The nongravitational interactions of dark matter in colliding galaxy clusters

Abstract

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...

FRONTIER FIELDS CLUSTERS:CHANDRAAND JVLA VIEW OF THE PRE-MERGING CLUSTER MACS J0416.1-2403

“…Crosses mark the centers of the dark matter halos of the NE and SW subclusters. 33 An analysis of the DM-gas offset in MACS J0416.1-2403 was carried out more recently by Harvey et al (2015). For the SW subcluster, their DM peak is offset by about 100 kpc (∼20″) from the DM peak determined by previous analyses (e.g., Grillo et al 2015;Jauzac et al 2015, models at https://archive.…”

Stellar streams as gravitational experiments

“…This paradigm provides a natural explanation to the apparent conspiracy between the observed baryonic distribution and the gravitational force in various types of galaxies (see, e. g. Famaey & McGaugh 2012;McGaugh et al 2016;Lelli et al 2016). Nevertheless, the successes of the paradigm are mostly limited to galactic scales (for problems at other scales, see, e.g., Skordis et al 2006;Clowe et al 2006;Angus 2008;Ibata et al 2011;Dodelson 2011;Clowe et al 2012;Harvey et al 2015). This could mean that it is either an effective way to predict how dark matter (DM) is arranged in galaxies, or that it is an effective modification of gravity resulting from a broader theory of the dark sector (e.g.…”

Direct detection portals for self-interacting dark matter

“…The effect of self-interactions can also be searched through various other astrophysical observations e.g. [3,[36][37][38][39][40][41][42][43][44][45][46][47][48][49]. Astrophysical tests of self-interactions can probe DM that is decoupled from the Standard Model (SM).…”