How to resuscitate a dead galaxy?
Title: How to resuscitate a dead galaxy?
SNIC Project: SNIC 2019/3-533
Project Type: SNIC Medium Compute
Principal Investigator: Nils Ryde <>
Affiliation: Lunds universitet
Duration: 2019-11-01 – 2020-11-01
Classification: 10305


Understanding the origin of galaxies in general, and of our Milky Way in particular, requires to describe how the gas produced during the Big Bang is converted into stars. A growing number of observations report that this process (rate, efficiency etc.) is strongly dependent on the galactic environment. For instance, elliptical galaxies are known to be deprived of gas, and thus to have ceased to form stars: they are often called "dead galaxies". However, new observations have reported a significant star formation activity in some ellipticals, which thus questions our current understanding of star formation and galaxy evolution. A possible explanation is that when an elliptical interacts with another galaxy, in theory it could gravitationally capture the gas of the companion gas and could thus "resuscitate" the star formation activity, which would explain the observations. If valid, this idea would drastically change our perspective on the very process of star formation. However, this process has never been modeled nor quantified and it is still uncertain that it could account for the entire activity reported. The proposed project aims, for the first time, at modeling numerically the gaseous fueling of a non star forming galaxy, and the restart of its star formation activity. By monitoring a large range of physical processes (flows, turbulence, cooling, radiative transfer, gravitational collapse etc.) at high resolution, we will propose a holistic scenario for the mechanism and test our hypotheses against observational data. We will run hydrodynamical simulations of interacting galaxies, in addition to control runs of galaxies in isolation. We will compare the evolution of the star formation rates and efficiencies in these two sets. We will start with relatively medium resolution simulations to first get a better understanding of the process, and then choose the best set of parameters (galaxy mass, size, orbit of the encounter, etc.) to run high resolution simulations that will fully capture the interplay of the physical processes involved. The final outcomes of this project will be (i) an understanding the details of the physical process, (ii) the finding of which model best matches the observations, (iii) a holistic theory on the topic, (iv) predictions to prepare new observation campaigns and (v) outreach material.