如何抑制星系巡天中的散粒噪声
How to suppress the shot noise in galaxy surveys
摘要：
星系巡天是提取宇宙学信息的最有力手段之一，对于给定的体积，可达到的精度取决于星系散粒噪声σ_n ^2相对于功率谱P的关系。通常假设散粒噪声是白色的，由数密度n的倒数给出。在本文中，我们认为，一个可能是能够大大改善后：在晕的宇宙结构图片的所有的暗物质是在不同质量的晕和星系内形成这些晕，但对于暗物质的质量和动量守恒保证，非线性效应不能开发一个大尺度上的暗物质功率谱的白噪声。这表明，通过适当的加权，可以为星系实现类似的效果，抑制它们的散粒噪声。我们用N体模拟来探索这一想法，通过对中心晕星系进行晕质量加权，发现由此产生的散粒噪声相对于预期值可以显著降低，在我们的模拟中，在最高数密度n=4*10^-3（Mpc/h）^[3]处得到了10-30的抑制。对于特定的应用，其他加权方案可能会获得更好的结果，对于n=3*10^-4（Mpc/h）^[3]，我们可以将sigma_n^2/P相对于均匀加权降低10倍。这些结果开辟了新的机会，以提取宇宙学信息的星系调查，如最近提出的多示踪剂的方法，以消除抽样方差，并可能有重要的后果，为规划未来的红移调查。充分利用这些发现可能需要更好地理解星系形成过程，以开发晕质量的精确示踪剂。
英文摘要：
Galaxy surveys are one of the most powerful means to extract the cosmological information and for a given volume the attainable precision is determined by the galaxy shot noise sigma_n^2 relative to the power spectrum P. It is generally assumed that shot noise is white and given by the inverse of the number density n. In this paper we argue one may be able to considerably improve upon this: in the halo picture of cosmological structure all of the dark matter is in halos of varying mass and galaxies are formed inside these halos, but for the dark matter mass and momentum conservation guarantee that nonlinear effects cannot develop a white noise in the dark matter power spectrum on large scales. This suggests that with a suitable weighting a similar effect may be achieved for galaxies, suppressing their shot noise. We explore this idea with N-body simulations by weighting central halo galaxies by halo mass and find that the resulting shot noise can be reduced dramatically relative to expectations, with a 10-30 suppression at the highest number density of n=4*10^-3 (Mpc/h)^3 resolved in our simulations. For specific applications other weighting schemes may achieve even better results and for n=3*10^-4(Mpc/h)^3 we can reduce sigma_n^2/P by up to a factor of 10 relative to uniform weighting. These results open up new opportunities to extract cosmological information in galaxy surveys, such as the recently proposed multi-tracer approach to cancel sampling variance, and may have important consequences for the planning of future redshift surveys. Taking full advantage of these findings may require better understanding of galaxy formation process to develop accurate tracers of the halo mass.
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