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| Project Details |
| Funding Scheme : | General Research Fund | ||||||||||||||||||||||||
| Project Number : | 17304519 | ||||||||||||||||||||||||
| Project Title(English) : | Astrophysical Applications of Gravitational Lensing: Nature of Dark Matter and Galaxies in the early Universe | ||||||||||||||||||||||||
| Project Title(Chinese) : | 引力透镜的天文学应用：理解暗物质和宇宙早期星系的性质 | ||||||||||||||||||||||||
| Principal Investigator(English) : | Prof Lim, Jeremy Jin Leong | ||||||||||||||||||||||||
| Principal Investigator(Chinese) : | |||||||||||||||||||||||||
| Department : | Dept of Physics | ||||||||||||||||||||||||
| Institution : | The University of Hong Kong | ||||||||||||||||||||||||
| E-mail Address : | jjlim@hku.hk | ||||||||||||||||||||||||
| Tel : | |||||||||||||||||||||||||
| Co - Investigator(s) : |
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| Panel : | Physical Sciences | ||||||||||||||||||||||||
| Subject Area : | Physical Sciences | ||||||||||||||||||||||||
| Exercise Year : | 2019 / 20 | ||||||||||||||||||||||||
| Fund Approved : | 837,408 | ||||||||||||||||||||||||
| Project Status : | Completed | ||||||||||||||||||||||||
| Completion Date : | 22-1-2023 | ||||||||||||||||||||||||
| Project Objectives : |
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| Abstract as per original application (English/Chinese): |
宇宙中大部分的质量以暗物质的形式存在。星系和星系团形成于暗物质晕中，而暗物质也保证了星系团有足够的引力维持其物质组成和形状。但是，究竟什么是暗物质？在暗物质晕中的气体是什么时候开始形成恒星，并演化为如今可见的星系的？而宇宙早期的星系又有怎样的性质？当我们通过引力透镜现象去研究暗物质的物理性质时，我们也同时在观测被引力透镜现象放大了的早期星系，因此我们可以同时研究以上提到的问题。 在过去的五年中，我们在香港大学组建了一个不断壮大的科研团队，训练了一批有才华的学生来研究引力透镜在天文学上的应用。我们的学生在本科期间就开始接受相关训练，并逐渐开展科研项目，最终将项目发展为研究生或博士阶段的课题。很多相关课题都由我们的研究组首次提出。我们的研究组利用大质量的星系或星系团形成的引力透镜，而数据则来源于哈勃望远镜，是目前世界上最高质量的相关数据。 （1） 暗物质的结构： 在进行2016RGC/GRF的研究工作时，我们已经为Hubble Frontier Fields（HFF）项目中的全部六个星系团和Cluster Lensing And Supernova survey with Hubble (CLASH)项目中的部分星系团创建了透镜模型。这些模型准确地描述了暗物质在大尺度上的空间结构。我们通过研究模型是否能准确预测透镜成像的位置和形状，成像光源的红移，以及不同的像之间的相对亮度来判断我们的透镜模型的准确度。我们的模型成功预测了首次被引力透镜折射形成多个像的超新星的成像位置和时间。我们计划将相关研究延续到Reionization Cluster Lensing Survey (RELICS)项目，这是哈勃望远镜最新的大范围引力透镜星团项目。 （2） 宇宙早期的星系： 宇宙早期的星系能谱与中红移的椭圆星系的能谱非常相似，因此很难分辨出宇宙早期的星系。当没有光谱红移的时候，单纯通过引力透镜几何结构来推测的红移能够比能谱更准确地推测出星系的红移。我们计划将进一步发展此项先驱研究，以准确测量RELICS项目中的遥远星系的几何红移。 （3） 测量超大质量黑洞的质量： 星系是否与星系中心的超大质量黑洞同步演化？要回答这个问题，我们需要先了解超大质量黑洞随着宇宙演化的质量分布。我们之前的研究已经验证了引力透镜现象可以被用来直接测量遥远星系中超大质量黑洞的质量。我们计划将这项研究扩展到新发现的适用的数据上。 （4） 暗物质的观测性测试： 我们已经证实了哈勃HFF项目中的引力透镜光源星系的空间密度在低亮度的范围内并不持续增加。相比冷暗物质，这一现象更加符合波暗物质的理论预测。相关测量还预测了玻色子的质量，并解释了在本星系群中的暗物质主导的低质量星系的核机构。我们还发现了在再电离阶段的中期，高亮度的星系的空间密度显著增加。我们计划用RELICS项目中更大量的远距离星系的数据来延展此项研究。 （5） 波暗物质的米粒组织： 组成波暗物质的玻色子在量子干涉下会形成米粒组织，这是波暗物质的独特性质。我们认为我们在此方面的先驱研究可以解释引力透镜现象中在小尺度上的亮度变化，并正在开展大质量星系和星系团透镜的观测研究来验证这一推测是否正确。 |
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| Realisation of objectives: | Objectives 1, 2, & 4: As explained in the mid-term report, an MPhil student was hired for this purpose - but after 6 months began to developed mental issues, and could not be persuaded to address his issues and return to work. He did not complete his degree. Work on Objectives 2 and 4 subsequently taken up by two other MPhil students, and continued (completed) in a follow-up RGC/GRF proposal. Objective 3: Work started during this grant period, and completed in a follow-up RGC/GRF proposal. See attached pdf file in Part C, Section 11, for realisation of objectives. | ||||||||||||||||||||||||
| Summary of objectives addressed: |
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| Research Outcome | |||||||||||||||||||||||||
| Major findings and research outcome: | Demonstration of the need for Dark Matter around galaxies based purely on the geometry of gravitational lensing (Chen et al. 2020, ApJ); dynamical arguments for Dark Matter (DM) can be equally well explained by alternative theories of gravity. Analytical framework for density modulations induced by quantum interference of ultra-light DM particles, and its effects on gravitational lensed images (Hiroki et al. 2022, ApJ). Computer simulations of gravitationally-lensed images generated by galaxies possessing halos comprising ultra-light DM particles - showing that this form of DM is able to resolve long-standing lensing anomalies left by lens models based on ultra-massive DM particles. The ability of such wave-like DM to resolve lensing anomalies, together with its success in reproducing other astrophysical observations, tilt the balance toward new physics invoking axions. The latter has been invoked to alleviate deficiencies in the Standard Model of particle physics, and is one of the two leading contenders (along with WIMPs) for DM (Alfred et al. 2023, Nature Astronomy). Also, the PI took a foray into a dynamical test of DM, so as to better understand the relative merits of different tests for the nature of DM (Pozo et al. 2021, MNRAS). | ||||||||||||||||||||||||
| Potential for further development of the research and the proposed course of action: |
We continue to lead work using gravitational lensing to test wave-like DM, as applied to other gravitationally-lensed systems involving galaxies as well as galaxy clusters, in a follow-up RGC/GRF grant as well as in a RGC/CRF grant. Work on Objective 2 was started in this proposal, and has been completed as part of a RGC/CRF grant (paper published). Work on Objective 3 was started in this proposal, and completed as part of a follow-up RGC/GRF grant. This work was part of an MPhil thesis (successfully defended), and a paper is in preparation. Work on Objective 4 was started in this proposal, and continues in a follow-up RGC/GRF grant as prt of an MPhil thesis. | ||||||||||||||||||||||||
| Layman's Summary of Completion Report: | Accepting the Theory of General Relativity as the correct theory of gravitation requires a leap of faith: that much of the matter in the Universe, whether it be located in galaxies, small groups of galaxies, large cluster of galaxies, or in cosmic filaments, is in a form that neither emits nor absorbs light and which interact with ordinary matter only through gravity (so far as we can tell). This invisible form of matter is known as Dark Matter (DM). Alternatives to the Theory of General Relativity that do away with DM (hence, for which all gravity is associated with visible matter) are plentiful, perhaps the best known of which is Modified Newtonian Dynamics (MOND). The existence, and nature, of DM are not just esoteric debates among astronomers: the existence of DM is the clearest evidence that the Standard Model for particle physics is far from complete. Theoretical extensions to address deficiencies in the Standard Model predict hypothetical particles (e.g., WIMPs and Axions) that have been proposed as candidates for DM. Identifying the nature of DM, widely regarded as the most important problem in all of physics, therefore promises to point the correct path towards New Physics. In our work, we showed the need for DM around galaxies based purely on the geometry of gravitational lensing. Next we developed an analytical framework for density modulations induced by quantum interference of ultra-light DM particles, and its effects on gravitational lensed images. Finally, we conducted computer simulations showing that such wave-like DM is able to resolve long-standing lensing anomalies left by lens models based on ultra-massive DM particles. The ability of such wave-like DM to resolve lensing anomalies, together with its success in reproducing other astrophysical observations, tilt the balance toward new physics invoking axions. | ||||||||||||||||||||||||
| Research Output | |||||||||||||||||||||||||
| Peer-reviewed journal publication(s) arising directly from this research project : (* denotes the corresponding author) |
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| Recognized international conference(s) in which paper(s) related to this research project was/were delivered : |
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| Other impact (e.g. award of patents or prizes, collaboration with other research institutions, technology transfer, etc.): |
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| SCREEN ID: SCRRM00542 |