The mass distribution of the Fornax dSph: constraints from its globular cluster distribution

Abstract

Uniquely among the dwarf spheroidal (dSph) satellite galaxies of the Milky Way, Fornax hosts globular clusters. It remains a puzzle as to why dynamical friction has not yet dragged any of Fornax's five globular clusters to the centre, and also why there is no evidence that any similar star cluster has been in the past (for Fornax or any other tidally undisrupted dSph). We set up a suite of 2800 N‐body simulations that sample the full range of globular cluster orbits and mass models consistent with all existing observational constraints for Fornax. In agreement with previous work, we find that if Fornax has a large dark matter core, then its globular clusters remain close to their currently observed locations for long times. Furthermore, we find previously unreported behaviour for clusters that start inside the core region. These are pushed out of the core and gain orbital energy, a process we call ‘dynamical buoyancy’. Thus, a cored mass distribution in Fornax will naturally lead to a shell‐like globular cluster distribution near the core radius, independent of the initial conditions. By contrast, cold dark matter‐type cusped mass distributions lead to the rapid infall of at least one cluster within Δt = 1–2 Gyr, except when picking unlikely initial conditions for the cluster orbits (∼2 per cent probability), and almost all clusters within Δt = 10 Gyr. Alternatively, if Fornax has only a weakly cusped mass distribution, then dynamical friction is much reduced. While over Δt = 10 Gyr this still leads to the infall of one to four clusters from their present orbits, the infall of any cluster within Δt = 1–2 Gyr is much less likely (with probability 0–70 per cent, depending on Δt and the strength of the cusp). Such a solution to the timing problem requires (in addition to a shallow dark matter cusp) that in the past the globular clusters were somewhat further from Fornax than today; they most likely did not form within Fornax, but were accreted.

Simple J-factors and D-factors for indirect dark matter detection

“…There is also strong observational evidence that some of the dwarf galaxies are mildly cusped or even cored. This includes the persistence of substructure in Ursa Minor [47], the survival of globular clusters in Fornax [48], and the kinematics of multiple populations [25,28,49,50]. Here, somewhat speculatively, we suppose that the J-profile can be mapped out as a function of θ and ask what can then be deduced about the dark halo structure.…”

Farthest Neighbor: The Distant Milky Way Satellite Eridanus II*

“…Crnojević et al (2016) show that EriII is possibly the least massive dwarf galaxy that is known to have an extended star formation history, and therefore its density profile may also be affected by baryons. The star cluster of EriII may offer potential to constrain the dark matter profile of EriII through survivability arguments (see, e.g., Cole et al 2012) and could provide an independent probe of the dark matter profile shape. A better understanding of the dark matter distribution at small scales will help us understand how the dwarf galaxies we observe today are linked to the primordial population of dark matter subhalos predicted by ΛCDM cosmology.…”

Understanding the Core-Halo Relation of Quantum Wave Dark Matter from 3D Simulations

“…The relative deficiency of the observed number of low-mass galaxies is a major problem for standard cold dark matter (CDM) [5][6][7], for which a steeply rising mass function is predicted [8]. Furthermore, the dwarf spheroidal galaxies [9][10][11][12][13][14][15][16][17][18][19][20] and low surface brightness galaxies [21,22] are generally inferred to have large flat cores of dark matter, at odds with the singular cores required by standard CDM [23,24]. Complicated baryonic physics such as supernova feedback is required to solve both issues in the CDM paradigm [25][26][27][28][29][30][31][32][33][34].…”