Environmental Effect on the Subhalo Abundance - a Solution to the Missing Dwarf Problem

Recent high-resolution simulations of the formation of dark-matter halos have shown that the distribution of subhalos is scale-free, in the sense that if scaled by the velocity dispersion of the parent halo, the subhalo velocity distribution functions of galaxy-sized and cluster-sized halos are identical. For cluster-sized halos, the simulation results agreed well with the observations. The simulations, however, predicted far too many subhalos for galaxy-sized halos. Our galaxy has several tens of known dwarf galaxies. On the other hand, simulated dark-matter halos contain thousands of subhalos. We performed a simulation of a single large volume, and measured the abundance of subhalos in all massive halos. We found that the variation of the subhalo abundance is very large, and those with the largest number of subhalos correspond to the simulated halos in previous studies. The subhalo abundance depends strongly on the local density of the background. Halos in high-density regions contain a large number of subhalos. Our Galaxy is in the low-density region. For our simulated halos in low-density regions, the number of subhalos is within a factor of four to that of our Galaxy. We argue that the missing dwarf problem is not a real problem, but is caused by biased selections of the initial conditions in previous studies, which were not appropriate for field galaxies.