Title: Not So Dark, Not So Dense: An Alternative Explanation for the Lensing Subhalo in the “Jackpot” Lens

Speaker: Dr. Qiuhan He (University of Groningen, Netherlands)

Time: 10:00am, July 23, 2026

Location: 5-516, PMO Xianlin Campus

Abstract: Strong gravitational lensing is a rare but fascinating phenomenon in which the gravity of a foreground galaxy bends and distorts the light from a background galaxy when the two happen to align along our line of sight. By studying these distorted images, we can measure the mass distribution of the foreground object, particularly that of dark matter, an invisible component that dominates the mass of our Universe.

For decades, astronomers have used this phenomenon to search for small dark matter subhaloes in lens galaxies — structures predicted to be abundant in the standard cold dark matter (CDM) model but otherwise extremely difficult to detect. One particularly intriguing example is the Jackpot Lens (SDSSJ0946+1006). Recent analyses suggested that a dark matter subhalo in this system has an extremely high concentration, which appears difficult to reconcile with predictions from the CDM model.

In this talk, I will revisit this system and explore an alternative possibility: the dark subhalo may not be completely “dark”, but instead could host a faint satellite galaxy. Once this possibility is included in the lens modeling, the system can be naturally explained as a satellite whose properties are fully consistent with CDM predictions.