The gap of stellar mass in galaxy groups: another perspective of the too-big-to-fail problem in the Milky Way

The Milky Way (MW) presents the too-big-to-fail (TBTF) problem that there are two observed satellite galaxies with maximum circular velocity larger than 55 km s(-1), and others have velocity less than 25 km s(-1), but the cold dark matter (CDM) model predicts that there should be more than 10 subhaloes with velocity larger than 25 km s(-1). Those massive subhaloes with 25 km s(-1) < V-max < 55 km s(-1) 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 MW, 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 has not accreted subhaloes with velocity within 25 < V-max < 55 km s(-1) although the probability of such a gap is lower as similar to 1 per cent and cannot be tested against observations. In this work, we study the gap in stellar mass of satellite galaxies using the Sloan Digital Sky Survey group catalogue and a semi-analytical model. We find that there are 1-2 per cent 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 MW is appropriate, we conclude that it is very likely that our MW has not accreted enough massive subhaloes to host those low-velocity satellites, and the TBTF problem is naturally avoided.