Lunch talk on Aug. 19, 2024

The role of angular momentum in dark matter formation and evolution

Speaker: Jie Li (University of Western Australia)

Venue: Video Conference

Time: 12:45 PM, Monday, Aug. 19, 2024

Abstract: 

Angular momentum (AM) is important in dark matter (DM) halo and galaxy dynamics, connecting gas and DM evolution within haloes, galaxy formation, and galactic properties. It is often assumed that gas and DM have identical specific AM (sAM) within virialised DM haloes. This assumption is widely used in analytical and semi-analytical models to derive the scale length of disc galaxies. However, cosmological hydrodynamical simulations have found that, on average, gas has more sAM than DM, and the sAM ratio between gas and DM has a large scatter. We use a non-radiative cosmological hydrodynamical simulation to investigate the spin distributions of gas and DM within virialized haloes exclusively under the influence of pressure. Our findings demonstrate that, statistically, the spin distribution of gas in the inner halo ( < 0. 6 of the virial radius) is greater than that of DM, suggesting that AM may be transferred from DM to gas. To further investigate this, we conduct a series of controlled simulations using the top-hat ellipsoidal collapse model and identify a mechanism through which DM transfers AM to gas during halo formation: asymmetric collapse. As a rotating, asymmetric proto-halo collapses, DM ellipsoid collapses more rapidly than the pressurized gas and spins ahead of the gas ellipsoid, generating torques and transferring AM to gas. This phenomenon can be observed in both cosmological and controlled simulations, and we develop an analytical model to link the results of the two types of simulations. On average, the asymmetric collapse can explain ~40% increase in the spin of gas.

Report PPT:  SWIFAR_Jie Li.pdf