Abstract:
We study the abundance, radial distribution, and orbits of satellites in Milky Way (MW)-mass halos formed in the LCDM cosmology. We follow the evolution of a dark halo with the Aquarius simulations and the evolution of the luminous galaxies with the GALFORM semi-analytic model of galaxy formation, assuming that luminous galaxies form only in halos that exceed a redshift-dependent "critical" virial mass close to the hydrogen-cooling limit (Mcrit ~ 5 x 10^9 Msun at z = 0). We find that even in the highest resolution simulation (Aq-L1, with particle mass mp ~ 10^3 Msun), a significant number of luminous subhalos are artificially disrupted in the main halo tidal field, giving rise to a population of "orphan" satellites tracked by GALFORM. Simulations able to resolve the critical mass yield a converged satellite luminosity function across different resolution levels. They also indicate that roughly half of all luminous satellites are orphans, even in Aq-L1. Most orphans are located in the central regions of the halo, resulting in a concentrated radial distribution that is consistent with the observed MW data without resorting to luminous galaxies populating subhalos below the hydrogen-cooling limit, as argued in recent work. We find that a satellite’s infall time largely determines whether it survives to the present or becomes an orphan. Orphans dominate the ultra-faint population, as well as the population of satellites with small pericentric orbital radii. Finally, we model the tidal stripping of stars, and estimate the final z = 0 stellar mass and half-mass radii of satellites. We predict that there should be dozens of luminous satellites at small distances from the MW’s centre which should be detectable in deep wide-field surveys.
