arXiv 0709.31762007-09-20astro-ph

附近的星系群是受引力束缚的物体吗？

Are the nearby groups of galaxies gravitationally bound objects?

Sami-Matias Niemi, Pasi Nurmi, Pekka Heinämäki, Mauri Valtonen

备注：: 13 figures and 7 tables, Accepted 2007 September 19. Received 2007 September 19; in original form 2007 April 2
DOI：: 10.1111/j.1365-2966.2007.12498.x

摘要：

我们比较了数值模拟和观测附近（< 40 Mpc）星系群（Huchra & Geller 1982和Ramella et al. 2002）。使用由Huchra和Geller（1982）开发的组发现算法来进行组识别。使用宇宙学N-体模拟代码与LambdaCDM宇宙学，我们表明，从模拟数据中确定的星系群的动力学性质，一般来说，在一个温和的，在2sigma，协议与观测目录的星系群。由于模拟提供了比观测更多的动力学信息，我们使用N体模拟数据来计算附近的星系群是否是引力束缚的物体，使用它们的维里比。我们表明，在一个LambdaCDM宇宙学约20%的附近星系群，确定了相同的算法，在观测的情况下，没有约束，而仅仅是在视觉上的群体。这是相当重要的，特别是因为在观测中对星系群质量的估计通常是基于一个假设，即通过朋友的朋友算法发现的星系群是引力束缚的物体。不同分辨率的模拟结果相同。我们还展示了当样品的视星等极限和宇宙学常数的值改变时，引力未束缚群的分数是如何变化的。通常，较大的Omega_Lambda值生成的未绑定组略多。

英文摘要：

We have compared numerical simulations to observations for the nearby (< 40 Mpc) groups of galaxies (Huchra & Geller 1982 and Ramella et al. 2002). The group identification is carried out using a group-finding algorithm developed by Huchra and Geller (1982). Using cosmological N-body simulation code with the LambdaCDM cosmology, we show that the dynamical properties of groups of galaxies identified from the simulation data are, in general, in a moderate, within 2sigma, agreement with the observational catalogues of groups of galaxies. As simulations offer more dynamical information than observations, we used the N-body simulation data to calculate whether the nearby groups of galaxies are gravitationally bound objects by using their virial ratio. We show that in a LambdaCDM cosmology about 20 per cent of nearby groups of galaxies, identified by the same algorithm as in the case of observations, are not bound, but merely groups in a visual sense. This is quite significant, specifically because estimations of group masses in observations are often based on an assumption that groups of galaxies found by the friends-of-friends algorithm are gravitationally bound objects. Simulations with different resolutions show the same results. We also show how the fraction of gravitationally unbound groups varies when the apparent magnitude limit of the sample and the value of the cosmological constant is changed. In general, a larger value of the Omega_Lambda generates slightly more unbound groups.

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