# Probing dark matter with star clusters: a dark matter core in the ultra-faint dwarf Eridanus II

## TL;DR

This paper introduces a new method to determine the dark matter density profile in galaxies by analyzing star cluster survival, applying it to Eridanus II, and finding evidence for a dark matter core.

## Contribution

The study develops a novel technique combining star cluster observations and simulations to distinguish between dark matter cores and cusps in dwarf galaxies.

## Key findings

- Eri II's star cluster is best explained by a dark matter core.
- A dense cusp would require implausible cluster orbits and positions.
- Results support the presence of a dark matter core in Eri II.

## Abstract

We present a new technique to probe the central dark matter (DM) density profile of galaxies that harnesses both the survival and observed properties of star clusters. As a first application, we apply our method to the `ultra-faint' dwarf Eridanus II (Eri II) that has a lone star cluster ~45 pc from its centre. Using a grid of collisional $N$-body simulations, incorporating the effects of stellar evolution, external tides and dynamical friction, we show that a DM core for Eri II naturally reproduces the size and the projected position of its star cluster. By contrast, a dense cusped galaxy requires the cluster to lie implausibly far from the centre of Eri II (>1 kpc), with a high inclination orbit that must be observed at a particular orbital phase. Our results, therefore, favour a dark matter core. This implies that either a cold DM cusp was `heated up' at the centre of Eri II by bursty star formation, or we are seeing an evidence for physics beyond cold DM.