The segregation of baryons and dark matter during halo assembly

TL;DR

This study uses non-radiative hydrodynamical simulations to show that baryons and dark matter can become segregated during halo formation, challenging the assumption of initial mixing in galaxy formation models.

Contribution

It demonstrates that baryons and dark matter can be initially segregated due to different physics, affecting angular momentum and challenging standard galaxy formation assumptions.

Findings

Approximately 25% of baryons and dark matter are segregated initially.

Baryons and dark matter often have misaligned angular momentum vectors.

Segregation persists during halo assembly, impacting galaxy formation models.

Abstract

The standard galaxy formation theory assumes that baryons and dark matter are initially well-mixed before becoming segregated due to radiative cooling. We use non-radiative hydrodynamical simulations to explicitly examine this assumption and find that baryons and dark matter can also be segregated because of different physics obeyed by gas and dark matter during the build-up of the halo. As a result, baryons in many haloes do not originate from the same Lagrangian region as the dark matter. When using the fraction of corresponding dark matter and gas particles in the initial conditions (the "paired fraction") as a proxy of the dark matter and gas segregation strength of a halo, on average about $25$ percent of the baryonic and dark matter of the final halo are segregated in the initial conditions. This is at odds with the assumption of the standard galaxy formation model. A consequence of this effect is that the baryons and dark matter of the same halo initially experience different tidal torques and thus their angular momentum vectors are often misaligned. The degree of the misalignment is largely preserved during later halo assembly and can be understood with the tidal torque theory. The result challenges the precision of some semi-analytical approaches which utilize dark matter halo merger trees to infer properties of gas associated to dark matter haloes.