Mass assembly history and infall time of the Fornax dwarf spheroidal galaxy

We use cosmological simulations to identify dark matter subhalo host candidates of the Fornax dwarf spheroidal galaxy using the stellar kinematic properties of Fornax. We consider cold dark matter (CDM), warm dark matter (WDM), and decaying dark matter (DDM) simulations for our models of structure formation. The subhalo candidates in CDM typically have smaller mass and higher concentrations at z = 0 than the corresponding candidates in WDM and DDM. We examine the formation histories of the ∼100 Fornax candidate subhaloes identified in CDM simulations and, using approximate luminosity–mass relationships for subhaloes, we find two of these subhaloes that are consistent with both the Fornax luminosity and kinematics. These two subhaloes have a peak mass over 10 times larger than their z = 0 mass. We suggest that in CDM the dark matter halo hosting Fornax must have been severely stripped of mass and that it had an infall time into the Milky Way of ∼9 Gyr ago. In WDM, we find that candidate subhaloes consistent with the properties of Fornax have a similar infall time and a similar degree of mass-loss, while in DDM we find a later infall time of ∼3–4 Gyr ago and significantly less mass-loss. We discuss these results in the context of the Fornax star formation history, and show that these predicted subhalo infall times can be linked to different star formation quenching mechanisms. This emphasizes the links between the properties of the dark matter and the mechanisms that drive galaxy evolution.