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）在足够分辨率的星系中心无处不在。在矮星中，它们往往会取代在大多数大质量星系中发现的超大质量黑洞（SMBH）。然而，在像我们的银河系这样的星系中（见图），它们与质量相似的SMBHs愉快地共存。NSCs是宇宙中密度最大的星团，其动态质量在106到108倍太阳质量之间，其半径不大于5%。这将它们牢牢地置于球状星团亮度函数的明亮端（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因素。尽管它们在20世纪70年代初首次被发现，但NSCs的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：一类是通过动态摩擦引起星团向内迁移，另一类是由星系核中存在的高密度气体触发的原位恒星形成。然而，不管它们的形成方式如何，如果nsc真的无处不在，那些在早期矮星系中形成的，在再电离时代之前，仍然可能存在于由这些矮星合并而成的当今星系的光晕中。这为球状星团（GC）的形成提供了一种假设，即NSCs的残余被剥夺了气体含量，阻止了它们在新恒星形成时恢复活力。此外，鉴于NSCs所处的极端环境，以及在相当大一部分星系的核心中有充分记录的NSCs和SMBHs共存，我们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，密集的NSC内恒星的早期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的其他恒星进一步生长。可以说，在这些问题上进展甚微的主要原因是，NSCs和gc（更不用说SMBHs）的直接建模是困难的，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的n体编码来正确地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实现这样的计算，特别是对于大型nsc（参见DiCintio等人（2021）最近对替代技术的回顾）。