宇宙最大質量結構之探究

Abstract

在此碩士論文中，我們研究在宇宙網尺度下及星系尺度下質量最大的結構，也就是超星系團和星系團最亮星系（簡稱BCG)。 在論文的第一部分，我們採用物理動機的超星系團定義:要求超星系團即使在暗能量驅動下膨脹的宇宙中也會最終塌縮。藉由運用friends-of-friends（FoF）演算法在從Subaru Hyper Suprime-Cam巡天資料建構的CAMIRA星系團樣本上，我們首次系統性地搜尋在紅移 z=0.5-1.0 的超星系團，並在1027平方度的範圍內找到了673個候選超星系團，FoF演算法的參數是藉由演化N-body模擬到遙遠的未來進行校正，以確保高純度。我們發現大部分高紅移超星系團有2到4個星系團成員，表明了在年輕宇宙重力束縛的極限。除此之外，我們研究星系團以及BCG在不同的大尺度環境下的性質，我們發現屬於超星系團的星系團豐度通常較高，然而BCG的性質對大尺度環境沒有顯著的依賴性。我們也比較了觀測到的超星系團豐度和從暗物質暈光錐中找到的超星系團的豐度，藉此發現光度紅移的不確定性限制了尋找超星系團的效能。 在論文的第二部分，我們對BCGs進行兩項互補的統計檢定來檢驗BCG是否是星系團星系的恆星質量分佈中的極端值，或是源自於另外一個獨特的分佈。這份研究中的BCG樣本是選自宇宙磁流體力學模擬，The Next Generation Illustris Project (TNG)。第一個統計檢定依靠的是星系團中質量最大的星系(BCG) 和質量次大的星系 (G2)之間的恆星質量差，第二個統計檢定則依賴BCG恆星質量和星系團恆星質量間的相關性以及一個模擬建立星系團樣本的演算法上。藉由兩種不同採樣的方法來研究BCG統計特性，我們發現這個兩個統計檢定以及兩種採樣方法都表明BCG在紅移z=1.0-4.0之間是特別的。此外，當我們根據暗物質暈的性質將BCG分成兩個子樣本，然後重複這些統計測試，我們發現在紅移大於2時低暈集中和高質量吸積率的BCG比較``不特別''，然而當根據BCG的性質(像是顏色、恆星形成率、merger次數）將BCG分成兩個子樣本重複進行統計檢定時，我們沒有發現顯著差異。除此之外，對於從暗物質暈質量大於10^{13.5} h^{-1} Msun選出的BCG和G2，我們分析了他們的前身暗物質暈質量、前身恆星形成率、形成時間和恆星形成歷史。

In this thesis, we investigate the most massive structures on the cosmic web scale and the galactic scale, namely, the galaxy superclusters and the brightest cluster galaxies. In the first part of the thesis, we adopt a physically motivated supercluster definition, requiring that superclusters should eventually collapse even in the presence of dark energy. Applying a friends-of-friends (FoF) algorithm to the CAMIRA cluster sample constructed using the Subaru Hyper Suprime-Cam survey data, we have conducted the first systematic search for superclusters at z=0.5-1.0 and identified 673 supercluster candidates over an area of 1027 deg^2. The FoF algorithm is calibrated by evolving N-body simulations to the far future to ensure high purity. We found that these high-z superclusters are mainly composed of 2-4 clusters, suggesting the limit of gravitationally bound structures in the younger Universe. In addition, we studied the properties of the clusters and brightest cluster galaxies (BCGs) residing in different large-scale environments. We found that clusters associated with superclusters are typically richer, but no apparent dependence of the BCG properties on large-scale structures is found. We also compared the abundance of observed superclusters with mock superclusters extracted from halo light cones, finding that photometric redshift uncertainty is a limiting factor in the performance of superclusters detection. In the second part of the thesis, we perform two complementary statistical tests on BCGs to examine whether they are statistically extreme of the cluster galaxy stellar mass function or other distinct populations. BCGs are selected from the cosmological magnetohydrodynamical (MHD) simulation suites, The Next Generation Illustris Project (TNG). The first statistical test relies on the stellar mass gap between the first-ranked galaxy (BCG) and the second-ranked galaxy (G2), and the second statistical test utilizes the BCG stellar mass - cluster stellar mass correlation along with an algorithm to construct mock cluster sample. Implementing two different BCG sampling methods to trace the time evolution of their statistical nature, we find that both tests and both sampling methods suggest that BCGs are special throughout z=1.0-4.0. Furthermore, we find that BCGs with lower halo concentration and higher mass accretion rate appear to be relatively ``less special'' at redshift ≳2 when repeating statistical tests with the BCG sample binned according to their parent halo properties. However, no significant differences are found when repeating the statistical tests with the BCG sample binned according to BCG properties such as color, star-formation rate, and total count of merger experienced. In addition, for BCGs and G2s selected from halos with mass M_{200m} ≳ 10^{13.5} h^{-1} Msun, we also analyze their progenitor halo mass, progenitor star formation rate, formation time, and star formation history.