Where Are the Missing Galactic Satellites?

According to the hierarchical clustering scenario, galaxies are assembled by merging and accretion of numerous satellites of different sizes and masses. This ongoing process is not 100% efficient in destroying all of the accreted satellites, as evidenced by the satellites of our Galaxy and of M31. Using published data, we have compiled the circular velocity (V_circ) distribution function (VDF) of galaxy satellites in the Local Group. We find that within the volumes of radius of 570 kpc (400 h^-1 kpc assuming the Hubble constant h = 0.7) centered on the Milky Way and Andromeda, the average VDF is roughly approximated as n(> V_circ) ≈ 55 ± 11(V_circ/10 km s^-1)^-1.4±0.4 h³ Mpc^-3 for V_circ in the range ≈10-70 km s^-1. The observed VDF is compared with results of high-resolution cosmological simulations. We find that the VDF in models is very different from the observed one: n(> V_circ) ≈ 1200(V_circ/10 km s^-1)^-2.75 h³ Mpc^-3. Cosmological models thus predict that a halo the size of our Galaxy should have about 50 dark matter satellites with circular velocity greater than 20 km s^-1 and mass greater than 3 × 10⁸ M_☉ within a 570 kpc radius. This number is significantly higher than the approximately dozen satellites actually observed around our Galaxy. The difference is even larger if we consider the abundance of satellites in simulated galaxy groups similar to the Local Group. The models predict ~300 satellites inside a 1.5 Mpc radius, while only ~40 satellites are observed in the Local Group. The observed and predicted VDFs cross at ≈50 km s^-1, indicating that the predicted abundance of satellites with V_circ ≳ 50 km s^-1 is in reasonably good agreement with observations. We conclude, therefore, that unless a large fraction of the Local Group satellites has been missed in observations, there is a dramatic discrepancy between observations and hierarchical models, regardless of the model parameters. We discuss several possible explanations for this discrepancy including identification of some satellites with the high-velocity clouds observed in the Local Group and the existence of dark satellites that failed to accrete gas and form stars either because of the expulsion of gas in the supernovae-driven winds or because of gas heating by the intergalactic ionizing background.