Extended Data Fig. 1: Gaia selection of candidate double quasars.

The black dots are the parent z > 2 SDSS quasars with single Gaia matches. The red and cyan circles are our final sample of 15 targets (there are two objects in cyan circles that overlap with each other), and the crosses are excluded objects based on apparent spectral features that indicate a star-quasar superposition (see details of target selection in Methods). Measurement uncertainties are negligible. Most of the objects at G > 19.1 with high astrometric excess noise are excluded from our extended target selection (only the cyan circles are selected). The four targets that have been followed up by HST are indicated by filled circles, and the remaining targets are indicated by open circles.