室女星系团中的超致密矮星系研究

For many decades, it was widely believed that galaxies and star clusters were completely unrelated populations in the manifold of stellar systems, characterized by distinct stellar populations, structural and dynamical properties. However, in the late 1990s, people discovered an apparently new type of compact stellar system with properties that seemingly bridged the gap between compact, low-mass galaxies and globular clusters (GCs). These objects had luminosities comparable to the faintest dwarf galaxies that known in the Local Group, but effective radii about an order of magnitude smaller. As a result, these rare systems came to be known as “ultra-compact dwarf” galaxies (UCDs). Generally speaking, the UCDs have continued to diminish the once-prominent gap between GCs and “normal”, low-mass galaxies, and thus blur the distinction between these families of stellar systems..More than 15 years after their discovery, the origin of UCDs remains surprisingly obscure. A number of scenarios have been proposed: i.e., UCDs could be: (1) luminous globular clusters; (2) end-products of the aggregation of young massive star clusters formed during the interaction of gas-rich galaxies; (3) the remnants of nucleated dwarf galaxies that have been stripped, or “threshed”, during tidal interactions..To better understand the origin of these puzzling systems, a large and carefully selected sample of UCDs is needed. As the nearest rich cluster of galaxies, Virgo is an obvious choice for a comprehensive UCD survey. The Next Generation Virgo Cluster Survey (NGVS) is a large program carried out with the 3.6m Canada-France-Hawaii Telescope (CFHT). Briefly, the survey used the MegaCam instrument to perform panoramic imaging of the Virgo cluster in the ugriz filters. Especially important for the study of UCDs, the NGVS image quality is uniformly excellent, with a median i-band seeing of FWHM about 0.54”. The NGVS survey is an ideal resource for the study of UCDs. .In this project, we will use NGVS images to carry out a systematic study of UCDs in the Virgo cluster. Moreover, the Virgo cluster is quite rich, containing 2000 cataloged member galaxies, tens of thousands of GCs, and large numbers of nucleated galaxies. Therefore, we can compare the properties of GCs, UCDs and dwarf nuclei within a single, homogenous survey, and study the origin of UCDs.

一直以来，球状星团和矮星系都被认为是完全不同的两类天体，它们的质量、半径、质光比等性质都有很大差异。然而，人们在2000年左右发现了一类新的天体：超致密矮星系（Ultra-Compact Dwarf galaxy, 简称UCD），它的各项性质均介于球状星团和矮星系之间。UCD的发现填补了球状星团和矮星系之间的空白，使得整个恒星系统（从球状星团到巨星系）变得更加完整，而UCD的相关研究也对完善星系形成和演化的总体框架有重要意义。.当前UCD研究的瓶颈在于缺少一个高完备度的样本。本项目中，申请人拟使用“下一代室女星系团巡天”的高质量图像，构建室女星系团中首个完备的UCD样本，系统地研究UCD的统计性质和标度率，并使用同质的大样本比较球状星团、UCD和矮星系（主要是有核矮椭圆星系）的性质，从而揭示UCD的起源问题。

超致密矮星系（简称UCD）是20年前发现的一类新的天体，这类天体的质量、半径、质光比等性质均介于球状星团和矮星系之间。因此，自发现至今，UCD的起源一直存在争议，争论的焦点就在于UCD究竟是比较大的球状星团还是由矮星系演化而来的。解决这一争论主要有两个途径，一是从统计上比较超致密矮星系，球状星团和矮星系（主要为有核矮星系）的性质，二是研究特殊的超致密矮星系。这两种研究途径都需要UCD大样本，大样本不仅可以使统计结果更可靠，还有更大的概率找到特殊的超致密矮星系。.基于下一代室女星系团巡天（NGVS）的多波段数据，我们在室女星系团中选取了612个超致密矮星系。这是首个星系团尺度上完备的超致密矮星系样本，也是迄今最大的超致密矮星系样本。同时，我们还基于同一数据源（NGVS）选取了球状星团和有核矮星系样本，用于跟超致密矮星系的性质进行统计比较。.由于样本的独特性，我们得以首次研究超致密矮星系的某些统计性质，如光度（质量）函数、空间分布、半光度半径分布等。经比较，超致密矮星系的颜色分布、半径分布等统计性质介于球状星团和有核矮星系之间，但空间分布、颜色梯度等性质则表现出与有核矮星系的紧密联系。此外，我们也找到了一批特殊的超致密矮星系，包括大质量、大半径以及有潮汐尾和恒星包层的超致密矮星系，这些超致密矮星系表现出与球状星团截然不同的性质以及与有核矮星系的紧密联系。.我们的研究结果表明，至少有部分超致密矮星系是由有核矮星系演化而来，矮星系的恒星包层在并合过程中逐渐被剥离，剩下的星系核即为我们观测到的超致密矮星系。因此，本项目部分解决了超致密矮星系的起源问题。