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arXiv 1302.0183astro-ph.CO
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IM3SHAPE:宇宙引力透镜的最极大似然星系切变测量代码

IM3SHAPE: A maximum-likelihood galaxy shear measurement code for cosmic gravitational lensing

Joe Zuntz, Tomasz Kacprzak, Lisa Voigt, Michael Hirsch, Barnaby Rowe, Sarah Bridle

摘要:

我们提出并描述im3shape,一个新的公开可用的星系形状测量代码弱引力透镜剪切。im3shape对有噪声的图像进行了膨胀加盘星系模型的极大似然拟合,并结合了应用的点扩散函数。我们详细介绍了面临的挑战和选择,在其设计和实施,然后讨论各种限制,影响这个和其他极大似然方法。我们评估了使用简单的无噪声图像拟合不正确的星系模型所产生的偏差,并发现它不应该成为当前宇宙剪切调查的一个问题。我们在GREAT08 Challenge图像模拟上测试im3shape,并在模拟被拟合模型所包含的情况下,使用图像噪声偏差的简单校正,满足即将到来的宇宙剪切调查的要求。对于GREAT08的基准分支,我们得到了一个可以忽略的加性剪切偏差和小于2%的乘性偏差,这是适合于分析当前的调查。我们没有达到即将进行的调查的百分之几级别的要求,我们将其归因于噪声偏差和我们的星系模型与GREAT08模拟中使用的模型之间的不匹配。我们满足GREAT08所有分支的当前调查要求,除了那些小或高噪声星系,我们将从我们的分析中删除。使用GREAT08度量,我们获得了可用分支的Q=717的分数,相对于未来实验的Q=1000的目标。该代码可从https://bitbucket.org/joezuntz/im3shape免费获取

英文摘要:

We present and describe im3shape, a new publicly available galaxy shape measurement code for weak gravitational lensing shear. im3shape performs a maximum likelihood fit of a bulge-plus-disc galaxy model to noisy images, incorporating an applied point spread function. We detail challenges faced and choices made in its design and implementation, and then discuss various limitations that affect this and other maximum likelihood methods. We assess the bias arising from fitting an incorrect galaxy model using simple noise-free images and find that it should not be a concern for current cosmic shear surveys. We test im3shape on the GREAT08 Challenge image simulations, and meet the requirements for upcoming cosmic shear surveys in the case that the simulations are encompassed by the fitted model, using a simple correction for image noise bias. For the fiducial branch of GREAT08 we obtain a negligible additive shear bias and sub-two percent level multiplicative bias, which is suitable for analysis of current surveys. We fall short of the sub-percent level requirement for upcoming surveys, which we attribute to a combination of noise bias and the mis-match between our galaxy model and the model used in the GREAT08 simulations. We meet the requirements for current surveys across all branches of GREAT08, except those with small or high noise galaxies, which we would cut from our analysis. Using the GREAT08 metric we we obtain a score of Q=717 for the usable branches, relative to the goal of Q=1000 for future experiments. The code is freely available from https://bitbucket.org/joezuntz/im3shape

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