The gap of stellar mass in galaxy groups: another perspective of the
  Too-big-To-Fail problem in the Milky Way
  The Milky Way presents the too-big-to-fail (TBTF) problem that there are two
observed satellite galaxies with maximum circular velocity larger than 55km/s,
and others have velocity less than 25km/s, but the cold dark matter model
predicts there should be more than 10 subhaloes with velocity larger than
25km/s. Those massive subhaloes with $25km/s<V_{max}<55km/s$ should not have
failed to form stars. The TBTF problem severely challenges the CDM model. Most
efforts are seeking the effects of baryonic feedback, decreasing the mass of
the Milky Way, changing the properties of dark matter, so as to assign the
observed low-velocity satellites into the massive subhaloes found in
simulations. However, the TBTF problem can be avoided if the MW have not
accreted subhaloes with velocity between $25km/s<V_{max}<55km/s$ although the
probability of such a gap is lower as $\sim 1\%$ and can not be tested against
observations. In this work we study the gap in stellar mass of satellite
galaxies using the SDSS group catalogue and a semi-analytical model. We find
that there are 1-2\% of galaxy groups with a large gap in the stellar mass of
their satellites. These 'big gap' groups have accreted less massive subhaloes
in their formation history and naturally display a gap between their satellite
galaxies. If extrapolating our results to the Milky Way is appropriate, we
conclude that it is very likely that our Milky Way has not accreted enough
massive subhaloes to host those low-velocity satellites, and the TBTF problem
is naturally avoided.
