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

Strong Constraints on Fuzzy Dark Matter from Ultrafaint Dwarf Galaxy Eridanus II

“…The central star cluster has a half light radius r h = 13 pc, age T EII = 3 → 12 Gyr and mass M = 2000 M . These values have been shown to be consistent with the expected dynamical evolution in the presence of a DM core, disfavouring a cuspy DM profile [44].…”

“…[49][50][51] computed using the sharp-k filtering method [49]. The sharp-k filtering model does not include stripping, and should be compared to the pre-infall mass of Eri II, 5 × 10 8 M [44]. The two models give comparable constraints on the FDM mass.…”

“…We relate the spatial and temporal fluctuations with the dispersion velocity v of the dark matter as r = vt. We neglect the intrinsic relaxation caused by the cluster stars to obtain an upper limit on the allowed amplitude of dark matter fluctuations. Two-body relaxation by cluster stars has been shown to naturally explain the observed radius of Eri II [44]. We derive the gravitational heating rate produced by a fluctuating density field from the force correlation function, closely following Ref.…”

Globular clusters as probes of dark matter cusp-core transformations

“…Goerdt et al 2006;Cole et al 2012;Amorisco 2017). Furthermore, our result linking large GC sizes to the presence of a central dark matter core is also seen in Contenta et al (2018) and Webb & Vesperini (2018). The key new result in our paper, however, is that GCs can survive and grow to large sizes also in a dwarf galaxy that has undergone a gradual cusp-core transformation.…”

A model for the minimum mass of bound stellar clusters and its dependence on the galactic environment

“…Likewise, young stellar clusters provide detailed information about the recent star formation conditions in their host galaxies (Portegies Zwart et al 2010;Longmore et al 2014;Chilingarian & Asa'd 2018). Clusters are also ideal tracers of gravity and probe the detailed mass distribution of dark matter haloes (Cole et al 2012;Erkal & Belokurov 2015;Alabi et al 2016;Contenta et al 2018;van Dokkum et al 2018). In order to fully exploit stellar clusters as tracers of galaxy and structure formation, we must understand how their birth environments give rise to their initial properties (including masses, ages, structure and chemical composition) and how these evolve across cosmic time.…”