共查询到18条相似文献，搜索用时 406 毫秒
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重谱即傅立叶空间的三点相关函数，是研究由引力不稳定性产生的大尺度结构的最低非高斯阶统计性质．在傅立叶空间中，发现二维和三维空间的统计性质是等价的，相对三维快速傅立叶，二维快速傅立叶变换可以节省很多计算机资源．结合二维和三维的快速傅立叶变换，利用高精度的初始谱指数为-1的N体数值模拟，测量了从弱线性区域到强非线性区域的重谱．在测量过程中，考虑并严格校正了作用于功率谱和重谱上的数值效应，如离散效应（particle discreteness）、软化因子（force softening）、模拟盒子的大小、混淆效应（alias effect）．发现约化重谱即使在强非线性区域仍然依赖于三角形的形状和尺度，并随着波数的增加而增加，这和前人的假设完全不同．在测量结果的基础上，拟合了重谱的一个适用于初始谱指数为-1的宇宙学模型的经验公式． 相似文献
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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。 相似文献
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CygX—1硬态高能光子的时延 总被引：1，自引：1，他引：0
硬X射线和软射线光子的时延时研究主能辐射的一个重要方面，傅里叶交叉谱常常被用于教育处硬软光子之间的时延，但是交叉说示能在高于30Hz的傅里叶频率上从CygX-1测量到有统计意义的快速光变，由李惕培在时域上发展的交叉相关函方法能有效测量不同时间尺度上的时延，利用此交叉相关分析方法，讨论了不同观测时期CygX-1硬态高能光子时延 的性质，分析结果表明，CygX-1硬态在短的尺度（＜0．1秒）上存存在有意义的时间延迟，并将短时间尺度上的观测结果和各种CygX-1硬态模型进行了比较讨论。 相似文献
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分析了Balastegui等（2001）应用神经网络划分出的两类长γ暴的平均傅里叶功率谱（PDS），两类暴的平均PDS都具有明显的幂律谱结构，对两类暴分别按亮度和能谱硬度分组，计算各组的平均功率谱，两类暴都具有平均功率谱随亮度和能谱硬度的增加而变平的趋势。 相似文献
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首先考察了谱线形成深度理论及其在诊断太阳大气中矢量磁场分层结构方面的应用,指出了分别与贡献函数和响应函数相关的方法的优缺点,然后用理论斯托克斯轮廓观察各轮廓参量沿视线方向梯度对轮廓尤其是它的对称性和极值点的影响,在此基础上提出了基本假设使偏振辐射转移方程在极值点得到极大的简化而创立了一种不依赖形成深度理论的新方法。它由四个操作构成,将其中三个操作循环处理便可得到位于所考虑的沿深度方向格点化的层次上 相似文献
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给出了4种分析类星体长周期光变的方法,用一个模拟的周期信号y=sinθ检验这4种分析方法,结果表明:(1)天体光变采样的数据点个数相对少到一定值时,Jurkevich方法、时间补偿离散傅里叶变换分析方法(DCDFT)、离散相关分析方法(DCF)和功率谱密度分析方法(PSD)的分析结果不一样,获取最短的连续数据点后,Jurkevich方法分析结果在4种方法中可能最为精确可靠,且计算方法简捷实用;(2)获得了Jurkevich分析方法的最佳参数,当m=9时分析结果最佳;(3)用m=9时的Jurkevich方法分析了类星体3C 279及3C 454.3的光变周期,得出3C 279的可能光变周期为(2.81±0.54)年,3C 454.3的可能光变周期为457 d。 相似文献
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观测到的偏振量与法拉第色散函数之间是傅里叶变换对,而法拉第色散函数反映了辐射区域和辐射传播途径的磁场结构。如何通过这一关系精确重构出法拉第色散函数对于研究银河系及河外星系磁场具有重要的作用。目前已提出了基于压缩感知的法拉第色散函数重构方法,模拟结果要优于传统方法,然而是否具有实用性仍然未知。主要探究该方法应用于实际观测频率范围时是否依然可行,并进行了大样本统计学实验。结果表明:重构结果受多种因素的影响,具有很大的随机性,对重构结果在峰值附近再次进行最小二乘拟合后,重构的法拉第深度更接近真实值。 相似文献
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《Chinese Astronomy and Astrophysics》2006,30(2):133-149
Bispectrum, or three-point correlation function in Fourier space, is the lowest order non-Gaussian statistic to study the large-scale structure produced by gravitational instability. In Fourier space, the 2-dimensional density field has the same statistical properties as the 3-dimensional field, and compared with the 3-dimensional fast Fourier transform (FFT), the 2-dimensional FFT can save a lot of computer resources. In a combination of 2- and 3-dimensional FFTs and using a high-resolution N-body numerical simulation with the initial spectral index -1, we have measured the bispectrum from the weakly linear regime to the strongly nonlinear regime. All possible effects of numerical artefacts on the power spectrum and bispectrum, such as particle discreteness, force softening, finite box size and alias effect, are considered and strictly corrected. In contrast with what previous researchers assumed, it is revealed that the reduced bispectrum depends on the triangle shape and scale even in the strongly nonlinear regime, and increases with the wave number. And based on the measured results, a new fitting formula for the bispectum suitable for cosmological models with an initial spectral index −1 is obtained. 相似文献
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S. Pires J.-L. Starck A. Amara R. Teyssier A. Réfrégier J. Fadili 《Monthly notices of the Royal Astronomical Society》2009,395(3):1265-1279
With increasingly large data sets, weak lensing measurements are able to measure cosmological parameters with ever-greater precision. However, this increased accuracy also places greater demands on the statistical tools used to extract the available information. To date, the majority of lensing analyses use the two-point statistics of the cosmic shear field. These can be either studied directly using the two-point correlation function or in Fourier space, using the power spectrum. But analysing weak lensing data inevitably involves the masking out of regions, for example to remove bright stars from the field. Masking out the stars is common practice but the gaps in the data need proper handling. In this paper, we show how an inpainting technique allows us to properly fill in these gaps with only N log N operations, leading to a new image from which we can compute straightforwardly and with a very good accuracy both the power spectrum and the bispectrum. We then propose a new method to compute the bispectrum with a polar fft algorithm, which has the main advantage of avoiding any interpolation in the Fourier domain. Finally, we propose a new method for dark matter mass map reconstruction from shear observations, which integrates this new inpainting concept. A range of examples based on 3D N -body simulations illustrates the results. 相似文献
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Xiaohu Yang H. J. Mo Frank C. van den Bosch Y. P. Jing 《Monthly notices of the Royal Astronomical Society》2005,357(2):608-618
We analyse the two-point correlation function (2PCF) of galaxy groups identified from the 2-degree Field Galaxy Redshift Survey with the halo-based group finder recently developed by Yang et al. With this group catalogue we are able to estimate the 2PCFs for systems ranging from isolated galaxies to rich clusters of galaxies. The real-space correlation length obtained for these systems ranges from ∼4 to ∼15 h −1 Mpc, respectively. The observed correlation amplitude (and the corresponding bias factor) as a function of group abundance is well reproduced by associating galaxy groups with dark matter haloes in the standard Λ-cold dark matter model. Redshift distortions are clearly detected in the redshift-space correlation function, the degree of which is consistent with the assumption of gravitational clustering and halo bias in the cosmic density field. In agreement with previous studies we find a strong increase of the correlation length with the mean intergroup separation. Although well-determined observationally, we show that current theoretical predictions are not yet accurate enough to allow for stringent constraints on cosmological parameters. Finally, we use our results to explore the power-law nature of the 2PCF of galaxies. We split the 2PCF into one- and two-group terms, equivalent to the one- and two-halo terms in halo occupation models, and show that the power-law form of the 2PCF is broken, when only including galaxies in the more massive systems. 相似文献
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A. F. Heavens 《Monthly notices of the Royal Astronomical Society》1998,299(3):805-808
The bispectrum of the microwave background sky is a possible discriminator between inflationary and defect models of structure formation in the Universe. The bispectrum, which is the analogue of the temperature three-point correlation function in harmonic space, is zero for most inflationary models, but non-zero for non-Gaussian models. The expected departures from zero are small, and easily masked by noise, so it is important to be able to estimate the bispectrum coefficients as accurately as possible, and to know the errors and correlations between the estimates so that they may be used in combination as a diagnostic to rule out non-Gaussian models. This paper presents a method for estimating in an unbiased way the bispectrum from a microwave background map in the near-Gaussian limit. The method is optimal, in the sense that no other method can have smaller error bars, and, in addition, the covariances between the bispectrum estimates are calculated explicitly. The method deals automatically with partial sky coverage and arbitrary noise correlations without modification. A preliminary application to the Cosmic Background Explorer 4-yr data set shows no evidence for non-Gaussian behaviour. 相似文献
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We perform a frequentist analysis of the bispectrum of the Wilkinson Microwave Anisotropy Probe first-year data. We find clear signal domination up to ℓ≈ 200 , with overall consistency with Gaussianity, except for the following features. There is a flat patch (i.e. a low χ2 region) in the same-ℓ components of the bispectrum spanning the range ℓ= 32–62 ; this may be interpreted as ruling out Gaussianity at the 99.6 per cent confidence level. There is also an asymmetry between the north and south inter-ℓ bispectrum components at the 99 per cent confidence level. The preferred asymmetry axis correlates well with the (ℓ, b ) = (57, 10) direction quoted in the literature for asymmetries in the power spectrum and three-point correlation function. However, our analysis of the quadrupole (its bispectrum and principal axes) fails to make contact with previously claimed anomalies. 相似文献
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Licia Verde Alan F. Heavens Sabino Matarrese 《Monthly notices of the Royal Astronomical Society》2000,318(2):584-598
We present a theoretical and exact analysis of the bispectrum of projected galaxy catalogues. The result can be generalized to evaluate the projection in spherical harmonics of any 3D bispectrum and therefore has applications to cosmic microwave background and gravitational lensing studies.
By expanding the 2D distribution of galaxies on the sky in spherical harmonics, we show how the three-point function of the coefficients can be used in principle to determine the bias parameter of the galaxy sample. If this can be achieved, it would allow a lifting of the degeneracy between the bias and the matter density parameter of the Universe, which occurs in linear analysis of 3D galaxy catalogues. In previous papers, we have shown how a similar analysis can be done in three dimensions, and we show here through an error analysis and by implementing the method on a simulated projected catalogue that ongoing three-dimensional galaxy redshift surveys (even with all the additional uncertainties introduced by partial sky coverage, redshift-space distortions and smaller numbers) will do far better than all-sky projected catalogues with similar selection function. 相似文献
By expanding the 2D distribution of galaxies on the sky in spherical harmonics, we show how the three-point function of the coefficients can be used in principle to determine the bias parameter of the galaxy sample. If this can be achieved, it would allow a lifting of the degeneracy between the bias and the matter density parameter of the Universe, which occurs in linear analysis of 3D galaxy catalogues. In previous papers, we have shown how a similar analysis can be done in three dimensions, and we show here through an error analysis and by implementing the method on a simulated projected catalogue that ongoing three-dimensional galaxy redshift surveys (even with all the additional uncertainties introduced by partial sky coverage, redshift-space distortions and smaller numbers) will do far better than all-sky projected catalogues with similar selection function. 相似文献