利用弱重力透鏡效應探測星系團暗物質暈的橢圓率

Abstract

Cosmological N-body simulations of Λ cold dark matter (ΛCDM) predicted the shape of DM halos to be triaxial, reflecting the collisionless nature of DM, and to be elongated in the preferential infall direction of materials. The predicted halo ellipticity is about 0.4(Jing & Suto (2002) [16]). Here we test this well-defined pre- dictions of the standard ΛCDM paradigm using wide-field weak shear lensing data of Umetsu et al.(2016) [41] obtained for a sample of 16 galaxy clusters targeted in the CLASH survey. We examine the mean halo ellipticity (projected halo aspheric- ity) in our cluster sample by measuring the quadrupole weak-lensing signal for each cluster, following the method of Clampitt & Jain(2015) [7]. First we assume the DM halos align with the BCGs. Our best-fit value for the ellipticity of an ensemble of CLASH clusters in this measurement is 0.26±0.07 corresponding to axis-ratio of 0.73±0.07. We also use the position angle of X-ray gas, assuming the dark matter halos align with the X-ray gas halo. In this measurement, we get the mean ellip- ticity is 0.29 ± 0.09 corresponding to axis-ratio of 0.71±0.10. The different between there two measurement is not significant but this give us a hint that the DM halo may be more likely align with X-ray gas. We will also discuss the estimates of the misalignment between central galaxies and halos from simulations and apply this correction to our result.