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I present the first three-dimensional (3D), full-sky dust map that reveals variations in the extinction curve, parameterized by R(V). Using Gaia XP spectra, we measure extinction curves for over 130 million stars—more than two orders of magnitude more than previously available—enabling us to trace dust properties across the Milky Way with unprecedented detail. Surprisingly, trends in this map suggest that much of the variation in the extinction curve is driven by polycyclic aromatic hydrocarbons, providing a feasible explanation for the enigmatic PAH-related signals observed in cosmic-dawn galaxies by JWST. This interpretation opens up a natural synergy with a dedicated radio survey of organic molecules using rotational spectroscopy that I am leading, designed to test PAH-related chemistry across diverse interstellar environments.

BIO

Xiangyu Zhang is a postdoctoral researcher at the Max Planck Institute for Astronomy (MPIA) and will join KIPAC/Stanford as a KIPAC Postdoctoral Fellow in September 2026. He received his PhD from MPIA/Heidelberg University in 2025 and his bachelor's degree from Tsinghua University in 2021. His research focuses on the chemical composition and evolution of the interstellar medium, combining large photometric and spectroscopic surveys with radio observations, particularly rotational spectroscopy. He develops and applies AI for Science methodologies to uncover new physical insights from large-scale astrophysical datasets. His work aims to understand the distribution and evolution of organic molecules across different Galactic and extragalactic environments.

Host: Fanyi Meng