Nuclear and globular star clusters: the missing link between supermassive black holes and their host galaxies?

核星团和球状星团：超大质量黑洞与其宿主星系之间缺失的联系？

• 批准号: 2888265
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2023
• 资助国家: 英国
• 起止时间: 2023 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2888265
• 关键词: Nuclear; globular; star; clusters; missing

Nuclear star clusters (NSC) are ubiquitously observed at the centre of sufficiently resolved galaxies. In dwarfs, they tend to replace the supermassive black hole (SMBH) detected in most massive galaxies. However, in galaxies like our own Milky Way (see picture), they happily co-exist with SMBHs of a similar mass. NSCs are the densest star clusters in the Universe with dynamic masses ranging from 106 to 108 solar masses enclosed in a radius no larger than 5pc. This places them firmly at the bright end of the globular cluster luminosity function (see e.g. the recent review by Neumayer et al, 2020, ARA&A, arXiv: 2001.03626). There exists clear evidence that their physical properties correlate with those of their host galaxies, which makes them key ingredients for our understanding galaxy formation and evolution. Although they were first detected in the early 1970s, the formation mechanism of NSCs is still debated. Crudely speaking, the formation channels put forward can be divided into two main categories: the ones which invoke an inward migration of star clusters through dynamical friction, and those that argue in favour of in-situ star formation triggered by high gas densities present in the galaxy nucleus. However, regardless of their mode of formation, if NSCs truly are ubiquitous, those that form in early dwarf galaxies, before the re-ionization epoch, could still be present in the halo of the present-day galaxy which results from the merger of these dwarfs. This provides a hypothesis for the formation of globular clusters (GC) as the remains of NSCs which have been stripped of their gas content, preventing them from being rejuvenated by new star formation.Moreover, given the extreme environment in which NSCs are located, and the well documented co-existence of NSCs and SMBHs in the nucleus of quite a large fraction of galaxies, it is quite natural to speculate that the formation and evolution of these two components are tightly linked. For instance, early collisions of stars within a dense NSC could easily provide seed black holes which could further grow from tearing apart other stars of the NCS. Arguably the main reason why little progress has been made on these issues is that the direct modelling of NSCs and GCs (not to mention SMBHs) is difficult because their behaviour is collisional (as opposed to the collisionless approach used to numerically simulate dark matter and ordinarily distributed stars in cosmological simulations of galaxy formation and evolution). This means that direct N-body codes must be used to properly track the dynamical evolution of these star clusters (example N-Body6 (Aarseth 2003)). However, such calculations are currently out of reach of even the most powerful super-computers, especially for massive NSCs (see e.g. DiCintio et al (2021) for a recent review of alternative techniques).

核星星团（NSC）是普遍观察到的中心足够清晰的星系。在矮星系中，它们往往会取代在大多数大质量星系中检测到的超大质量黑洞（SMBH）。然而，在像我们银河系这样的星系中（见图），它们很高兴地与类似质量的SMBH共存。NSC是宇宙中最大的星星团，其动态质量在106到108太阳质量之间，半径不超过5 pc。这使它们坚定地处于球状星团光度函数的亮端（例如，参见Neumayer et al，2020，ARA&A，arXiv：2001.03626）。有明确的证据表明，它们的物理性质与其宿主星系的物理性质相关，这使它们成为我们理解星系形成和演化的关键因素。尽管神经干细胞早在20世纪70年代初就被发现，但其形成机制仍存在争议。粗略地说，提出的形成通道可以分为两大类：一种是通过动力学摩擦引起星星团向内迁移的通道，另一种是主张由星系核中的高气体密度触发原位星星形成的通道。然而，不管它们的形成模式如何，如果NSC真的无处不在，那么在再电离时代之前形成于早期矮星系中的NSC仍然可能存在于今天星系的晕中，这是这些矮星系合并的结果。这为球状星团的形成提供了一个假说，即球状星团是NSC的残余物，NSC被剥离了气体成分，阻止了新星星的形成。此外，鉴于NSC所处的极端环境，以及NSC和SMBH在相当大一部分星系核中共存的证据，很自然地推测，这两个组成部分的形成和演化是紧密相连的。例如，在密集的NSC中，早期的恒星碰撞可以很容易地提供种子黑洞，这些种子黑洞可以通过撕裂NCS中的其他恒星而进一步成长。可以说，在这些问题上几乎没有取得进展的主要原因是，NSC和GC（更不用说SMBH）的直接建模是困难的，因为它们的行为是碰撞的（与用于数值模拟暗物质和星系形成和演化的宇宙学模拟中通常分布的恒星的无碰撞方法相反）。这意味着必须使用直接N体编码来正确地跟踪这些星星团的动力学演化（例如N-Body 6（Aarseth 2003））。然而，这种计算目前甚至是最强大的超级计算机也无法实现，特别是对于大规模的NSC（例如，DiCintio等人（2021）最近对替代技术的评论）。