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).

核星团（NSC）在充分解析的星系中心随处可见。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），它们很高兴与质量相似的超大质量黑洞共存。 NSC 是宇宙中最密集的星团，其动态质量范围为 106 至 108 个太阳质量，包围在半径不大于 5pc 的范围内。这将它们牢牢地置于球状星团光度函数的明亮端（例如，参见 Neumayer 等人最近的评论，2020，ARA&A，arXiv：2001.03626）。有明确的证据表明它们的物理特性与其宿主星系的物理特性相关，这使它们成为我们理解星系形成和演化的关键要素。尽管 NSC 于 20 世纪 70 年代初首次被发现，但其形成机制仍存在争议。粗略地说，所提出的形成通道可以分为两大类：一类是通过动力摩擦引起星团向内迁移，另一类是支持由星系核中存在的高气体密度引发的原位恒星形成。然而，无论它们的形成方式如何，如果 NSC 确实无处不在，那么那些在再电离时代之前的早期矮星系中形成的 NSC 仍然可能存在于当今星系的晕中，这是这些矮星系合并的结果。这为球状星团（GC）的形成提供了一个假设，即球状星团（GC）是NSC的残余物，这些NSC的气体含量已被剥夺，从而阻止它们通过新的恒星形成而恢复活力。此外，考虑到NSC所处的极端环境，以及有据可查的NSC和SMBH在相当大一部分星系的核心中共存的情况，推测球状星团的形成和演化是很自然的。 这两个组成部分紧密相连。例如，密集的国家宇宙线内恒星的早期碰撞很容易提供种子黑洞，这些黑洞可以通过撕裂国家宇宙线的其他恒星而进一步生长。可以说，在这些问题上几乎没有取得进展的主要原因是，对 NSC 和 GC（更不用说SMBH）的直接建模很困难，因为它们的行为是碰撞的（与在星系形成和演化的宇宙学模拟中用于数值模拟暗物质和普通分布恒星的无碰撞方法相反）。这意味着必须使用直接 N 体代码来正确跟踪这些星团的动态演化（例如 N-Body6 (Aarseth 2003)）。然而，目前即使是最强大的超级计算机也无法进行此类计算，尤其是对于大型 NSC（参见 DiCintio 等人（2021）最近对替代技术的回顾）。