用强引力透镜研究暗晕中心的物质分布

摘要

对矮星系和晚型LSB 星系的最新高分辨率自转曲线进行观测,揭示出由CDM 主导的物质密度分布中心具有常数密度核.在ΛCDM 宇宙学框架下,纯CDM 宇宙学N 体模拟给出普适的、带尖点的NFW模型,解决尖点-核问题的最佳方案是考虑暗晕中心重子物理过程对暗晕密度分布的影响.到目前为止,考虑重子物理过程后,不管是解析模型还是数值模拟,给出的暗晕密度分布都能很好地符合自转曲线的观测结果,但是却不能通过强引力透镜观测的检验.最后,对基于这两种观测到底能不能用一个统一的暗晕密度分布模型来描述作了简要的讨论,并认为需要更多的强引力透镜样本和分辨率更高的数值模拟,以解决上述问题.%The most recent high resolution observations of the rotation curves for cold dark matter(CDM)dominated dwarf galaxies and low surface brightness(LSB)galaxies imply that these galactic CDM halos have a density profile with a flat central core,whereas pure CDM cosmological N-body structure formation simulations predict a divergent(cuspy)density profile at the center.This is known as cusp-core problem.Gravitational lensing provides another independent powerful tool to detect CDM,although it can not distinguish whether the lensing mass is baryonic or dark matter,but rather simply depends on the total mass.For a certain given mass of a lensing galaxy or galaxy cluster,strong lensing efficiency is very sensitive to the slope γ of the central total mass density profile(ρ ∝ r-γ).For example,a cored density profile for a reasonable value of the core radius rc(usually determined by rotation curves,within which γ ?0)would lead to an extremely low lensing rate compared with lensing observations,whereas a singular isothermal sphere(SIS,γ = 2,for elliptical galaxies)matches observations well.In ΛCDM cosmology,a main type of solutions to the cusp-core problem is including baryons as astrophysical processes originally in collision-less CDM N-body simulations.The baryons have two opposite effects on the central mass density of CDM halos.While stellar feedback and dynamical friction can induce expansion of the CDM halos and produce a core,the adiabatic contractions can steepen central density to the singular isothermal sphere(SIS)type(which is cuspy).Consequently,simulations have always been designed to tackle a certain specific problem(usually determined by observations).For example,disk galaxies and giant ellipticals are often simulated independently,usually among very different communities.Unfortunately,on one hand,a cored density profile would lead to an extremely low lensing rate compared with observations.On the other hand,when SIS profile matches the observations of strong lensing by giant elliptical galaxies,it fails in fitting the inner parts of rotation curves which indicate a central core.In this paper,we review several simulations,for which,the density profiles are in good agreement with observations of rotation curves,but all fail in predicting the available strong gravitational lensing observational samples.We conclude that,it is difficult for current simulations to reconcile the DM distributions derived from the observations of rotation curves and that from strong lensing.