Origins of galactic discs

NAISS 2023/5-144


NAISS Medium Compute

Principal Investigator:

Oscar Agertz


Lunds universitet

Start Date:


End Date:


Primary Classification:

10305: Astronomy, Astrophysics and Cosmology



The origin of galaxies is still an unsolved problem in modern astrophysics. Galaxies come in many shapes and sizes, from tiny dwarf galaxies orbiting spiral galaxies like our own Milky Way, to giant elliptical galaxies situated in the centers of entire cluster of galaxies. The nature of disc galaxies has been found to be complex; the Milky Way is known to feature not only one disc of stars, but possibly two, with one being almost three times thicker than the other. Precisely how distinct these discs are, i.e wether they are well separated entities or a matter of gradual thickening, is currently debated. Regardless, such geometric separations of thick and thin discs has also been observed for external galaxies, and seems to be an almost generic outcome of the galaxy formation process. It is today well established that at least in our Galaxy, the thick disc of stars is significantly older (billions of years) than the thin disc, in addition to being chemically distinct, having a high abundance of so called ‘alpha elements’ produced in supernovae type II explosions (e.g. O, Ne, Mg, Si, Ti etc.), as well as being kinematically distinct, with older stars moving at higher vertical velocities through the plane of the disc. These features are clues to the formation history of our Galaxy, and observational data keeps improving the above picture, thanks to e.g. current and future missions such as GAIA and 4MOST. Several physical mechanisms are thought to contribute, albeit with no consensus on which matter the most. Thanks to the previous allocation 2022/5-136 (Medium Compute) + associated storage allocation 2022/6-75 , we have several papers in preparation including researchers and students in Lund. This work needs to continue by running more numerical simulations with a more varied set of galaxy formation physics. In this project I will use the state-of-the-art hydrodynamical + N-body code RAMSES to model entire disc galaxies, consisting of stars, gas and dark matter, representing different stages of the Milky Way’s past. I plan to model at least 10 galaxies, with varying formation histories, at state-of-the-art resolution, each for several billion years in simulation time. As a result of my simulations I will determine (1) if gravitational scattering in current day galactic conditions can explain the observed kinematic heating of the Milky Way’s young disc, (2) if this thickening in the early Universe gives rise to a distinct thick disc, (3) wether the contribution from star formation in turbulent gas can explain thick discs, and (4) the role of radial migration of stars (5) the life-cycle of gas in the interstellar medium For the last item on the list, a new technology called "tracer particles", which is developed by a postdoc in Lund (Corentin Cadiou), will be used.