Dark Matter Cusps Research Articles

Surviving the Waves: evidence for a dark matter cusp in the tidally disrupting Small Magellanic Cloud

Surviving the Waves: evidence for a dark matter cusp in the tidally disrupting Small Magellanic Cloud

Dark matter, black holes, and gravitational waves

The formation and growth of black holes can strongly influence the distribution of dark matter around them. I discuss here the different types of dark matter overdensities around black holes, including dark matter cusps, spikes, mounds, crests, and gravitational atoms. I then review recent results on the evolution of a black holes binary in presence of dark matter, focusing on the energy transfer between binary and dark matter induced by dynamical friction. Finally, I present the prospects for studying dark matter with gravitational wave observations, and argue that future interferometers might be able to detect and characterize dark matter overdensities around black holes.

The diversity of rotation curves of simulated galaxies with cusps and cores

ABSTRACTWe use ΛCDM cosmological hydrodynamical simulations to explore the kinematics of gaseous discs in late-type dwarf galaxies. We create high-resolution 21-cm ‘observations’ of simulated dwarfs produced in two variations of the EAGLE galaxy formation model: one where supernova-driven gas flows redistribute dark matter and form constant-density central ‘cores’, and another where the central ‘cusps’ survive intact. We ‘observe’ each galaxy along multiple sightlines and derive a rotation curve for each observation using a conventional tilted-ring approach to model the gas kinematics. We find that the modelling process introduces systematic discrepancies between the recovered rotation curve and the actual circular velocity curve driven primarily by (i) non-circular gas orbits within the discs; (ii) the finite thickness of gaseous discs, which leads to overlap of different radii in projection; and (iii) departures from dynamical equilibrium. Dwarfs with dark matter cusps often appear to have a core, whilst the inverse error is less common. These effects naturally reproduce an observed trend which other models struggle to explain: late-type dwarfs with more steeply rising rotation curves appear to be dark matter-dominated in the inner regions, whereas the opposite seems to hold in galaxies with core-like rotation curves. We conclude that if similar effects affect the rotation curves of observed dwarfs, a late-type dwarf population in which all galaxies have sizeable dark matter cores is most likely incompatible with current measurements.

A probabilistic deep learning model to distinguish cusps and cores in dwarf galaxies

ABSTRACT Numerical simulations within a cold dark matter (DM) cosmology form haloes whose density profiles have a steep inner slope (‘cusp’), yet observations of galaxies often point towards a flat central ‘core’. We develop a convolutional mixture density neural network model to derive a probability density function (PDF) of the inner density slopes of DM haloes. We train the network on simulated dwarf galaxies from the NIHAO and AURIGA projects, which include both DM cusps and cores: line-of-sight velocities and 2D spatial distributions of their stars are used as inputs to obtain a PDF representing the probability of predicting a specific inner slope. The model recovers accurately the expected DM profiles: $\sim 82{{\ \rm per\ cent}}$ of the galaxies have a derived inner slope within ±0.1 of their true value, while $\sim 98{{\ \rm per\ cent}}$ within ±0.3. We apply our model to four Local Group dwarf spheroidal galaxies and find results consistent with those obtained with the Jeans modelling based code GravSphere: the Fornax dSph has a strong indication of possessing a central DM core, Carina and Sextans have cusps (although the latter with large uncertainties), while Sculptor shows a double peaked PDF indicating that a cusp is preferred, but a core cannot be ruled out. Our results show that simulation-based inference with neural networks provide a innovative and complementary method for the determination of the inner matter density profiles in galaxies, which in turn can help constrain the properties of the elusive DM.

The Effect of Adiabatic Compression on Dark Matter Halos and the Radial Acceleration Relation

Abstract We use a semiempirical model to investigate the radial acceleration relation (RAR) in a cold dark matter (CDM) framework. Specifically, we build 80 model galaxies covering the same parameter space as the observed galaxies in the SPARC database, assigning them to dark matter (DM) halos using abundance-matching and halo mass–concentration relations. We consider several abundance-matching relations, finding some to be a better match to the kinematic data than others. We compute the unavoidable gravitational interactions between baryons and their DM halos, leading to an overall compression of the original Navarro–Frenk–White (NFW) halos. Before halo compression, high-mass galaxies lie approximately on the observed RAR, whereas low-mass galaxies display up-bending “hooks” at small radii due to DM cusps, making them deviate systematically from the observed relation. After halo compression, the initial NFW halos become more concentrated at small radii, making larger contributions to rotation curves. This increases the total accelerations, moving all model galaxies away from the observed relation. These systematic deviations suggest that the CDM model with abundance matching alone cannot explain the observed RAR. Further effects (e.g., feedback) would need to counteract the compression with precisely the right amount of halo expansion, even in high-mass galaxies with deep potential wells where such effects are generally predicted to be negligible.

The baryon cycle of Seven Dwarfs with superbubble feedback

We present results from a high-resolution, cosmological, ΛCDM simulation of a group of field dwarf galaxies with the “superbubble” model for clustered SN feedback, accounting for thermal conduction and cold gas evaporation. We compared our results to a previous simulation which has the same initial condition and galaxy formation physics (other than SN feedback), but adopts a delayed-cooling model for supernova. The simulated luminous galaxies have blue colors, low star formation efficiencies and metallicities, and high cold gas content, reproducing the observed scaling relations of dwarf galaxies in the Local Volume. Bursty star formation histories and superbubble-driven outflows lead to the formation of kpc-size dark matter (DM) cores when stellar masses reaches M* > 106 M⊙, similar to previous findings. However, the superbubble model appears more effective in destroying DM cusps than the delayed-cooling model in the previous study, reflecting a higher coupling efficiency of SN energy with the ISM. On larger scale, superbubble-driven outflows have a more moderate impact: galaxies have higher gas content, more extended stellar discs, and a smaller metal-enriched region in the circumgalactic medium (CGM). The two halos with Mvir ∼ 109 M⊙, which formed ultra-faint dwarf galaxies with the delayed-cooling mode, remain dark due to the different impact of metal-enriched galactic winds from two nearby luminous galaxies, indicating that the formation of faint dwarfs is highly dependent on feedback and environmental effects. The column density distributions of H I, Si II, C IV and O VI as a function of the scaled impact parameter (b/Rvir) are in good agreement with recent observations of CGM around isolated dwarf galaxies. While H I is ubiquitous with a covering fraction of unity within the CGM, low and intermediate ions like Si II and C IV are less extended (typically confined within 0.2 − 0.3 Rvir), and non-detections are common. O VI is more extended with column density N(O VI) ≳ 1013.5 cm−2 within Rvir, but its mass is only 11% of the total CGM oxygen budget, as the diffuse CGM is highly ionised by the UV background. Superbubble feedback produces C IV and O VI column densities that are an order of magnitude higher than those in the previous study using delayed-cooling feedback. Thus, the CGM and DM cores are most sensitive probes of feedback mechanisms.

Flattening of Dark Matter Cusps during Mergers: Model of M31

Abstract We run high-resolution fully GPU N-body simulations to demonstrate that the dark matter (DM) distribution in M31 is well fitted by a core-like profile. Rich observational data especially on the giant stellar stream provides stringent constraints on the initial conditions of our simulations. We demonstrate that the accretion of a satellite on a highly eccentric orbit heats up the central parts of M31, causes an outward migration of DM particles, flattens the central cusp over more than a decade in scale, and generates a new model-independent DM profile that is well fitted by a core. Our results imply that core-like central profiles could be a common feature of massive galaxies that have been initially cuspy but have accreted satellites on nearly radial orbits.

Core formation in high-z massive haloes: heating by post-compaction satellites and response to AGN outflows

ABSTRACT Observed rotation curves in star-forming galaxies indicate a puzzling dearth of dark matter in extended flat cores within haloes of mass ≥1012M⊙ at z ∼ 2. This is not reproduced by current cosmological simulations, and supernova-driven outflows are not effective in such massive haloes. We address a hybrid scenario where post-compaction merging satellites heat up the dark-matter cusps by dynamical friction, allowing active galactic nucleus (AGN)-driven outflows to generate cores. Using analytic and semi-analytic models (SatGen), we estimate the dynamical friction heating as a function of satellite compactness for a cosmological sequence of mergers. Cosmological simulations (VELA) demonstrate that satellites of initial virial masses >1011.3M⊙, which undergo wet compactions, become sufficiently compact for significant heating. Constituting a major fraction of the accretion on to haloes ≥1012M⊙, these satellites heat up the cusps in half a virial time at z ∼ 2. Using a model for outflow-driven core formation (CuspCore), we demonstrate that the heated dark-matter cusps develop extended cores in response to removal of half the gas mass, while the more compact stellar systems remain intact. The mergers keep the dark matter hot, while the gas supply, fresh and recycled, is sufficient for the AGN outflows. AGNs indeed become effective in haloes ≥1012M⊙, where the black hole growth is no longer suppressed by supernovae and its compaction-driven rapid growth is maintained by a hot circumgalactic medium. For simulations to reproduce the dynamical friction effects, they should resolve the compaction of the massive satellites and avoid artificial tidal disruption. AGN feedback could be boosted by clumpy black hole accretion and clumpy response to AGN.

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.

Fuzzy Dark Matter and Dark Matter Halo Cores

Abstract Whereas cold dark matter (CDM) simulations predict central dark matter cusps with densities that diverge as ρ(r) ∼ 1/r, observations often indicate constant-density cores with finite central densities ρ 0 and a flat density distribution within a core radius r 0. This paper investigates whether this core–cusp problem can be solved by fuzzy dark matter (FDM), a hypothetical particle with a mass of the order of m ≈ 10−22 eV and a corresponding de Broglie wavelength on astrophysical scales. We show that galaxies with CDM halo virial masses M vir ≤ 1011 M ⊙ follow two core-scaling relations. In addition to the well-known universal core column density Σ0 ≡ ρ 0 × r 0 = 75 pc−2, core radii increase with virial masses as r 0 ∼ with γ of order unity. Using the simulations by Schive et al. we demonstrate that FDM can explain the r 0–M vir scaling relation if the virial masses of the observed galaxy sample scale with the formation redshift z as M vir ∼ (1 + z)−0.4. The observed constant Σ0 is however in complete disagreement with FDM cores which are characterized by a steep dependence Σ0 ∼ r , independent of z. More high-resolution simulations are now required to confirm the simulations of Schive et al. and explore the transition region between the soliton core and the surrounding halo. If these results hold, FDM can be ruled out as the origin of observed dark matter cores and other physical processes are required to account for their formation.

Probing the nature of dark matter with accreted globular cluster streams

ABSTRACT The steepness of the central density profiles of dark matter (DM) in low-mass galaxy haloes (e.g. dwarf galaxies) is a powerful probe of the nature of DM. We propose a novel scheme to probe the inner profiles of galaxy subhaloes using stellar streams. We show that the present-day morphological and dynamical properties of accreted globular cluster (GC) streams – those produced from tidal stripping of GCs that initially evolved within satellite galaxies and later merged with the Milky Way (MW) – are sensitive to the central DM density profile and mass of their parent satellites. GCs that accrete within cuspy cold dark matter (CDM) subhaloes produce streams that are physically wider and dynamically hotter than streams that accrete inside cored subhaloes. A first comparison of MW streams ‘GD-1’ and ‘Jhelum’ (likely of accreted GC origin) with our simulations indicates a preference for cored subhaloes. If these results hold up in future data, the implication is that either the DM cusps were erased by baryonic feedback, or their subhaloes naturally possessed cored density profiles implying particle physics models beyond CDM. Moreover, accreted GC streams are highly structured and exhibit complex morphological features (e.g. parallel structures and ‘spurs’). This implies that the accretion scenario can naturally explain the recently observed peculiarities in some of the MW streams. We also propose a novel mechanism for forming ‘gaps’ in stellar streams when the remnant of the parent subhalo (which hosted the GC) later passes through the GC stream. This encounter can last a longer time (and have more of an impact) than the random encounters with DM subhaloes previously considered, because the GC stream and its parent subhalo are on similar orbits with small relative velocities. Current and future surveys of the MW halo will uncover numerous faint stellar streams and provide the data needed to substantiate our preliminary tests with this new probe of DM.

Search for Axionic Dark Matter Using the Magnetar PSR J1745-2900

We report on a search for dark matter axion conversion photons from the magnetosphere of the Galactic Center magnetar PSR J1745-2900 using spectra obtained from the Karl G. Jansky Very Large Array (the National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.). No significant spectral features are detected. Using a hybrid model for PSR J1745-2900 and canonical assumptions about the dark matter density profile, we exclude axion models with axion-photon coupling g_{aγγ}>6-34×10^{-12} GeV^{-1} with 95%confidence over the mass ranges 4.2-8.4, 18.6-26.9, 33.0-41.4, 53.7-62.1, and 126.0-159.3 μeV. If there is a dark matter cusp, the limits reduce to g_{aγγ}>6-34×10^{-14} GeV^{-1}, which overlap some axion models for the observed mass ranges >33 μeV. These limits may be improved by modeling the stimulated emission that can boost the axion-photon conversion process.

New Limits on Axionic Dark Matter from the Magnetar PSR J1745-2900

Abstract Axions are a promising dark matter candidate that were motivated to solve the strong charge-parity problem and that may also address the cosmological matter−antimatter asymmetry. Axion−photon conversion is possible in the presence of the strong magnetic fields, and the photon so produced will have energy equal to the axion mass. Here we report new limits on axionic dark matter obtained from radio spectra of the Galactic Center magnetar PSR J1745−2900. The magnetar has a magnetic field of 1.6 × 1014 G that interacts with a dark matter density 2 × 105 to 2 × 109 times greater than the local dark matter encountered by terrestrial haloscopes, depending on the Galactic dark matter profile. No significant spectral features are detected across 62% of the axion mass range 4.1–165.6 μeV (1–40 GHz). The interpretation of flux limits into limits on the two-photon coupling strength g aγγ depends on the magnetospheric conversion model and on the dark matter density at the Galactic Center. For a standard dark matter profile, we exclude axion models with g aγγ > 6–34 ×10−12 GeV−1 with 95% confidence over the mass ranges 4.2–8.4, 8.9–10.0, 12.3–16.4, 18.6–26.9, 33.0–62.1, 70.1–74.3, 78.1–80.7, 105.5–109.6, 111.6–115.2, 126.0–159.3, and 162.5–165.6 μeV. For the maximal dark matter cusp allowed by stellar orbits near Sgr A*, these limits reduce to g aγγ > 6–34 × 10−14 GeV−1, which exclude some theoretical models for masses >33 μeV. Limits may be improved by modeling stimulated axion conversion, by ray-tracing conversion pathways in the magnetar magnetosphere, and by obtaining deeper broad-band observations of the magnetar.

The effect of fluctuating fuzzy axion haloes on stellar dynamics: a stochastic model

ABSTRACT Fuzzy dark matter of ultralight axions has gained attention, largely in light of the galactic scale problems associated with cold dark matter. But the large de Broglie wavelength, believed to possibly alleviate these problems, also leads to fluctuations that place constraints on ultralight axions. We adapt and extend a method, previously devised to describe the effect of gaseous fluctuations on cold dark matter cusps, in order to determine the imprints of ultralight axion haloes on the motion of classical test particles. We first evaluate the effect of fluctuations in a statistically homogeneous medium of classical particles, then in a similar system of ultralight axions. In the first case, one recovers the classical two body relaxation time (and diffusion coefficients) from white noise density fluctuations. In the second situation, the fluctuations are not born of discreteness noise but from the finite de Broglie wavelength; correlation therefore exists over this scale, while white noise is retained on larger scales, elucidating the correspondence with classical relaxation. The resulting density power spectra and correlation functions are compared with those inferred from numerical simulations, and the relaxation time arising from the associated potential fluctuations is evaluated. We then apply our results to estimate the heating of discs embedded in axion dark haloes. We find that this implies an axion mass $m \gtrsim 2 \times 10^{-22} \, {\rm eV}$. We finally apply our model to the case of the central cluster of Eridanus II, confirming that far stronger constraints on m may in principle be obtained, and discussing the limitations associated with the assumptions leading to these.

The OSIRIS Lens-amplified Survey (OLAS). I. Dynamical Effects of Stellar Feedback in Low-mass Galaxies at z ∼ 2

Abstract We introduce the OSIRIS Lens-Amplified Survey (OLAS), a kinematic survey of gravitationally lensed galaxies at cosmic noon taken with Keck adaptive optics. In this paper, we present spatially resolved spectroscopy and nebular emission kinematic maps for 17 star-forming galaxies with stellar masses 8 < log(M */M ⊙) < 9.8 and redshifts 1.2 < z < 2.3. OLAS is designed to probe the stellar mass (M *) and specific star formation rate (sSFR) range where simulations suggest that stellar feedback is most effective at driving gaseous outflows that create galaxy-wide potential fluctuations, which can generate dark matter cores. We compare our kinematic data with the trend among sSFR, M *, and Hα velocity dispersion, σ, from the Feedback In Realistic Environments (FIRE) simulations. Our observations reveal a correlation between sSFR and σ at fixed M * that is similar to the trend predicted by simulations: feedback from star formation drives star-forming gas and newly formed stars into more dispersion-dominated orbits. The observed magnitude of this effect is in good agreement with the FIRE simulations, in which feedback alters the central density profiles of low-mass galaxies, converting dark matter cusps into cores over time. Our data support the scenario that stellar feedback drives gaseous outflows and potential fluctuations, which in turn drive dark matter core formation in dwarf galaxies.

Globular clusters as probes of dark matter cusp-core transformations

Abstract Bursty star formation in dwarf galaxies can slowly transform a steep dark matter cusp into a constant density core. We explore the possibility that globular clusters (GCs) retain a dynamical memory of this transformation. To test this, we use the nbody6df code to simulate the dynamical evolution of GCs, including stellar evolution, orbiting in static and time-varying potentials for a Hubble time. We find that GCs orbiting within a cored dark matter halo, or within a halo that has undergone a cusp-core transformation, grow to a size that is substantially larger (Reff > 10 pc) than those in a static cusped dark matter halo. They also produce much less tidal debris. We find that the cleanest signal of an historic cusp-core transformation is the presence of large GCs with tidal debris. However, the effect is small and will be challenging to observe in real galaxies. Finally, we qualitatively compare our simulated GCs with the observed GC populations in the Fornax, NGC 6822, IKN, and Sagittarius dwarf galaxies. We find that the GCs in these dwarf galaxies are systematically larger (〈Reff〉 ≃ 7.8 pc), and have substantially more scatter in their sizes than in situ metal-rich GCs in the Milky Way and young massive star clusters forming in M83 (〈Reff〉 ≃ 2.5 pc). We show that the size, scatter, and survival of GCs in dwarf galaxies are all consistent with them having evolved in a constant density core, or a potential that has undergone a cusp-core transformation, but not in a dark matter cusp.

Can bars erode cuspy halos?

AbstractOne of the major and widely known small scale problem with the Lambda CDM model of cosmology is the “core-cusp” problem. In this study we investigate whether this problem can be resolved using bar instabilities. We see that all the initial bars are thin (b/a < 0.3) in our simulations and the bar becomes thick ( b /a > 0.3) faster in the high resolution simulations. By increasing the resolution, we mean a larger number of disk particles. The thicker bars in the high resolution simulations transfer less angular momentum to the halo. Hence, we find that in the high resolution simulations it takes around 7 Gyr for the bar to remove inner dark matter cusp which is too long to be meaningful in galaxy evolution timescales. Physically, the reason is that as the resolution increases, the bar buckles faster and becomes thicker much earlier on.

Dark matter heats up in dwarf galaxies

Gravitational potential fluctuations driven by bursty star formation can kinematically 'heat up' dark matter at the centres of dwarf galaxies. A key prediction of such models is that, at a fixed dark matter halo mass, dwarfs with a higher stellar mass will have a lower central dark matter density. We use stellar kinematics and HI gas rotation curves to infer the inner dark matter densities of eight dwarf spheroidal and eight dwarf irregular galaxies with a wide range of star formation histories. For all galaxies, we estimate the dark matter density at a common radius of 150pc, $\rho_{\rm DM}(150\,\mathrm{pc})$. We find that our sample of dwarfs falls into two distinct classes. Those that stopped forming stars over 6Gyrs ago favour central densities $\rho_{\rm DM}(150\,\mathrm{pc})>10^8\,{\rm M}_\odot\,{\rm kpc}^{-3}$, consistent with cold dark matter cusps, while those with more extended star formation favour $\rho_{\rm DM}(150\,\mathrm{pc})<10^8\,{\rm M}_{\odot}\,{\rm kpc}^{-3}$, consistent with shallower dark matter cores. Using abundance matching to infer pre-infall halo masses, $M_{200}$, we show that this dichotomy is in excellent agreement with models in which dark matter is heated up by bursty star formation. In particular, we find that $\rho_{\rm DM}(150\,\mathrm{pc})$ steadily decreases with increasing stellar mass-to-halo mass ratio, $M_*/M_{200}$. Our results suggest that, to leading order, dark matter is a cold, collisionless, fluid that can be kinematically 'heated up' and moved around.

The case for a cold dark matter cusp in Draco

We use a new mass modelling method, GravSphere, to measure the central dark matter density profile of the Draco dwarf spheroidal galaxy. Draco's star formation shut down long ago, making it a prime candidate for hosting a 'pristine' dark matter cusp, unaffected by stellar feedback during galaxy formation. We first test GravSphere on a suite of tidally stripped mock 'Draco'-like dwarfs. We show that we are able to correctly infer the dark matter density profile of both cusped and cored mocks within our 95% confidence intervals. While we obtain only a weak inference on the logarithmic slope of these density profiles, we are able to obtain a robust inference of the amplitude of the inner dark matter density at 150pc, $\rho_{\rm DM}(150\,{\rm pc})$. We show that, combined with constraints on the density profile at larger radii, this is sufficient to distinguish a $\Lambda$ Cold Dark Matter ($\Lambda$CDM) cusp $-$ that has $\rho_{\rm DM}(150\,{\rm pc}) > 1.8 \times 10^8\,{\rm M}_\odot \,{\rm kpc}^{-3}$ $-$ from alternative dark matter models that have lower inner densities. We then apply GravSphere to the real Draco data. We find that Draco has an inner dark matter density of $\rho_{\rm DM}(150\,{\rm pc}) = 2.4_{-0.6}^{+0.5} \times 10^8\,{\rm M}_\odot \,{\rm kpc}^{-3}$, consistent with a $\Lambda$CDM cusp. Using a velocity independent SIDM model, calibrated on $\Lambda$SIDM cosmological simulations, we show that Draco's high central density gives an upper bound on the SIDM cross section of $\sigma/m < 0.57\,{\rm cm}^2\,{\rm g}^{-1}$ at 99% confidence. We conclude that the inner density of nearby dwarf galaxies like Draco provides a new and competitive probe of dark matter models.

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

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.