EDGE: two routes to dark matter core formation in ultra-faint dwarfs

Abstract

In the standard Lambda cold dark matter paradigm, pure dark matter simulations predict dwarf galaxies should inhabit dark matter haloes with a centrally diverging density ‘cusp’. This is in conflict with observations that typically favour a constant density ‘core’. We investigate this ‘cusp-core problem’ in ‘ultra-faint’ dwarf galaxies simulated as part of the ‘Engineering Dwarfs at Galaxy formation’s Edge’ project. We find, similarly to previous work, that gravitational potential fluctuations within the central region of the simulated dwarfs kinematically heat the dark matter particles, lowering the dwarfs’ central dark matter density. However, these fluctuations are not exclusively caused by gas inflow/outflow, but also by impulsive heating from minor mergers. We use the genetic modification approach on one of our dwarf’s initial conditions to show how a delayed assembly history leads to more late minor mergers and, correspondingly, more dark matter heating. This provides a mechanism by which even ultra-faint dwarfs ($M_* \lt 10^5\, \text{M}_{\odot }$), in which star formation was fully quenched at high redshift, can have their central dark matter density lowered over time. In contrast, we find that late major mergers can regenerate a central dark matter cusp, if the merging galaxy had sufficiently little star formation. The combination of these effects leads us to predict significant stochasticity in the central dark matter density slopes of the smallest dwarfs, driven by their unique star formation and mass assembly histories.

EDGE: the puzzling ellipticity of Eridanus II's star cluster and its implications for dark matter at the heart of an ultra-faint dwarf

“…We also include, for comparison, the lower density cored potential from Contenta et al (2018). This is more cored than any of the EDGE dwarfs presented in Orkney et al (2021), but has properties similar to the cored ultra-faint dwarf presented and discussed in Pontzen et al (2021). We discuss whether such a cored model is likely in a ΛCDM cosmology in §4.…”

“…Detailed analysis was performed using the T database package (Pontzen & Tremmel 2018) and the P analysis package (Pontzen et al 2013). We analyse four haloes previously described in Rey et al (2019); Orkney et al (2021), all of which have a final stellar mass of O (10 5 ) M and are selected from the EDGE suite to be most similar to the observed properties of EriII (see Table 1). Halo1459 GM:Later and Halo1459 GM:Latest are based upon the initial conditions of Halo1459, but have been modified with our GM treatment.…”

“…Note that, although it is most similar to EriII's observed properties (see Table 1), we do not consider SC survival within Halo1459 GM:Latest. This is because, due to a late, low stellar mass, cuspy merger, its final density profile ends up being rather cuspy (see the discussion of this in Orkney et al 2021). As such, its behaviour in terms of SC survival is already covered by Halo1445.…”

“…However, there are some notable exceptions. In recent work, Orkney et al (2021) studied cusp-core transformations for ultra-faint dwarfs drawn from the 'Engineering Dwarfs at Galaxy Formation's Edge' (EDGE) simulation project (with a mass, baryonic mass and spatial resolution of 120 M , 20 M and 3 pc, respectively, sufficient to resolve dark matter cores in ultra-faints larger than ∼20 pc). They found that ultra-faints can lower their inner dark matter density by up to a factor ∼2 through a combination of early heating due to star formation, followed by late-time heating from minor mergers.…”

“…As such, we set out in this paper to answer the question: can we form a SC as large, offset and elliptical as that in EriII within realistic ultra-faints in ΛCDM? To address this question, we first select a number of 'ultra-faint' dwarfs from the EDGE simulation project (Rey et al 2019;Agertz et al 2020;Pontzen et al 2021;Rey et al 2020;Orkney et al 2021;Prgomet et al 2021;Rey et al 2021), chosen to be most similar to EriII. We then set up a suite of 960 collisional 6 follow-up simulations that model SCs orbiting within spherically symmetric potentials fit to these dwarfs, sampling over a grid of initial properties and orbits.…”