Limitations on shapelet-based weak-lensing measurements
Limitations on shapelet-based weak-lensing measurements
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DOI：
10.1051/0004-6361/200912785
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发表时间：
2009-06
影响因子：
6.5
通讯作者：
Peter Melchior;A. Boehnert;M. Lombardi;M. Bartelmann
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文献类型：
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作者：
Peter Melchior;A. Boehnert;M. Lombardi;M. Bartelmann
目标。我们试图了解在两种现实条件下圆形和椭圆形状模型对形状估计量的影响：(a)模型中只有有限数量的形状模型可用，(b)星系的固有形状并不局限于形状模型。方法。我们创建了一组简单的模拟，其中星系的形状遵循Sersic轮廓。通过改变Sersic指数和应用剪切，我们量化了由于建模不足而产生的剪切估计偏差的数量。另外还讨论了PSF卷积、像素化和像素噪声的复杂性。结果。高度椭圆的星系形状不能在圆形基础系统中精确地建模，并且偏向于不那么细长的形状。这个问题可以通过允许椭圆基函数来解决。另一个问题出现在星系，其径向轮廓与圆形和椭圆形小块基中使用的高斯加权函数有很大不同。对于具有大Sersic指数的星系，由于偏爱圆形模型，即使在较小的表观椭圆度下，圆形小颗粒的剪切估计也有较低的偏差，而椭圆小颗粒的剪切估计主要依赖于精确的椭圆度先验。与估计器的特定形式无关，偏差取决于真实的本征星系形态，但也取决于PSF的大小和形状。结论。只要这里讨论的问题没有得到解决，我们就质疑shapelet方法能否为所有可观测星系（独立于它们的Sersic指数）提供弱透镜测量，并达到即将到来的任务和调查所要求的精度。需要针对正在调查的数据集进行准确可靠的校准，但很难从数据中进行一致的推断。
Aims. We seek to understand the impact on shape estimators obtained from circular and elliptical shapelet models under two realistic conditions: (a) only a limited number of shapelet modes are available for the model, and (b) the intrinsic galactic shapes are not restricted to shapelet models. Methods. We create a set of simplistic simulations, in which the galactic shapes follow a Sersic profile. By varying the Sersic index and applied shear, we quantify the amount of bias on shear estimates that arises from insufficient modeling. Additional complications from PSF convolution, pixelation, and pixel noise are also discussed. Results. Highly elliptical galaxy shapes cannot be accurately modeled within the circular shapelet basis system and are biased towards less elongated shapes. This problem can be cured by allowing elliptical basis functions. Another problem occurs for galaxies whose radial profile differs strongly from the Gaussian weighting function employed in both the circular and the elliptical shapelet bases. For galaxies with large Sersic indices, shear estimates from circular shapelets are biased low even for small apparent ellipticities because of the preference for round models, and shear estimates from elliptical shapelets depend critically on accurate ellipticity priors. Independent of the particular form of the estimator, the bias depends on the true intrinsic galaxy morphology, but also on the size and shape of the PSF. Conclusions. As long as the issues discussed here are not solved, we question that the shapelet method can provide weak-lensing measurements for all observable galaxies (independent of their Sersic index) with an accuracy demanded by upcoming missions and surveys. An accurate and reliable calibration, specific for the dataset under investigation, would be required but is difficult to infer consistently from the data.