# 理论物理交流平台系列报告—哥廷根大学杜小龙博士 (德国)

应物理学院刘玉孝教授邀请，哥廷根大学杜小龙博士来我院访问，并做学术报告，欢迎广大师生届时参加！

题目：Structure Formation with Ultralight Axion Dark Matter (超轻轴子暗物质模型下的结构形成)

时间：2018年10月11日星期四，上午11:00

地点：齐云楼525报告厅

摘要：

Ultralight axion (ULA) is a scalar field with an extremely small mass $sim 10^{-22}$ eV. When the self interaction can be neglected, it is often called fuzzy dark matter (FDM). On large scales, ULA dark matter behaves like cold dark matter (CDM), thus can produce the correct large scale structure of the Universe. But on scales below the Jeans length, the quantum pressure counters the gravity, leading to a large suppression in the structure formation on small scales. Unlike CDM halos which have cuspy profiles, ULA dark matter halos have flat cores whose density profiles can be approximated by soliton solutions. These unique features of ULA dark matter model make it possible to constrain the mass of axion from observations such as UV luminosity function, number density of ultra-faint galaxies, reionizaion, Lyman-alpha forest, and gravitational lensing. Different methods have been developed to study the cosmic structure formation with ULA dark matter. Most of these methods have been concentrated on direct simulations either by solving the Schr?dinger-Poisson equations or its fluid formalism, thus are limited by the computational power.

In this talk, I will introduce some of my work on another approach, i.e. the semi-analytical model (SAM). First, I will show how we implement some aspects of ULA dark matter physics into the publicly available semi-analytic code for galaxy formation, GALACTICUS. Using the modified SAM code, we are able to investigate the (sub) halo mass function for ULA dark matter with different masses and fractions,which cannot be done even with the state-of-art numerical simulations. Comparing to the standard CDM, the (sub) halo mass function are found to be largely suppressed at lower mass end. The larger the axion mass, the smaller the suppression scale is. Thus observations on the number density of ultra-faint galaxies put a lower bound on the mass of axion particles. Then I will show our work on idealized simulation of tidal disruption of subhalo cores. We find that the compact core of ULA dark matter subhalo can be disrupted by the tidal force from the host. Unlike the classical case, the mass inside the tidal radius can also by stripped away due to the “tunneling” effect. The core mass loss rate from our simulations is broadly consistent with the previous semi-classical treatment by Hui et al. (2017), but with some reinterpretations. We implement the core mass loss rate into the SAM and find it is important for computing the subhalo mass function.

Finally, I will show how how our results can be applied to substructure lensing observations to get constraints on the axion mass.

报告人简介：

After obtaining his Bachelor’s degree in Theoretic Physics in Nankai University, Xiaolong Du went to Lanzhou University to continue his Master’s study under the supervision of Prof. Yuxiao Liu. He had great interests in modified gravity, numerical relativity and cosmic structure formation. In 2014, he obtained his Master’s degree in Theoretic Physics and got an opportunity to go to one of the most famous University in Germany, G?ttingen University. There he worked on his doctoral project, “Ultralight axion dark matter model”, with Prof. Jens Niemeyer. During his Ph.D. study, together with Prof. Jens Niemeyer, Christoph Behrens, Bodo Schwabe, he investigated in details the semi-analytic model of structure formation in the scenario of ultralight axion dark matter. His work can be applied to many aspects of studies on ULA dark matter models, both in the case of dark matter-only study and the case including also the baryons. His pseudo-spectral code, which is first developed to study the tidal disruption of solitonic cores, is now used in cosmological simulations of axion miniclusters.

In this summer, he finished his study in Gottingen University and got his Ph.D. He will go to Carnegie Observatories to do his postdoctoral research with the famous SAM expert Andrew Benson.