Globular cluster selection process and cuts for UDG 13. Red dots are all the sources detected in the 20R_e × 20R_e cutout centered on the galaxy. The green filled dots are all the sources matching the GC selection criteria described in Sec. 4. The blue empty circles around the green filled dots identify GC candidates within 5R_e of the UDG. Black-filled five-pointed stars mark bright point sources, likely MW stars. Upper left: g-band CLASS_STAR vs. magnitude. The thin solid blue line at 26 mag represents the TOM, the blue dotted line is the the 3σ_GCLF brighter selection cut. The horizontal blue dashed line shows the adopted star/galaxy cut. Upper right:g-band concentration index CI vs. magnitude. Vertical lines are the same as the upper left panel, while horizontal blue dashed lines show the ±0.1 mag region around the local CI_g sequence of point sources. Bottom left: Color–magnitude diagram. The dashed blue lines show the 0.25 ≤ (g − r)₀ ≤ 1.25 mag color cut. The other lines are as in previous panels. Bottom right : UDG 13 cutout field. Concentric shaded blue circles represent the 1, 1.5, 3, and 5 R_e regions.