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

核星团(NSC)在足够分辨率的星系中心被无处不在地观测到。在矮星中，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，在我们银河系这样的星系中(见图)，它们很高兴地与质量相似的SMBH共存。NSCs是宇宙中密度最大的星团，其动态质量从106到108个太阳质量不等，半径不超过5pc。这将它们牢牢地置于球状星团光度函数的亮端(例如，参见Neumayer等人最近的评论，2020年，ARA&A，arxiv：2001.03626)。有明确的证据表明，它们的物理性质与它们所在星系的物理性质有关，这使它们成为我们理解星系形成和演化的关键因素。虽然它们最早在20世纪70年代初被发现，但神经干细胞的形成机制仍然存在争议。粗略地说，提出的形成通道可以分为两大类：一种是通过动力摩擦引发星团向内迁移的通道，另一种是支持由星系核中存在的高气体密度引发的就地恒星形成的通道。然而，无论它们的形成模式如何，如果NSC真的无处不在，那么那些在再电离时代之前形成于早期矮小星系的NSC，可能仍然存在于由这些矮星合并而成的今天星系的光环中。这为球状星团(GC)的形成提供了一种假说，球状星团(GC)是去除了气体含量的NSCs的残余物，防止了新恒星的形成使它们变得年轻。此外，考虑到NSCs所处的极端环境，以及相当大一部分星系的核心中NSCs和SMBH的共存，很自然地推测这两个组成部分的形成和演化是紧密联系的。例如，在致密的NSC中恒星的早期碰撞很容易产生种子黑洞，这些黑洞可能会在撕裂NCS中的其他恒星后进一步增长。可以说，在这些问题上进展甚微的主要原因是，NSC和GC(更不用说SMBH)的直接模拟很困难，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正常分布的恒星的无碰撞方法相反)。这意味着必须使用直接的N体编码来适当地跟踪这些星团的动态演化(例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这样的计算，特别是对于大规模的NSC(参见DiCintio等人(2021年)最近对替代技术的审查)。

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).