Luminous Satellites versus Dark Subhaloes: Clustering in the Milky Way

TL;DR

This study investigates the clustering of Milky Way satellite galaxies compared to simulated dark matter subhaloes, revealing discrepancies that challenge current Cold Dark Matter models on small scales.

Contribution

It introduces a novel clustering analysis of Milky Way satellites and compares it with simulated subhaloes, highlighting potential tensions with Cold Dark Matter theory.

Findings

No significant substructure detected in Milky Way satellites.

Simulated luminous subhaloes are more strongly clustered than observed satellites.

Discrepancies suggest a potential small-scale tension with Cold Dark Matter models.

Abstract

The observed population of the Milky Way satellite galaxies offer a unique testing ground for galaxy formation theory on small-scales. Our novel approach was to investigate the clustering of the known Milky Way satellite galaxies and to quantify the amount of substructure within their distribution using a two-point correlation function statistic in each of three spaces: configuration space, line-of-sight velocity space, and four-dimensional phase-space. These results were compared to those for three sets of subhaloes in the Via Lactea II Cold Dark Matter simulation defined to represent the luminous dwarfs. We found no evidence at a significance level above 2-sigma of substructure within the distribution of the Milky Way satellite galaxies in any of the three spaces. The "luminous" subhalo sets are more strongly clustered than are the Milky Way satellites in all three spaces and over a broader range of scales in four-dimensional phase-space. Each of the "luminous" subhalo sets are clustered as a result of substructure within their line-of-sight velocity space distributions at greater than 3-sigma significance, whereas the Milky Way satellite galaxies are randomly distributed in line-of-sight velocity space. While our comparison is with only one Cold Dark Matter simulation, the inconsistencies between the Milky Way satellite galaxies and the Via Lactea II subhalo sets for all clustering methods suggest a potential new 'small-scale' tension between Cold Dark Matter theory and the observed Milky Way satellites. Future work will obtain a more robust comparison between the observed Milky Way satellites and Cold Dark Matter theory by studying additional simulations.