The existence of satellite galaxy planes poses a major challenge for the standard picture of structure formation with non-baryonic dark matter. Recently Tully et al. (2015) reported the discovery of two almost parallel planes in the nearby Cen A group using mostly high-mass galaxies (M$_B$ $<$ -10 mag) in their analysis. Our team detected a large number of new group member candidates in the Cen A group (Müller et al. 2016). This dwarf galaxy sample combined with other recent results from the literature enables us to test the galaxy distribution in the direction of the Cen A group and to determine the statistical significance of the geometric alignment. Taking advantage of the fact that the two galaxy planes lie almost edge-on along the line of sight, the newly found 13 group members by Crnojevic et al. (2014, 2016) and our 16 new Cen A group candidates (Müller et al. 2016) can be assigned relative to the two planes. We use various statistical methods to test whether the distribution of galaxies follows a single normal distribution or shows evidence of bimodality as it has been reported earlier. We confirm that the data used for the Tully et al. (2015) study support the picture of a bimodal structure. However, when the new galaxy samples are included, the gap between the two galaxy planes is closing and the significance level of the bimodality is reduced. Instead, the plane that contains Cen A becomes more prominent. We found evidence that the galaxy system around Cen A is made up of only one plane of satellites. This plane is almost orthogonal to the dust plane of Cen A. Accurate distances to the new dwarf galaxies will be required to measure the precise 3D distribution of the galaxies around Cen A.
Alignment between satellite and central galaxies in the SDSS DR7: dependence on large-scale environment
卫星星系和中央星系之间的排列已经在观测和理论工作中得到了详细的研究。普遍接受的事实是，卫星优先居住在其中央星系的主轴上。然而，这种对齐的起源和大规模环境依赖性仍然是未知的。为了找出这些问题，我们使用SDSS DR 7构建的数据来研究这种对齐的大规模环境依赖性，重点是检查对齐对中央星系颜色的依赖性。我们发现一个非常强大的大规模的环境依赖的卫星中心对准组与蓝色的中心。结中蓝心的排列方向优先垂直于心的长轴，排列角度随环境变化而减小，即从结到空洞。对齐角度强烈依赖于中心的${}^{0.1}(g-r)$颜色。我们认为，卫星中心排列是大尺度结构内卫星吸积与宿主晕内星系演化竞争的结果。对于包含红色中心星系的星系团，星心排列主要由演化效应决定，而对于蓝色中心星系团，大尺度结构的作用更为重要，特别是在星系结中。我们的研究结果提供了一个解释如何在不同的大尺度环境中的卫星中心对准的形式。垂直的情况下，在集团和结与蓝色的中心也可能提供深入了解类似的极性安排，如银河系和半人马座A的卫星系统的形成。
Do halos that form early, have high concentration, are part of a pair, or contain a central galaxy potential host more pronounced planes of satellite galaxies?
Luminous Satellites versus Dark Subhaloes: Clustering in the Milky Way
观测到的银河系卫星星系群为小尺度上的星系形成理论提供了一个独特的试验场。我们的新方法是调查已知的银河系卫星星系的集群和量化的数量的子结构内的分布使用两点关联函数统计在三个空间：配置空间，视线速度空间，和四维相空间。这些结果进行了比较的三套subhaloes在通过Lactea II冷暗物质模拟定义为代表发光的矮星。我们没有发现任何证据在一个显着性水平以上的2西格玛的子结构内的银河系卫星星系的分布在任何三个空间。在四维相空间中，在所有三个空间和更广泛的尺度范围内，“发光”的次晕集合比银河系卫星更强烈地聚集在一起。每个“发光”的subhalo集聚类的结果，其视线内的速度空间分布的子结构在大于3西格玛的意义，而银河系的卫星星系是随机分布在视线的速度空间。虽然我们的比较是只有一个冷暗物质模拟，银河系卫星星系和所有聚类方法的Via Lactea II subhalo集之间的不一致性表明冷暗物质理论和观测到的银河系卫星之间存在潜在的新的“小尺度”紧张关系。未来的工作将通过研究额外的模拟来获得观测到的银河系卫星和冷暗物质理论之间更可靠的比较。
Semi-analytic Inference of Satellite Densities in the Cold Dark Matter Model Part I. Comparison to Ultra-faint Dwarf Kinematics
Leinweber Institute for Theoretical Physics, University of California, Berkeley（加州大学伯克利分校莱因韦伯理论物理研究所）；Theoretical Physics Group, Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室理论物理组）；Department of Physics, Princeton University（普林斯顿大学物理系）；Department of Physics and Astronomy, University of California - Irvine（加州大学尔湾分校物理与天文学系）；Center for Computational Astrophysics, Flatiron Institute（平顿研究所计算天体物理学中心）
