The chosen few: the low-mass haloes that host faint galaxies

Since reionization prevents star formation in most haloes less massive than 3 10 9 M , dwarf galaxies only populate a fraction of existing dark matter haloes. We use hydrodynamic cosmological simulations of the Local Group to study the discriminating factors for galaxy formation in the early Universe and connect them to the present-day properties of galaxies and haloes. A combination of selection effects related to reionization, and the subsequent evolution of haloes in different environments, introduces strong biases between the population of haloes that host dwarf galaxies, and the total halo population. Haloes that host galaxies formed earlier and are more concentrated. In addition, haloes more affected by tidal stripping are more likely to host a galaxy for a given mass or maximum circular velocity, v max , today. Consequently, satellite haloes are populated more frequently than field haloes, and satellite haloes of 10 8 -10 9 M or v max of 12-20 km s -1 , compatible with stellar kinematics of Local Group dwarf spheroidals, have experienced a greater than average reduction in both mass and v max after infall. They are on closer, more radial orbits with higher infall velocities and earlier infall times. Together, these effects make dwarf galaxies highly biased tracers of the underlying dark matter distribution.