Mind the Gap: Is the Too Big to Fail Problem Resolved?

Abstract

The faintness of satellite systems in galaxy groups has contributed to the widely discussed “missing satellite” and “too big to fail” issues. Using techniques based on Tremaine & Richstone, we show that there is no problem with the luminosity function computed from modern codes per se, but that the gap between first and second-brightest systems is too big given the luminosity function, and the same large gap found in modern, large-scale baryonic ΛCDM simulations such as EAGLE and IllustrisTNG, is even greater in dark-matter-only simulations. Additionally, this is most likely due to gravitationally induced merging caused by classical dynamical friction. Quantitatively, the gap is larger in the computed simulations than in the randomized ones by 1.79 ± 1.04, 1.51 ± 0.93, 3.43 ± 1.44, and 3.33 ± 1.35 mag in the EAGLE, IllustrisTNG, and dark-matter-only simulations of EAGLE and IllustrisTNG, respectively. Furthermore, the anomalous gaps in the simulated systems are even larger than those in the real data by over half a magnitude and are still larger in the dark-matter-only simulations. Briefly stated, ΛCDM does not have a problem with an absence of “too big to fail” galaxies. Statistically significant large gaps between first and second-brightest galaxies are to be expected.