# The Galaxy Clustering Crisis in Abundance Matching

## TL;DR

This paper examines the limitations of mass-based sub-halo abundance matching models in predicting galaxy clustering, highlighting the superiority of velocity-based models and exploring potential remedies and their shortcomings.

## Contribution

The study critically evaluates the failure of $M_{peak}$ based SHAM models and discusses possible solutions, emphasizing the need for additional components or alternative predictors.

## Key findings

- $V_{peak}$ models better predict galaxy clustering than $M_{peak}$ models.
- Mass-based models require orphan galaxies or post-accretion growth to match observations.
- Popular $M_{peak}$ models are incomplete and cannot fully explain galaxy clustering phenomena.

## Abstract

Galaxy clustering on small scales is significantly under-predicted by sub-halo abundance matching (SHAM) models that populate (sub-)haloes with galaxies based on peak halo mass, $M_{\rm peak}$. SHAM models based on the peak maximum circular velocity, $V_{\rm peak}$, have had much better success. The primary reason $M_{\rm peak}$ based models fail is the relatively low abundance of satellite galaxies produced in these models compared to those based on $V_{\rm peak}$. Despite success in predicting clustering, a simple $V_{\rm peak}$ based SHAM model results in predictions for galaxy growth that are at odds with observations. We evaluate three possible remedies that could "save" mass-based SHAM: (1) SHAM models require a significant population of "orphan" galaxies as a result of artificial disruption/merging of sub-haloes in modern high resolution dark matter simulations; (2) satellites must grow significantly after their accretion; and (3) stellar mass is significantly affected by halo assembly history. No solution is entirely satisfactory. However, regardless of the particulars, we show that popular SHAM models based on $M_{\rm peak}$ cannot be complete physical models as presented. Either $V_{\rm peak}$ truly is a better predictor of stellar mass at $z\sim 0$ and it remains to be seen how the correlation between stellar mass and $V_{\rm peak}$ comes about, or SHAM models are missing vital component(s) that significantly affect galaxy clustering.