What it takes to measure a fundamental difference between dark matter and baryons: the halo velocity anisotropy

Host, Ö.; Hansen, Steen H.

doi:10.1088/1475-7516/2007/06/016

Cited by 17 publications

(8 citation statements)

References 48 publications

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“…Dark matter behaves as collisionless particles on the timescale of Gigayears, the dynamical timescale of galaxy clusters, and this implies an order-of-magnitude upper limit to the self-interaction cross-section per unit mass of σ /m 1 cm 2 g −1 , similar to what was found for the Bullet Cluster [13] and only slightly larger than the value proposed for selfinteracting dark matter [14]. According to numerical simulations, the Galactic dark matter halo is also anisotropic, and this anisotropy will influence direct detection rates at the 5-10% level [15] and is potentially measurable in directional searches [16].…”

supporting

confidence: 81%

Measurement of the dark matter velocity anisotropy profile in galaxy clusters

Host¹

2009

Proceedings of Identification of Dark Matter 2008 — PoS(idm2008)

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Dark matter particles form halos that contribute the major part of the mass of galaxy clusters. The formation of these cosmological structures have been investigated both observationally and in numerical simulations, which have confirmed the existence of a universal mass profile [1]. However, the dynamic behaviour of dark matter in halos is not as well understood. We have used observations of 16 equilibrated galaxy clusters to show that the random velocities of dark matter particles are larger on average along the radial direction than along the tangential, and that the magnitude of this velocity anisotropy is radially varying [2]. Our measurement implies that the collective behaviour of dark matter particles is fundamentally different from that of normal particles and the radial variation of the anisotropy velocity agrees with the predictions of numerical simulation [3]. PoS(idm2008)052Measurement of the dark matter velocity anisotropy profile in galaxy clusters Ole HostDark matter is most often assumed to be a new type of particle which couples only very weakly to itself or other particle species [4]. Unlike a collisional gas, dark matter structures may therefore have a non-zero velocity anisotropy defined as β = 1 − σ 2 t /σ 2 r , where σ 2 t and σ 2 r are the 1-dimensional tangential and radial velocity dispersions of the dark matter. Here we summarize the measurement of the radial velocity anisotropy profile in a sample of galaxy clusters, as presented in [2].Most of the ordinary baryonic matter in galaxy clusters is found in the intracluster medium (ICM), a hot plasma emitting X-rays. We consider a sample of 16 relaxed galaxy clusters for which the radial ICM density and temperature profiles have been obtained from X-ray observations in earlier work. The clusters are selected to appear close to circular in projection, have smooth gas temperature and density profiles, and reconstructed total density profiles which are monotonically declining. The sample consists of eleven highly relaxed cool-core clusters at low redshift observed with XMM-Newton (A262, A496, A1795, A1837, A2052, A4059, Sérsic 159−3, MKW3s, MKW9, NGC533, and 2A0335+096) [5,6], and five intermediate redshift clusters observed with Chandra (RXJ1347.5, A1689, A2218, A1914, and A611) [7]. Data for A2052 and Sérsic 159−3 were also used in an earlier analysis [8] where a constant velocity anisotropy was assumed.We determine the velocity anisotropy for the 16 clusters of our sample using the method outlined in [2], and in almost all cases the anisotropy profile is radially increasing from close to zero in the center to about 0.5 or greater in the outer parts. Since the qualitative behaviour of the profiles are similar, we combine all our data into a single 'stacked' profile depending on radius in units of r 2500 . Figure 1 shows the resulting velocity anisotropy profile which varies between 0.1 and 0.5 in the radial range 0.03-1.0 r 2500 , where r 2500 is the scale radius enclosing a mean density 2500 times greater than the critical density at the...

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confidence: 81%

Measurement of the dark matter velocity anisotropy profile in galaxy clusters

Host¹

2009

Proceedings of Identification of Dark Matter 2008 — PoS(idm2008)

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“…Exploring equilibrium dark matter phase space distributions, particularly anisotropic distributions, may give further insight into the behavior of N-body simulations and may explain the inner slope of ρ(r). These effects are unlikely to be directly observable in early direct detection experiments, but may be seen in future directional dark matter detection experiments [51].…”

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confidence: 98%

Dark matter at the end of the Galaxy

et al. 2011

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Dark matter density profiles based upon ΛCDM cosmology motivate an ansatz velocity distribution function with fewer high velocity particles than the Maxwell-Boltzmann distribution or proposed variants. The high velocity tail of the distribution is determined by the outer slope of the dark matter halo -the large radius behavior of the Galactic dark matter density. N-body simulations of Galactic halos reproduce the high velocity behavior of this ansatz. Predictions for direct detection rates are dramatically affected for models where the threshold scattering velocity is within 30% of the escape velocity.

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“…It may also be possible to detect anisotropy in the velocity distribution. For example, Host and Hansen [249] showed that with 10 4 events a 32 S target with 100 keV energy threshold could measure the velocity anisotropy parameter, β = 1 − σ 2 t /σ 2 r where σ r,t are the radial and tangential velocity dispersions, with a precision of ∼0.03. Furthermore, it may be possible to take a non-parametric approach-analogous to the methods used in Section 7, but for the full 3-dimensional velocity distribution.…”

Section: Directional Detectionmentioning

confidence: 99%

WIMP physics with ensembles of direct-detection experiments

Peter

Gluscevic

Green

et al. 2014

Physics of the Dark Universe

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a b s t r a c tThe search for weakly-interacting massive particle (WIMP) dark matter is multi-pronged. Ultimately, the WIMP-dark-matter picture will only be confirmed if different classes of experiments see consistent signals and infer the same WIMP properties. In this work, we review the ideas, methods, and status of directdetection searches. We focus in particular on extracting WIMP physics (WIMP interactions and phasespace distribution) from direct-detection data in the early discovery days when multiple experiments see of order dozens to hundreds of events. To demonstrate the essential complementarity of different directdetection experiments in this context, we create mock data intended to represent the data from the nearfuture Generation 2 experiments. We consider both conventional supersymmetry-inspired benchmark points (with spin-independent and -dependent elastic cross sections just below current limits), as well as benchmark points for other classes of models (inelastic and effective-operator paradigms). We also investigate the effect on parameter estimation of loosening or dropping the assumptions about the local WIMP phase-space distribution. We arrive at two main conclusions. Firstly, teasing out WIMP physics with experiments depends critically on having a wide set of detector target materials, spanning a large range of target nuclear masses and spin-dependent sensitivity. It is also highly desirable to obtain data from low-threshold experiments. Secondly, a general reconstruction of the local WIMP velocity distribution, which will only be achieved if there are multiple experiments using different target materials, is critical to obtaining a robust and unbiased estimate of the WIMP mass.

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What it takes to measure a fundamental difference between dark matter and baryons: the halo velocity anisotropy

Cited by 17 publications

References 48 publications

Measurement of the dark matter velocity anisotropy profile in galaxy clusters

Measurement of the dark matter velocity anisotropy profile in galaxy clusters

Dark matter at the end of the Galaxy

WIMP physics with ensembles of direct-detection experiments

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