Collisions between galaxy clusters provide a test of the non-gravitational forces acting on dark matter. Dark matter's lack of deceleration in the 'bullet cluster collision' constrained its self-interaction cross-section σ DM /m < 1.25 cm 2 /g (68% confidence limit) for long-ranged forces. Using the Chandra and Hubble Space Telescopes we have now observed 72 collisions, including both 'major'and 'minor' mergers. Combining these measurements statistically, we detect the existence of dark mass at 7.6σ significance. The position of the dark mass has remained closely aligned within 5.8±8.2 kpc of associated stars: implying a self-interaction cross-section σ DM /m < 0.47 cm 2 /g (95% CL) and disfavoring some proposed extensions to the standard model.Many independent lines of evidence now suggest that most of the matter in the Universe is in a form outside the standard model of particle physics. A phenomenological model for cold dark matter (1) has proved hugely successful on cosmological scales, where its gravitational influence dominates the formation and growth of cosmic structure. However, there are several 1 arXiv:1503.07675v2 [astro-ph.CO] 13 Apr 2015 challenges on smaller scales: the model incorrectly predicts individual galaxy clusters to have more centrally concentrated density profiles (2), larger amounts of substructure (3, 4), and the Milky Way to have more satellites able to produce stars (5) than are observed. These inconsistencies could be resolved through astrophysical processes (6), or if dark matter particles are either warm (7) or self-interact with cross-section 0.1 ≤ σ DM /m ≤ 1 cm 2 /g (8-10). Following (11), we define the momentum transfer per unit mass σ DM /m, integrating over all scattering angles and assuming that individual dark matter particles are indistinguishable.Self-interaction within a hidden dark sector is a generic consequence of some extensions to the standard model. For example, models of mirror dark matter (12) and hidden sector dark matter (12-16) all predict anisotropic scattering with σ DM /m ≈ 1 barn/GeV = 0.6 cm 2 /g, similar to nuclear cross-sections in the standard model. Note that couplings within the dark sector can be many orders of magnitude larger than those between dark matter and standard model particles, which is at most of order picobarns (17).In terrestrial collider experiments, the forces acting on particles can be inferred from the trajectory and quantity of emerging material. Collisions between galaxy clusters, which contain dark matter, provide similar tests for dark sector forces. If dark matter's particle interactions are frequent but exchange little momentum (via a light mediator particle that produces a longranged force and anisotropic scattering), the dark matter will be decelerated by an additional drag force. If the interactions are rare but exchange a lot of momentum (via a massive mediator that produces a short-ranged force and isotropic scattering), dark matter will tend to be scattered away and lost (11,18,19).The dynamics of colliding dark matter...

Galaxy cluster Abell 3827 hosts the stellar remnants of four almost equally bright elliptical galaxies within a core of radius 10 kpc. Such corrugation of the stellar distribution is very rare, and suggests recent formation by several simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and VLT/MUSE integral field spectroscopy of a gravitationally lensed system threaded through the cluster core. We find that each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars. The best-constrained offset is 1.62 +0.47 −0.49 kpc, where the 68% confidence limit includes both statistical error and systematic biases in mass modelling. Such offsets are not seen in field galaxies, but are predicted during the long infall to a cluster, if dark matter self-interactions generate an extra drag force. With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments -but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section σ DM /m ∼ (1.7 ± 0.7) × 10 −4 cm 2 /g ×(t infall /10 9 yrs) −2 , where t infall is the infall duration.

We analyse the performance of 12 different implementations of Smoothed Particle Hydrodynamics (SPH) using seven tests designed to isolate key hydrodynamic elements of cosmological simulations which are known to cause the SPH algorithm problems. In order, we consider a shock tube, spherical adiabatic collapse, cooling flow model, drag, a cosmological simulation, rotating cloud‐collapse and angular momentum transport. In the implementations special attention is given to the way in which force symmetry is enforced in the equations of motion. We study in detail how the hydrodynamics are affected by different implementations of the artificial viscosity including those with a shear‐correction modification. We present an improved first‐order smoothing‐length update algorithm that is designed to remove instabilities that are present in simple forward prediction algorithms. Gravity is calculated using the adaptive particle–particle, particle–mesh algorithm. For all tests we find that the artificial viscosity is the single most important factor distinguishing the results from the various implementations. The shock tube and adiabatic collapse problems show that the artificial viscosity used in the hydra code prior to version 4.0 performs relatively poorly for simulations involving strong shocks when compared to a more standard artificial viscosity. The shear‐correction term is shown to reduce the shock‐capturing ability of the algorithm and to lead to a spurious increase in angular momentum in the rotating cloud‐collapse problem. For the disc stability test, the shear‐corrected and previous hydra artificial viscosities are shown to reduce outward angular momentum transport. The cosmological simulations produce comparatively similar results, with the fraction of gas in the hot and cold phases varying by less than 10 per cent amongst the versions. Similarly, the drag test shows little systematic variation amongst versions. The cooling flow tests show that implementations using the force symmetrization of Thomas & Couchman are more prone to accelerate the overcooling instability of SPH, although the problem is generic to SPH. The second most important factor in code performance is the way force symmetry is achieved in the equation of motion. Most results favour a kernel symmetrization approach. The exact method by which SPH pressure forces are included in the equation of motion appears to have comparatively little effect on the results. Combining the equation of motion presented by Thomas & Couchman with a modification of the Monaghan & Gingold artificial viscosity leads to an SPH scheme that is both fast and reliable.

We examine the temperature structure of the intergalactic medium (IGM) due to the passage of individual ionization fronts using a radiative transfer (RT) code coupled to a particle‐mesh N‐body code. Multiple simulations were performed with different spectra of ionizing radiation: a power law (∝ ν−0.5), miniquasar, starburst, and a time‐varying spectrum that evolves from a starburst spectrum to a power law. The RT is sufficiently resolved in time and space to correctly model both the ionization state and the temperature across the ionization front. We find that the post‐ionization temperature of the reionized IGM is sensitive to the spectrum of the source of ionizing radiation, which may be used to place strong constraints on the nature of the sources of reionization. RT effects also produce large fluctuations in the He ii to H i number density ratio η. The spread in values is smaller than measured, except for the time‐varying spectrum. For this case, the spread evolves as the spectral nature of the ionizing background changes. Large values for η are found in partially ionized He ii as the power‐law spectrum begins to dominate the starburst, suggesting that the large η values measured may be indicating the onset of the He ii reionization epoch.