Local Group star formation in warm and self-interacting dark matter cosmologies

The nature of the dark matter can affect the collapse time of dark matter haloes, and can therefore be imprinted in observables such as the stellar population ages and star formation histories of dwarf galaxies. In this paper we use high resolution hydrodynamical simulations of Local Group-analogue (LG) volumes in cold dark matter (CDM), sterile neutrino warm dark matter (WDM) and self-interacting dark matter (SIDM) models with the EAGLE galaxy formation code to study how galaxy formation times change with dark matter model. We are able to identify the same haloes in different simulations, since they share the same initial density field phases. We show that the stellar mass varies systematically with resolution by over a factor of two, in a manner that depends on the final stellar mass. The evolution of the stellar populations in SIDM is largely identical to that of CDM, but in WDM early star formation is instead suppressed. The time at which LG haloes can begin to form stars through atomic cooling is delayed by $\sim$200~Myr in WDM models compared to CDM. 70~per~cent of WDM haloes of mass $>10^{8}M_{\odot}$ collapse early enough to form stars before $z=6$, compared to 90~per~cent of CDM and SIDM galaxies. It will be necessary to measure stellar ages of old populations to a precision of better than 100~Myr, and to address degeneracies with the redshift of reionization, in order to use these observables to distinguish between dark matter models.