The Formation of M101-alike Galaxies in the Cold Dark Matter Model
  The population of satellite galaxies in a host galaxy is a combination of the
cumulative accretion of subhaloes and their associated star formation
efficiencies, therefore, the luminosity distribution of satellites provides
valuable information of both dark matter properties and star formation physics.
Recently, the luminosity function of satellites in nearby Milky Way-mass
galaxies has been well measured to satellites as faint as Leo I with $M_{V}
\sim -8$. In addition to the finding of the diversity in the satellite
luminosity functions, it has been noticed that there is a big gap among the
magnitude of satellites in some host galaxies, such as M101, where the gap is
around 5 in magnitude, noticeably larger than the prediction from the halo
abundance matching method. The reason of this gap is still unknown. In this
paper, we use a semi-analytical model of galaxy formation, combined with
high-resolution N-body simulation, to investigate the probability and origin of
such big gap in M101-alike galaxies. We found that, although M101 analogues are
very rare with probability of \sim 0.1%-0.2% in the local universe, their
formation is a natural outcome of the CDM model. The gap in magnitude is mainly
due to the mass of the accreted subhaloes, not from the stochastic star
formation in them. We also found that the gap is correlated with the total
satellite mass and host halo mass. By tracing the formation history of M101
type galaxies, we find that they likely formed after $z \sim 1$ due to the
newly accreted bright satellites. The gap is not in a stable state, and it will
disappear in ~7 Gyr due to mergers of bright satellites with the central
galaxy.
