Research

Research Interests

  • Dark Matter
    • numerical simulation
    • dynamical modelling
    • weak lensing
    • indirect detection
  • Astrostatistics

Publication List on NASA ADS

Research Highlights

My research interests lie broadly in dark matter, mostly around dark matter halos, involving the use of large astronomical data sets, advanced statistics and high performance computation.

Starting from numerical simulations, I work on describing and understanding the internal structure of dark matter halos, in particular dark matter subhalos, as well as the connection between internal and external properties of halos. I wrote a physics-motivated subhalo finder HBT+ that identifies subhalos by tracking while also producing subhalo merger trees of the highest quality. I have built a model to describe the universal spatial distribution of subhalos, as well as its origin in the unbiased accretion picture. I have also built the first multi-dimensional description of halo bias with the help of a machine learning technique, connecting the largescale halo distribution to multiple internal halo properties.

The dark matter halos in N-body simulations form as a result of pure gravity. However, the simple physics of gravity is far from being well-understood in a N-body system. In the context of modelling the dynamics of a steady-state N-body system, I've built a first principle method called orbital Probability Distribution Function (oPDF), which can be regarded as the unbinned limit of the Schwarzschild method. Applying this method to simulated halos reveals that dark matter halos are only approximately in a steady-state (an almost trivial fact), leading to a limited (!) amount of dynamical information that can be easily exhausted by steady-state models and hence causing significant systematic uncertainties. (Ways out include adopting better dynamical tracers and empirical information brought-in from simulations.)

The knowledge from numerical simulations have to be confronted with observations. I have worked on weak gravitational lensing study of dark matter halo masses in galaxy groups, in which I've revived the maximum likelihood approach as a more efficient and accurate lensing method compared against stacked lensing. I've also tried the more exotic science of searching for dark matter annihilation in gamma-rays, inspired by the potentially dominating annihilation contribution from dark matter subhalos in galaxy clusters.

My current major research interest is to carry out comprehensive and systematic studies of the dynamical structure of dark matter halos, including their equilibrium structure and the structure arising from interactions. This has to lead to more thorough understanding and characterization of dark matter halos and their substructures, including subhalos and streams. I'm also making effort to use current generation lensing data to get more observational constraints on the subhalo population, as well as the connection between internal halo structure and largescale environment. The long term goal is to identify some unique astrophysical signals of dark matter that we can search for in the universe, to unambiguously decode the nature of dark matter, while enjoying the pleasure of hunting along the way.

Along this direction, very recently we have just discovered a new characterisation of the halo boundary, called the depletion radius, which is the simple yet powerful physical radius separating a growing halo from the draining environment. Following this, the first measurement of the depletion radius in our Universe has been made for our own galaxy, the Milky Way. Hopefully this new characterisation can remove a lot of artificial complexities from previous generation of halo models, and I believe there are a few more such revolutions awaiting before we eventually get back to the simplicity and beauty that physics always promises us.

Activities:

Conferences that I co-chaired at SJTU

Talks:

Some records of my talks available on the internet:

This is a popular science article (in Chinese) about the depletion radius:

 

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