# MIND THE GAP: Is The Too Big To Fail Problem Resolved?

## TL;DR

This paper investigates the 'too big to fail' problem in galaxy groups, showing that large luminosity gaps are consistent with gravitational merging effects and are not indicative of a failure in the $ ext{Lambda}$CDM model.

## Contribution

It demonstrates that large luminosity gaps are expected in $ ext{Lambda}$CDM simulations due to dynamical friction, resolving the 'too big to fail' issue.

## Key findings

- Large luminosity gaps are consistent with simulations and observations.
- Dynamical friction causes merging, explaining the gaps.
- No fundamental problem with $ ext{Lambda}$CDM regarding galaxy luminosity gaps.

## 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 (1977), 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 {\it given} the luminosity function, that the same large gap is found in modern, large scale baryonic $\Lambda$CDM simulations such as EAGLE and IllustrisTNG, is even greater in dark matter only simulations, and finally, that 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 $1.79 \pm 1.04$, $1.51 \pm 0.93$, $3.43 \pm 1.44$ and $3.33 \pm 1.35$ magnitudes 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 in the real data by over half a magnitude and are still larger in the dark matter only simulations. Briefly stated, $\Lambda$CDM 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.