Combining optical and X-ray observations of galaxy clusters to constrain cosmological parameters

Galaxy clusters present unique advantages for cosmological study.Here we collect a new sample of 10 lensing galaxy clusters with X-ray observations to constrain cosmological parameters.The redshifts of the lensing clusters lie between 0.1 and 0.6,and the redshift range of their arcs is from 0.4 to 4.9.These clusters are selected carefully from strong gravitational lensing systems which have both X-ray satellite observations and optical giant luminous arcs with known redshifts.Giant arcs usually appear in the central region of clusters,where mass can be traced with luminosity quite well.Based on gravitational lensing theory and a cluster mass distribution model,we can derive a ratio using two angular diameter distances.One is the distance between lensing sources and the other is that between the deflector and the source. Since angular diameter distance relies heavily on cosmological geometry,we can use these ratios to constrain cosmological models.Moreover,X-ray gas fractions of galaxy clusters can also be a cosmological probe.Because there are a dozen parameters to be fitted,we introduce a new analytic algorithm,Powell＇s UOBYQA（Unconstrained Optimization By Quadratic Approximation） ,to accelerate our calculation.Our result demonstrates that this algorithm is an effective fitting method for such a continuous multi-parameter constraint.We find an interesting fact that these two approaches are separately sensitive toΩΛandΩM.By combining them,we can get reasonable fitting values of basic cosmological parameters：ΩM=0.26 ＋0.04 -0.04,andΩΛ=0.82 ＋0.14 -0.16.

Statistical Cluster-QSO Weak Lensing in the Sloan Digital Sky Survey

We investigate the cross-correlation between galaxy clusters and QSOs using Sloan Digital Sky Survey （SDSS） DR4 - 5000 deg^2 data. With photometric redshifts of galaxies, we select galaxy clusters based on the local projected densities of LRGs brighter than Mr′ = -22. The QSOs are from the main sample of SDSS QSO spectroscopic survey to i′ = 19. A significant positive correlation is found between the clusters and QSOs. Under the assumption that the signal is caused by gravitational lensing, we fit the signal with singular isothermal sphere （SIS） model and NFW profile halo model. The velocity dispersion σv = 766 km s^-1 is derived for the best-fit of SIS model. Best-fit for the NFW model requires the dark matter halo mass within 1.5 h^-1 Mpc to be 4.6 × 10^14 h^-1 M⊙. The mass parameter Ωcl of the cluster sample is deduced as 0.077 with the SIS model and 0.083 with the NFW model. Our results of Ωcl are smaller than those given by Croom ＆ Shanks and by Myers et al.

The effects of BCGs on the statistics of large-separation lensed quasars by clusters

We study the statistics of large-separation multiply-imaged quasars lensed by clusters of galaxies. In particular, we examine how the observed brightest cluster galaxies （BCGs） affect the predicted numbers of wide-separation lenses. We model the lens as an NFW-profiled dark matter halo with a truncated singular isothermal sphere to represent the BCG in its center. We mainly make predictions for the Sloan Digital Sky Survey Quasar Lens Search （SQLS） sample from the Data Release 5 （DRS） in two standard ACDM cosmological models： a model with matter density ΩM = 0.3 and δ8 = 0.9, as is usually adopted in the literature （ACDM1）, and a model suggested by the WMAP seven-year （WMAPT） data with ΩM = 0.266 and δ8 = 0.801. We also study the lensing properties for the WMAP3 cosmology in order to compare with the previous work. We find that BCGs in the centers of clusters significantly enhance the lensing efficiency by a factor of 2 - 3 compared with that of NFW-profiled pure dark matter halos. In addition, the dependence of mass ratios of BCGs to their host halos on the host halo masses reduces the lensing rate by - 20% from assuming a constant ratio as in previous studies, but considering the evolution of this ratio with redshift out to z - 1 would reduce it by - 3%. Moreover, we predict that the numbers of lensed quasars with image separations larger than 10″ in the statistical sample of SQLS from DR5 are 1.22 and 0.47, respectively for ACDM1 and WMAP7 and 0.73 and 0.33 for separations between 10″ and 20″, which are consistent with the only observed cluster lens with such a large separation in the complete SQLS sample.

Resolving the Mass Discrepancy between Strong and Weak Lensing Methods

We readdress the outstanding cluster mass discrepancy between strong and weak gravitational lensing techniques utilizing updated data of both giant arcs and weak lensing measurements from the literature. We find that the systematically higher values of cluster masses revealed by strong lensing can be attributed to the oversimplification of the lensing model when estimating the cluster mass enclosed within the giant arcs. This arises because inhomogeneities and substructures in the central cores of clusters may invalidate the spherical symmetry assumption used widely in previous applications. When a more realistic modeling of the arcs is used, then the masses by strong lensing agree fairly well with those given by weak lensing when both are extrapolated to the same cluster regions. We conclude that as of now no significant discrepancy has been found among different cluster mass estimators including optical galaxies, X-ray gas and lensing.

A Better Candidate for Dark Matter is Cosmic Plasma

In the ΛCDM cosmological model, based on observations of supernovae Ia, the cosmic dark energy density is assumed to be Ω_(Λ)~ 0.70 and the gravitational mass density is assumed to be Ω_(m)~ 0.30. Based on the assumption that the observed cosmic microwave background(CMB) is a thermal relic of the early hot universe, the cosmic plasma density should be small, i.e., Ω_(b)~ 0.05(otherwise the Sunyaev-Zeldovich effect of the cosmic plasma would ruin the observed CMB's perfect blackbody spectrum). To fill the gap between Ω_(m) and Ω_(b), non-baryonic dark matter Ω_(c)~ 0.25 is introduced into the ΛCDM model. If the CMB is the result of a partial thermal equilibrium between cosmic radiation and cosmic plasma, then the observed perfect blackbody spectrum of the CMB can coexist with cosmic plasma. In this case, it is not necessary to introduce non-baryonic cold dark matter into cosmological models. A better candidate for dark matter is the cosmic plasma.

WMAP第3年的宇宙学参数对星系团中巨弧数目的影响

该文利用高精度的大尺度N体数值模拟,研究了WMAP(Wilkinson Microwave Anisotropy Probe)第3年的观测结果对星系团中的巨弧数目的影响,发现预言的巨弧数目比主流的ACDM模型预言的要低80%多。加入绝热气体几乎不会改变巨弧的发生概率,而引入恒星形成和超新星反馈后得到的概率不会大于原来的1.3倍。如此小的贡献是因为虽然有恒星的数值模拟得到的星系团具有更高的中心密度,但同时星系团的形状更趋球对称,这是影响巨弧概率的2个相反的因素。以前基于ACDM模型预言的巨弧概率还不能很好地与观测相符,WMAP第3年的观测结果将更难以解释。

光锥中强引力透镜的数值模拟

根据高精度宇宙学N体数值模拟输出的星系团以及星系团X射线光度和质量的经验关系:L-M,在红移区间(0.14,0.3)内构建了一个X射线波段流量限为3×10^(-19)J·s^(-1)·cm^(-2)(如Local Cluster Substructure Survey,缩写为LoCuSS )的大质量星系团样本。利用高分辨率的Ray-tracing数值模拟对包含真实观测信息(即红移、大小和形状)的COSMOS背景星系源进行单个透镜的成像模拟,并统计星系团不同投影方向的巨弧(即L/W>10)产生效率。依据数值模拟立方体的尺度建立观测光锥,再根据光锥内不同红移区间所占立方体的体积比例用对应数量、红移的透镜对光锥进行随机填充,通过平均它们的透镜效率,最终计算得到光锥内的平均强引力透镜效率为3.22_(-1.47)^(+2.73)×10^(-2)。