Colliding Galaxies in a Nutshell
Title: Colliding Galaxies in a Nutshell
SNIC Project: SNIC 2020/5-515
Project Type: SNIC Medium Compute
Principal Investigator: Nils Ryde <>
Affiliation: Lunds universitet
Duration: 2020-11-01 – 2021-11-01
Classification: 10305


Collisions pace the evolution of galaxies, but the richness of parameters (orbit, mass ratio, inclination, etc.) is such that every event is unique. One of the most spectacular collisions occurs when a small galaxy is totally destroyed by flying through by a bigger companion. The remnants of the small galaxy can then form fascinating shells, like the stunning example of Arp 227. What are the physical conditions in shells? Like other interacting galaxies, do shells also favor the formation of massive star clusters? Shell galaxies are relatively rare. Is it because the conditions of their formation are unlikely, or because they are short-lived and thus detectable during only a short period? This project will start by producing a numerical model of the galaxy Arp 227, run with the RAMSES code (already well tested and optimised on Tetralith). Two galaxies will be setup on a collision course susceptible to lead to the formation of shells. Tracing the hydrodynamics of the systems (with a Riemann solver) will allow use to explore the response of the interstellar medium to the interaction, and potentially probe the formation of massive star clusters, as suggested by recent observations. This will be the first time that hydrodynamics of shell galaxies are modeled. After having established the formation mechanism of the shells, we will use the early results to design follow-up simulations in which the orbital parameters of the two galaxies will be varied to explore their impact on the resulting system. The scientific strength of this project will be the comparisons with other interacting systems that we have already modeled and analyzed (see e.g. Renaud et al. 2019). The post-processing and the analysis of the simulation results will be performed on a small dedicated supercomputer in-house (in Lund). In particular, we will run diagnostics on the star (cluster) formation activity (rate, location, efficiency etc) and compare to that of other galaxies. This will complement on-going work led by our collaborators in Stockholm (PI: Ostlin, SNIC 2020/6-191). We will also produce mock observations (in the form of photometric maps) to assess the detectability of the faint shell structures with modern instruments. The same pipeline will be applied to all cases of our series of simulations (about 10), which will allow us to answer the questions above, and finally understand the importance of shells in the formation history of galaxies. Possibly, we will be able to assess if shells like those of Arp 227 have ever existed around our Milky Way. Part of this project will be conducted as part of a Master project.