnetic field lines in tiny circles, like the orbits of stars within a galactic disk,” says Hamilton. “The orbits of the electrons change over time because of electromagnetic perturbations of different length scales. We used exactly this idea to describe how stars’ orbits evolve under gravitational perturbations in a galactic disk.” In plasma physics, the mathematical framework used to understand the electrons’ orbits is called “gyrokinetics.” In honor of the connection that they have established, Hamilton, Modak, and Tremaine have titled their paper “Galactokinetics.”
