2 MASS聚类偶极是否收敛？
Is the 2MASS clustering dipole convergent?
摘要：
关于星系角分布的偶极矩的收敛性（即所谓的成团偶极）一直存在争议。本文研究了星系团偶极的增长随2微米全天巡天（2 MASS）样品极限通量的变化。我们发现，在2 MASS扩展源表的完备性极限之前，即在近红外K_s波段13.5 mag（相当于300 Mpc/h的有效距离），偶极子不收敛。我们将观察到的偶极子增长与理论预期的有条件增长（即，给定本星系群相对于宇宙微波背景辐射的速度），对于由WMAP约束的LambdaCDM功率谱和宇宙学参数。观察到的增长原来是在1-sigma置信水平的理论对应的2 MASS通量限制目录的适当的观测窗口包括在内。作为对比，如果采用的窗口是顶帽，那么预测的偶极子增长明显更快，并且在大约300 Mpc/h的距离内收敛到其最终值。通过比较观测窗口，我们表明，对于一个给定的通量限制和相应的距离限制，2 MASS通量加权窗口通过更少的大尺度信号比顶帽之一。我们的结论是，2 MASS偶极子的有效距离大于200 Mpc/h的增长是显而易见的。另一方面，对于80 Mpc/h的距离（2 MASS红移巡天的平均深度）和LambdaCDM功率谱，真正的偶极子预计只能达到其最终值的80%。最后，由于对于2 MASS的窗口函数，预测的增长与观察到的增长一致，我们可以比较两者以评估beta =（Omega_m）^0.55 / b。结果是beta = 0.38+-0.04，这导致密度参数Omega_m的估计值= 0.20+-0.08。
英文摘要：
There is a long-standing controversy about the convergence of the dipole moment of the galaxy angular distribution (the so-called clustering dipole). We study the growth of the clustering dipole of galaxies as a function of the limiting flux of the sample from the Two Micron All Sky Survey (2MASS). Contrary to some earlier claims, we find that the dipole does not converge before the completeness limit of the 2MASS Extended Source Catalog, i.e. up to 13.5 mag in the near-infrared K_s band (equivalent to an effective distance of 300 Mpc/h). We compare the observed growth of the dipole with the theoretically expected, conditional one (i.e., given the velocity of the Local Group relative to the CMB), for the LambdaCDM power spectrum and cosmological parameters constrained by WMAP. The observed growth turns out to be within 1-sigma confidence level of its theoretical counterpart once the proper observational window of the 2MASS flux-limited catalog is included. For a contrast, if the adopted window is a top-hat, then the predicted dipole grows significantly faster and converges to its final value for a distance of about 300 Mpc/h. By comparing the observational windows, we show that for a given flux limit and a corresponding distance limit, the 2MASS flux-weighted window passes less large-scale signal than the top-hat one. We conclude that the growth of the 2MASS dipole for effective distances greater than 200 Mpc/h is only apparent. On the other hand, for a distance of 80 Mpc/h (mean depth of the 2MASS Redshift Survey) and the LambdaCDM power spectrum, the true dipole is expected to reach only ~80% of its final value. Eventually, since for the window function of 2MASS the predicted growth is consistent with the observed one, we can compare the two to evaluate beta = (Omega_m)^0.55 / b. The result is beta = 0.38+-0.04, which leads to an estimate of the density parameter Omega_m = 0.20+-0.08.
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