We investigate the effect of orientation-dependent selection effects on galaxy clustering in redshift space. It is found that if galaxies are aligned by large-scale tidal fields, then these selection effects give rise to a dependence of the observed galaxy density on the local tidal field, in addition to the well-known dependences on the matter density and radial velocity gradient. This alters the galaxy power spectrum in a way that is different for Fourier modes parallel to and perpendicular to the line of sight. These tidal galaxy alignments can thus mimic redshift space distortions, and thus result in a bias in the measurement of the velocity power spectrum. If galaxy orientations are affected only by the local tidal field, then the tidal alignment effect has exactly the same scale and angular dependence as the redshift space distortions in the linear regime, so it cannot be projected out or removed by masking small scales in the analysis. We consider several toy models of tidal alignments and orientation-dependent selection, normalize their free parameter (an amplitude) to recent observations, and find that they could bias the velocity amplitude f(z)G(z) by 5--10 per cent in some models, although most models give much smaller contamination. We conclude that tidal alignments may be a significant systematic error in redshift space distortion measurements that aim to test general relativity via the growth of large-scale structure. We briefly discuss possible mitigation strategies.
An observational signal of the void shape correlation and its link to the cosmic web
宇宙空洞的形状很容易被外部潮汐力扭曲，因为它们的低密度意味着较低的内部阻力。这种敏感性的空隙形状的潮汐扭曲，使他们作为一个指标的大规模的潮汐和密度场，尽管在实际中很难定义它们。使用Pan等人（2012）从斯隆数字巡天第七次数据发布（SDSS DR 7）中构建的空洞目录，我们检测到20 Mpc/h尺度上空洞形状空间相关性的清晰4 sigma信号，并表明该信号对空洞形状在天空平面上的投影具有鲁棒性。通过构造一个简单的空泡形状相关性分析模型，在潮汐力矩理论框架内，我们证明了空泡形状关联函数与线性密度场的两点关联函数成线性比例关系。我们还发现了空洞形状与大尺度速度剪切场的互相关的直接观测证据，Lee等人（2014）从SDSS DR 7线性重建。我们讨论了利用空泡形状关联函数来打破密度参数和功率谱振幅之间的简并性以及独立约束中微子质量的可能性。
When galaxies align: intrinsic alignments of the progenitors of elliptical galaxies in the Horizon-AGN simulation
今天的椭圆星系似乎与宇宙的大尺度结构成一直线，但它们何时获得这种排列仍然是一个悬而未决的问题。目前的观测数据不足以提供约束的时间演化的内在的路线，因此现有的模型范围从假设星系获得一些原始的排列在形成，建议他们立即作出反应的潮汐相互作用的大尺度结构。利用宇宙学流体动力学模拟Horizon-AGN，我们测量了星系长轴与潮汐场本征矢量之间的相对排列随红移的变化。我们专注于约束的大质量$z=0$椭圆星系，在低红移的主要弱透镜污染物的主要祖先的排列的时间演化。我们表明，这个人口，在$z=0$有一个恒星质量以上$10^{10.4}$ M $_\odot$，过渡到没有对齐的潮汐场在$z=3$，一个显着的对齐由$z=1$。从$z=0.5$开始，它们以近似恒定的水平保持其对齐，直到$z=0$。我们发现了一个质量依赖性的椭圆祖细胞的对齐信号，即椭圆，是质量较低的今天（$10^{10.4}<M/{\rm M}_\odot<10^{10.7}$）不成为对齐，直到后来的红移（$z<2$），相比更大的同行。我们还提出了一个扩展的研究祖先的路线在参数空间的恒星质量和星系动力学，形状定义和潮汐场平滑的影响。
