Measuring Extragalactic Microlens Masses and Motions in Strongly Lensed Quasar Systems with Intensity Interferometry
The direct measurement of masses and transverse motions of individual stars and compact objects in distant galaxies remains an outstanding challenge, leaving extragalactic stellar populations constrained only through model-dependent population fits. We show that time-resolved intensity interferometry of strongly lensed quasars, combined with flux-ratio photometry, can break the source-size, microlens-mass, and transverse-velocity degeneracies of conventional light-curve microlensing. Using the quadruply lensed quasar B1422+231 as a fiducial system and a next-generation interferometer ($\sim$800\,m$^2$ collecting area, spectral resolving power of 20,000, and few-picosecond timing), we forecast the precision on the quasar source size and on the mass, position, and transverse motion of an individual stellar-mass microlens in the lens galaxy. A single epoch determines the accretion disk size to $4\%$ in favorable geometries ($10\%$ for a typical detectable configuration) and the microlens mass to order unity for $m_l \gtrsim 0.1\,M_\odot$. An eight-year, 33-epoch campaign reaches $\lesssim1\%$ on the disk size, localizes the microlens' impact parameter to $\sim0.1$-$0.5,\mu\mathrm{as}$, and resolves its proper motion. Such observations would enable direct measurements of individual extragalactic compact-object masses and transverse motions at moderate redshift. They also yield sub-$\mu\mathrm{as}\,\mathrm{yr}^{-2}$ sensitivity to light-centroid acceleration, which can help separate stellar microlensing from dark matter substructure lensing, the subject of a companion paper.