Title: Dark Matter under the Gravitational Lens

Speaker: Jeremy Lim

Abstract: Dark Matter (DM) is the clearest sign that the Standard Model of particle physics is incomplete. A determination of the DM particle mass will rule out entire classes of hypothetical extensions to the Standard Model, thus pointing the correct path towards New Physics.  In this talk, I describe how gravitational lensing can differentiate between the two top contenders for DM: ultra-massive (WIMPs) versus ultra-light (Axion or Axion-like) particles, both hypothesized in different theoretical extensions to the Standard Model.  Specifically, I show how DM in the form of ultra light particles (mass ~10-22 eV) can resolve a two-decade old problem in gravitational lensing, whereby galaxy DM models based on ultra-massive particles leave discrepancies between the predicted and observed properties of multiply-lensed images.  The increasing success of ultra-light DM particles in explaining astronomical observations, naturally predicting cores in dwarf galaxies and a suppression of low-mass halos thus resolving the missing satellite problem, together with observational evidence for solitonic cores in galaxies, is starting to tilt the scale to new physics involving ultra-light particles.

Bio: Prof. Jeremy Lim obtained his PhD at Macquarie University, Sydney, Australia, and then held positions as a postdoctoral fellow at University of Maryland and Caltech before taking up a position at ASIAA (Taiwan).  Moving to the University of Hong Kong, his team focuses on a wide range of research topics spanning star formation and AGNs in nearby galaxies, X-ray cooling flows in galaxy clusters, and the astrophysical applications of gravitational lensing primarily to study the nature of Dark Matter.  As an observational astronomer, Dr. Lim uses radio telescopes (e.g., VLA, SMA, ALMA) and optical-infrared telescopes (e.g., CFHT, HST, JWST).

In the context of gravitational lensing, his team has pioneered several applications including the direct mass measurement of a supermassive black hole in a lensing galaxy for the first time.  Most recently, they have made the first theoretical predictions for gravitational lensing by ultralight Dark Matter.  Comparing these with predictions from the standard massive particle Dark Matter paradigm, and using high-resolution lensing observations with VLBI, this work led to the first credible solution for a longstanding puzzle of position and brightness anomalies in the field of lensing.  This work was featured on the cover of Nature Astronomy’s June 2023 issue.

Time: 14:00-15:00, 9/Dec, Monday

Venue: Room 506 (Large seminar room), Department of Astronomy