同等质量下盘状星系的颜色-尺度关系

挑选Sloan数字巡天第7次释放数据(SDSS DR7)的主星系样本中近邻的、面向的盘状星系作为星系样本,统计研究了在恒星总质量相等的情况下盘状星系的颜色和尺度之间的相关性,并对相关性的真实性进行了检验.发现对于同等质量的盘状星系,u-r颜色与尺度相关性很弱,而g-r、r-i、r-z颜色与尺度负相关,即星系的尺度越大,颜色越蓝.该结果意味着盘状星系的质量分布对其恒星形成历史影响很大,物质分布越延展的星系,其演化越慢.     

重构Ly-α线丛的功率谱约束对数正态(LN)模型参数

选择3个类星体Keck观测样本,使用Gauss化方法重构宇宙质量密度扰动场,并计算相应的小波功率谱．同时,设定对数正态(LN)模型中的形状因子Γ和平滑因子r为自由参数,并通过数值模拟生成模拟样本,与观测样本的统计结果做拟合,来确定Γ和r．结合观测的辐射流量功率谱和数值模拟的结果可以唯一确定Γ的值,最后得到Γ≈0．50,r≈0．0g．类星体的辐射流量是非Gauss的,使用传统的Gauss化方法无法消除非Gauss特征．但是观测样本和数值模拟样本的偏斜谱、峰度谱以及尺度-尺度相关等非Gauss特征的表征量在研究的尺度(>100kpc)上表现一致,这就支持了拟合的结果．     

大大质量星系的尺度演化研究进展

星系(恒星质量M_*10~(10)M_⊙)的尺度(re)与M*之间有着明显的re_∝Mα_*关系。已有的研究揭示晚型星系(LTGs)和早型星系(ETGs)的α值分别对应0.22±0.03和0.75±0.05。另外,星系的尺度r_e与红移z之间存在演化关系:re_∝(1+z)~β。对于z3的ETGs来说,它们在静止光学波段的尺度随时间生长的指数β=-1.5～-1.2,其在LTGs中对应β=-0.8～-0.6。介绍了星系形成理论模型,星系尺度的测量,星系恒星质量与星系尺度之间的关系,不同类型星系的尺度演化,以及阐述了该领域未来的研究方向。     

DII类射电双源子源尺度的演化

本文对60个DII类射电双源的子源尺度进行了统计,发现: (一)与双源主轴方向垂直的子源线径(d_⊥)有一定的范围,其上限为150Kpc,下限约为4至6 Kpc. (二)对红移值(Z)在0.35至0.6之间的13个样品进行相关分析,得到中心天体绝对星等(M_v)与两子源的平均线径(d_⊥)的关系可用方程M_v=—19.75-2.75log(d_⊥/d_0)拟合,其相关系数R=0.8,其中d_0选为1Kpc. (三)全部样品在M_v—d_⊥图上的分布有上限.分布范围的边界约可表达为M_v=—19.75—2log(d_⊥ /d_0). 本文据此认为可能存在随中心天体能量耗损,M_v升高而d_⊥减小的演化过程,并提出了一个能量按指数衰减的非膨胀射束模型来解释这种演化过程.     

CDFS天区中喑蓝星系的测光红移研究

从COMBO-17数字巡天数据里,选择了CDFS(Chandra Deep Field South)天区中1231个测光红移在0.1～0.3之间的暗蓝星系作为样本,研究了这些星系分别在只有光学波段和光学加近红外波段数据情况下做测光红移得到的红移分布,以及这些星系在静止参考系下的能谱分布(Spectral Energy Distributions,SEDs)特征.结果表明有183个星系在利用光学加近红外波段数据做测光红移时得到的红移大于1.2,它们的误差为0.046,提高测光的信噪比也有利于区分这类被光学波段误认为低红移的星系.这些暗蓝星系中高红移星系的观测近红外流量相对于光学流量有上升的趋势,而低红移星系的观测近红外流量相对于光学流量有下降的趋势.     

双稳系统中的随机共振乘性信噪作用研究

方波信号和双值噪声在科学领域广泛应用,而乘性信号在工程中广泛存在。但是,乘性方波信号和乘性双值噪声对双稳系统的非线性影响方面的研究还较少,因此,本文研究乘性方波信号和乘性双值噪声作用下双稳系统中的随机共振现象。基于绝热近似条件,推导系统信噪比的表达式,并基于该公式进行了不同输入条件的仿真研究。研究结果表明,信噪比随着乘性方波信号和乘性双值噪声幅度的变化可以取得一个最大值和一个最小值。大的乘性方波信号幅值会抑制信噪比,对于噪声相关率λ=0.65的情形,方波幅值A=1.0时的系统输出幅度比A=0.3时的输出幅值下降约29%;而较大的乘性双值噪声强度可以提升系统的输出性能,当系统参数b=3.8,乘性双值噪声强度ε=3.5时的系统输出幅度为ε=0.25时的输出幅值的2.5倍。随着乘性白噪声、加性白噪声及系统参数的变化,信噪比曲线出现一个峰值。     

三点相关统计的理论与测量

本文分别从数值模拟、理论和观测的角度研究和分析了三点相关函数以及双谱对尺度、形状和光度等的依赖性,并比较了数值模拟与二阶扰动理论以及暗晕模型的差别。我们发现二阶扰动理论即使在线性尺度上也是与数值模拟存在明显偏差的,它并不足以很好地描述暗物质双谱在大尺度上的行为。如果要与数值模拟更好地吻合,我们需要引入更高阶的修正。我们用其他人的半解析模型构建了对应于我们数值模拟的模拟星系样本,并计算了不同光度星系样本的双谱从而得到了相应的星系偏袒值。我们发现,对星系密度场的泰勒二阶展开是可以适用到准线性尺度k(?)0.15h/Mpc上的,但是要通过三点相关来获得准确的星系偏袒值,我们需要拥有对暗物质密度场的准确估计,这却是二阶非线性扰动理论所无法做到的。暗晕模型在定性描述双谱行为方面是十分有效的,但是它与数值模拟的定量比较还是存在很多不同。要用暗晕模型来精确描述双谱,我们需要对模型的各种设定做出精细的调节和改进。最后,我们还利用了SDSS的最新观测数据测量了红移空间的三点相关函数以及投影三点相关函数,并研究它们对于星系光度、颜色和恒星质量等性质的依赖关系。我们发现,不同于之前的许多工作,星系的归约三点相关函数对光度存在明显的依赖性,而这种依赖性却是与三角形的形状和尺度耦合在一起。三点相关对恒星质量的依赖与光度依赖性十分类似。而颜色的依赖性在小尺度上则比光度和恒星质量更显著一些。     

基于小波方法的平滑算法在引力透镜中的应用

基于离散小波变换(DWT)方法,提出了一种可用于计算三维数值模拟样本面密度的平滑算法.为检验方法的有效性,利用该算法研究了两组不同质量解析度的引力透镜数值模拟样本,样本采用了暗物质晕的等温椭球模型,使用蒙特卡罗方法生成.计算结果表明此算法能够在很高的精度上构建引力透镜模拟样本的面密度分布轮廓,由面密度计算出来的透镜的临界曲线和焦散曲线也能较好地和理论曲线吻合,结果是令人满意的.同时比较了三组不同的小波基的计算结果,包括Daub4,Daub6和B-spline 3th,给出了最优的选择.在不损失平滑效果的同时,此算法具有非常高的速度,非常适合于处理以后更高精度的N体数值模拟.     

基于线性数据重构的天线动力学模型辨识方法

射电望远镜天线伺服控制系统中的非线性特性, 对系统动力学特性辨识有着显著的影响, 会提高辨识难度, 增加辨识模型的复杂程度. 系统非线性特性的测量与补偿也会增加系统辨识工作量. 针对上述问题, 提出了一种基于非线性采样数据的线性重构方法, 用于动力学特性建模. 通过提取原采样数据的相位与幅值, 对受到噪声与非线性畸变影响的系统采样数据进行线性重构, 降低待辨识模型的复杂度. 搭建了半实物实验平台, 以平台实际采样为基础, 重构线性数据, 利用奇异值法与自回归神经网络评估并辨识平台动力学模型. 实验结果表明, 建模数据奇异值拐点从100阶下降至40阶, 仅用10个神经网络节点200次训练即实现了模型辨识.     

基于Stacking集成学习的恒星/星系分类研究

机器学习在当今诸多领域已经取得了巨大的成功,但是机器学习的预测效果往往依赖于具体问题.集成学习通过综合多个基分类器来预测结果,因此,其适应各种场景的能力较强,分类准确率较高.基于斯隆数字巡天(Sloan Digital Sky Survey,SDSS)计划恒星/星系中最暗源星等集分类正确率低的问题,提出一种基于Stacking集成学习的恒星/星系分类算法.从SDSS-DR7(SDSS Data Release 7)中获取完整的测光数据集,并根据星等值划分为亮源星等集、暗源星等集和最暗源星等集.仅针对分类较为复杂且困难的最暗源星等集展开分类研究.首先,对最暗源星等集使用10折嵌套交叉验证,然后使用支持向量机(Support Vector Machine,SVM)、随机森林(Random Forest,RF)、XGBoost(eXtreme Gradient Boosting)等算法建立基分类器模型;使用梯度提升树(Gradient Boosting Decision Tree,GBDT)作为元分类器模型.最后,使用基于星系的分类正确率等指标,与功能树(Function Tree,FT)、SVM、RF、GBDT、XGBoost、堆叠降噪自编码(Stacked Denoising AutoEncoders,SDAE)、深度置信网络(Deep Belief Network,DBN)、深度感知决策树(Deep Perception Decision Tree,DPDT)等模型进行分类结果对比分析.实验结果表明,Stacking集成学习模型在最暗源星等集分类中要比FT算法的星系分类正确率提高了将近10%.同其他传统的机器学习算法、较强的提升算法、深度学习算法相比,Stacking集成学习模型也有较大的提升.     

Discriminating Dark Energy Models by Statistics of the Peak Count and Scale-scale Correlation of Weak Gravitational Lensing

In order to study the weak gravitational lensing effect under different cosmologicalmodels, the 2-dimensional κ-field samples are generated by using the ray-tracing method with high-resolution N-body simulation data. These samples correspond to three models with different parameters of dark-energy equation of state, i.e., ω = −0.8, ω = −1.0 and ω = −1.2, and have the same field of view of 3° × 3°. It is assumed that all galaxies, as background sources, are distributed on the plane of z = 1. The statistics of peak count and scale-scale correlation are performed on these samples. The statistical result of peak counts indicates that in the noisy κ field, some differences of peak distributions exist among various models. The noise has changed the distributions of the peaks with medium and low amplitudes, but has nearly no effect on high-amplitude peaks. However, after denoising the differences of high-amplitude peak distributions among various models become very clear. For the statistics of scale-scale correlation, the cumulative probability distribution functions of scale-scale correlation coefficients of different models are analyzed. It is found that the differences between the model of ω = −1.2 and the models of ω = −1.0 and ω = −0.8 can reach 20% and 30%, respectively. Hence the statistics of scale-scale correlation combining with the statistics of peak count can be taken as a new method to determine the dark energy equation-of-state parameter.     

Source–lens clustering effects on the skewness of the lensing convergence

The correlation between source galaxies and lensing potentials causes a systematic effect on measurements of cosmic shear statistics, known as the source–lens clustering (SLC) effect. The SLC effect on the skewness of lensing convergence, S ₃, is examined using a non-linear semi-analytic approach and is checked against numerical simulations. The semi-analytic calculations have been performed in a wide variety of generic models for the redshift distribution of source galaxies and power-law models for the bias parameter between the galaxy and dark matter distributions. The semi-analytic predictions are tested successfully against numerical simulations. We find the relative amplitude of the SLC effect on S ₃ to be of the order of  5–40  per cent. It depends significantly on the redshift distribution of sources and on the way in which the bias parameter evolves. We discuss possible measurement strategies to minimize the SLC effects.     

Mixed three-point correlation functions of the non-linear integrated Sachs–Wolfe effect and their detectability

In this paper, I investigate the family of mixed three-point correlation functions  〈τ ^q γ^{3− q} 〉, q = 0, 1, 2  , between the integrated Sachs–Wolfe (iSW) temperature perturbation τ and the galaxy overdensity γ as a tool for detecting the gravitational interaction of cosmic microwave background (CMB) photons with the potentials of non-linearly evolving cosmological structures. Both the iSW-effect and the galaxy overdensity are derived in hyperextended perturbation theory to second order and I emphasize the different parameter sensitivities of the linear and non-linear iSW-effect. I examine the configuration dependence of the relevant bispectra, quantify their sensitivities and discuss their degeneracies with respect to the cosmological parameters  Ω_m, σ₈, h   and the dark energy equation of state parameter w . I give detection significances for combining Planck CMB data and galaxy sample of a survey like Dark UNiverse Explorer (DUNE) by using a quadratic approximation for the likelihood with Λ cold dark matter (ΛCDM) as the fiducial cosmology: the combination of Planck with DUNE should be able to reach a cumulative signal-to-noise ratio of ≃0.6 for the bispectrum  〈τγ²〉  up to ℓ= 2000, which is too weak to be detected. The most important noise source is the primary CMB fluctuations as the Poisson noise in the galaxy number density is almost negligible for a survey like DUNE.     

Probing quintessence: reconstruction and parameter estimation from supernovae

We explore the prospects for using future supernova observations to probe the dark energy. We focus on quintessence, an evolving scalar field that has been suggested as a candidate for the dark energy. After simulating the observations that would be expected from the proposed SuperNova / Acceleration Probe satellite ( SNAP ), we investigate two methods for extracting information concerning quintessence from such data. First, by expanding the quintessence equation of state as   w _Q ( z ) = w _Q (0) −α ln(1 + z )  , to fit the data, it is possible to reconstruct the quintessence potential for a wide range of smoothly varying potentials. Secondly, it will be possible to test the basic properties of the dark energy by constraining the parameters  Ω _Q , w _Q   and α. We show that it may be possible, for example, to distinguish between quintessence and the cosmological constant in this way. Furthermore, when supernova data are combined with other planned cosmological observations, the precision of reconstructions and parameter constraints is significantly improved, allowing a wider range of dark energy models to be distinguished.     

Dark energy and future singularity of the universe in Kaluza-Klein space time

The dark energy model with the equation of state \(p_{\mathit{DE}} = {-} \rho_{\mathit{DE}} - A\rho_{\mathit{DE}}^{\alpha} \) is studied in Kaluza-Klein space time. The model comprises and provides realization of several types of singularities in different parameter regimes. We discuss the finite-time singularities into four classes and explicitly present the models which give rise to these singularities by assuming the form of the equation of state of dark energy. Also, we discussed the models in terms of the cosmological redshift and some observational parameters.     

Large-scale cosmic microwave background anisotropies and dark energy

In this paper we investigate the effects of perturbations in a dark energy component with a constant equation of state on large-scale cosmic microwave background (CMB) anisotropies. The inclusion of perturbations increases the large-scale power. We investigate more speculative dark energy models with   w < −1  and find the opposite behaviour. Overall the inclusion of perturbations in the dark energy component increases the degeneracies. We generalize the parametrization of the dark energy fluctuations to allow for an arbitrary constant sound speed, and we show how constraints from CMB experiments change if this is included. Combining CMB with large-scale structure, Hubble parameter and supernovae observations we obtain   w =−1.02 ± 0.16 (1σ)  as a constraint on the equation of state, which is almost independent of the sound speed chosen. With the presented analysis we find no significant constraint on the constant speed of sound of the dark energy component.     

Smoothing supernova data to reconstruct the expansion history of the Universe and its age

We propose a non-parametric method of smoothing supernova data over redshift using a Gaussian kernel in order to reconstruct important cosmological quantities including   H ( z )  and   w ( z )  in a model-independent manner. This method is shown to be successful in discriminating between different models of dark energy when the quality of data is commensurate with that expected from the future Supernova Acceleration Probe ( SNAP ). We find that the Hubble parameter is especially well determined and useful for this purpose. The look-back time of the Universe may also be determined to a very high degree of accuracy (≲0.2 per cent) using this method. By refining the method, it is also possible to obtain reasonable bounds on the equation of state of dark energy. We explore a new diagnostic of dark energy – the ' w -probe'– which can be calculated from the first derivative of the data. We find that this diagnostic is reconstructed extremely accurately for different reconstruction methods even if Ω_{0 m} is marginalized over. The w -probe can be used to successfully distinguish between Λ cold dark matter and other models of dark energy to a high degree of accuracy.     

The Abundance of QSOs in Various Cold Dark Matter Models

Based on the statistics of the peaks in the random Gaussian field, we put forward a method to model the evolution of quasars, which are assumed to be a phenomenon of the early stages for some massive galaxies. Our calculations show that the cold-hot mixed dark matter model with _h = 0.3 cannot match the observed quasar abundance at redshift z = 4.5, and other prevailing CDM models, such as the mixed dark matter model with _h = 0.2, the tilted CDM models with n = 0.7 or 0.8, and the flat CDM models with = 0.6 or 0.7, are compatible with the observation of the abundance of QSOs on the range of z = 2-5.     

measurement of noise power spectrum in photon counting PMT's and the effect of the observed (1/f) noise on astronomical photometry

In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques.     

De Sitter universe described by a binary mixture with a variable cosmological constant $\lambda $

We have constructed a self-consistent system of Bianchi Type VI₀ cosmology, and mingling of perfect fluid and dark energy in five dimensions. The usual equation of state \(p = \gamma \rho \) with \(\gamma \in [0, 1]\) is chosen by the perfect fluid. The dark energy assumed to be chosen is taken into consideration to be either the quintessence or Chaplygin gas. The same solutions pertaining to the corresponding the field equations of Einstein are obtained as a quadrature. State parameter’s equations for dark energy \(\omega \) is found to be consistent enough with the recent observations of SNe Ia data (SNe Ia data with CMBR anisotropy) and galaxy clustering statistics. Here our models predict that the rate of expansion of Universe would increase with passage of time. The cosmological constant \(\varLambda \) is traced as a declining function of time and it gets nearer to a small positive value later on which is supported by the results from the current supernovae Ia observations. Also a detail discussion is made on the physical and geometrical aspects of the models.