Anomalous flux ratios between lensed images can provide a key test of the dark matter sub-halo population, and hence the nature of dark matter. However, the observed anomalous flux ratios at radio frequencies can also be the result of systematics associated with our lack of knowledge about the source structure, source variability, propagation effects and microlensing of compact objects within the lensing galaxy. Removing or ruling-out these systematic effects is crucial for confirming and improving existing constraints on dark matter. In some cases, very long baseline interferometry imaging shows evidence of extended source structure, which provides a more accurate determination of the image magnification compared to when assuming a point source. From high cadence monitoring, we can rule out any intrinsic source variability for the systems. However, we do find evidence for a frequency-dependent flux ratio in at least one case, likely from free-free absorption within the lensing galaxy, and scattering of the lensed images by the ionized medium in the lens. I will also show a lens system with anomalous flux ratios consistent with large fractions of intermediate-mass black hole dark matter.
BIO
Di Wen received his Ph.D. in Astronomy from the University of Illinois at Urbana-Champaign in 2020, after which he became a postdoctoral researcher in the University of Groningen. His research interests include gravitational lensing, dark matter, interstellar medium in galaxies, and galaxy clustering statistics.
