The alignment of satellite galaxies in gasdynamic simulations

卫星被限制在星系的极地区域，至少40年来一直困扰着天文学家。在其间的四十年里，无论是在外部星系(霍姆伯格1969年首次发现)还是在我们的银河系中，对这种效应的观察都没有得到充分的解释。事实上，最近在SDSS和2dFGRS等大规模观测中对卫星星系排列的观察与所谓的Holmberg效应相矛盾，并发现卫星往往会将自己与其宿主星系的平面对齐。不管各向异性分布的意义是什么，卫星并不均匀地分布在它们的母晕中，这导致一些作者强调，这与标准协调宇宙学的简单预测不一致。到目前为止，理论描述一直试图使在模拟中发现的卫星的各向同性分布与观测结果相一致。许多作者只使用暗物质模拟来研究本星团中卫星的排列。虽然在理解卫星为什么是各向异性分布方面取得了一些进展，但卫星相对于中央星系扁平化的感觉几乎回避了分析。原因很简单：在大的宇宙体积中运行流体动力学模拟是昂贵的，而且对于确定中央星系盘的方向是必要的。通过使用有史以来最大的流体动力学模拟之一，我们希望解决一个40年来未解的谜团：银河系的极地排列是一个明显的例外吗？或者它的扁平化是类似星系的典型特征。

The confinement of satellites to the polar regions of galaxies has confounded astronomers for at least 40 years. Neither the observations of this effect in external galaxies (first seen by Holmberg 1969) nor in our own Milky Way have been adequately explained in the intervening four decades. In fact, more recent observations of the alignment of satellite galaxies in large scale surveys such as the SDSS and in the 2dFGRS have contradicted the so-called Holmberg effect and found that satellites tend to align themselves with the plane of their host galaxies. Regardless of what the sense of the anisotropic distribution is, that satellites are not uniformly distributed within their parent halos has caused a number of authors to highlight that this is in disagreement with the simple predictions of the standard concordance cosmology. Until now, theoretical descriptions have tried to reconcile the isotropic distributions of satellites found in simulations with the observations. Many authors have employed dark matter only simulations in a bid to study the alignment of satellites in the local group. Although some progress has been made in understanding why satellites are anisotropically distributed, the sense of their flattening with respect to the central galaxy has all but evaded analysis. The reason for this is simple: running hydrodynamic simulations in a large cosmological volume is expensive and is necessary for determining the orientation of a central galactic disc. By using one of the largest hydrodynamical simulation ever run we wish to address a 40 year old unsolved mystery: is the Milky Way a gross exception with its polar alignment? or is its flattening typical of similar galaxies.