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Einstein’s Microscope: Caustic Transiting Stars in Lensing Clusters as Probes of Dark Matter Granular Structure

Physics/Astro Auditorium

Liang Dai (Princeton IAS)

February 28 @ 4:00 PM - 5:00 PM

The physical nature of the Dark Matter (DM) has remained a mystery. Despite the triumph of the Cold Dark Matter (CDM) paradigm on large scales, empirical evidence for the DM structure on sub-galactic scales is scarce. Thanks to progress in observations, strong gravitational lensing has provided a promising avenue to address this fundamental question. As has recently been uncovered by the Hubble Space Telescope (HST), massive galaxy clusters, Nature’s most powerful gravitational magnifiers, can lens individual luminous stars at z~ 1–2 that lie fortuitously near the lensing caustic in a caustic straddling lensed galaxy, by extreme magnification factors ~ 10^2–10^3. I will discuss several important aspects about this remarkable phenomenon. First, these highly magnified stars have dramatically enhanced observability because they are inevitably subject to intermittent microlensing due to intracluster stars, and they are sensitive to stellar-mass compact DM objects inside the cluster halo. Second, their image positions in the critical curve vicinity can be perturbed by intervening low-mass DM subhalos inside the cluster halo, making them powerful targets for testing a key prediction of the CDM model. In addition, at the culmination of microlensing events with a magnification factor ~ 10^3 – 10^4, the star’s flux becomes susceptible to minuscule fluctuations in the lens surface density, which allows to probe mini-halos if the DM is made of the QCD axion. In the forthcoming years, the James Webb Space Telescope and possibly the giant ground-based optical/IR telescopes will play a huge role in fully realizing the scientific potential in the study of caustic transiting stars.