Bailing out the Milky Way: variation in the properties of massive dwarfs among galaxy-sized systems

Abstract

Recent kinematical constraints on the internal densities of the Milky Way's dwarf satellites have revealed a discrepancy with the subhalo populations of simulated Galaxy-scale halos in the standard cold dark matter model of hierarchical structure formation. In particular, the Via Lactea II and Aquarius simulations both have large subhalos with internal densities that are larger than the constraints inferred for any Milky Way dwarf satellites. This has been dubbed the ``too big to fail'' problem, with reference to the improbability of large and invisible companions existing in the Galactic environment.In this paper, we argue that both the Milky Way observations and simulated subhalos are consistent with the predictions of the standard model for structure formation. Specifically, we show that there is significant variation in the properties of subhalos among distinct host halos of fixed mass and suggest that this can reasonably account for the deficit of dense satellites in the Milky Way. We exploit well-tested analytic techniques to predict the properties in a large sample of distinct host halos with a variety of masses spanning the range expected of the Galactic halo. Such techniques render the problem of estimating the variance in subhalo properties computationally feasible. The analytic model produces subhalo populations consistent with both Via Lactea II and Aquarius, and our results suggest that natural variation in subhalo properties suffices to explain the discrepancy between Milky Way satellite kinematics and these numerical simulations. At least ∼ 10% of Milky Way-sized halos host subhalo populations for which there is no ``too big to fail'' problem, even when the host halo mass is as large as Mhost = 1012.2 h−1 M⊙. Follow-up studies consisting of high-resolution simulations of a large number of Milky Way-sized hosts are necessary to confirm our predictions. In the absence of such efforts, the ``too big to fail'' problem does not appear to be a significant challenge to the standard model of cold dark matter halos undergoing hierarchical formation.