Public Seminar of Research Postgraduate Programmes Student:- Probing the Nature of Dark Matter by Modelling Flux Ratios in Cosmic Horseshoe (J1148+1930)
Cold Dark Matter(CDM) has been widely regarded as a likely candidate of DM. The predictive power of CDM on a large scale has been demonstrated by the power spectrum of cosmic microwave background(CMB) and large-scale distributions of galaxies. However, there are persisting small-scale problems to CDM, most notably the ‘missing satellite problem’. Cosmological simulation of CDM demonstrates an abundance of small substructures surrounding the larger CDM halo. Those substructures are challenging to detect because they are composed of mostly DM with minimal electromagnetic signature. So far there is no well-supported observational evidence of CDM substructures.
The inconsistency of CDM in small scale provokes the formulation of Fuzzy Dark Matter (FDM) or equivalently Wave Dark Matter(ψDM). FDM is composed by a hypothetical ultra-light non-relativistic bosons(axions), with de Broglie wavelength in the order of a galaxy size. The axions are strongly subject to wave and quantum phenomena, such as interference and uncertainty principle. Its quantum nature suppresses small structures formation and produces a sizeable flat core in the centre of galaxies, solving the small-scale problems of CDM naturally. Therefore, it is considered as an appealing alternative to CDM. High-resolution FDM simulation reveals that FDM halos consist of many granular structures, which represents small-scale mass density fluctuations. However, similar to CDM substructures, there is yet no direct observation on FDM granulation.
Taking advantage of strong gravitational lensing, the effect of CDM substructures or FDM granulation on the mass density fluctuations can be revealed through constructing a robust lens model. Both substructures and granulation will introduce slight perturbation to the magnification of lensed images, and hence the brightness of the images will deviate from one would expect from a smooth-lens. However, there is yet no observational study FDM’s effect on lensed images, in particular, extended source. The research done by the speaker aims to address how likely the observed flux ratios of a system of extended sources lensed by the most massive lensing galaxy known so far, the Cosmic Horseshoe, can be explained by CDM structures and FDM granulation respectively.
講者：Lai Yee LEE(香港大學)