2025MNRAS.539..674H

Recent studies of ultra-diffuse galaxies (UDGs) have shown their globular cluster (GC) systems to be central in unveiling their remarkable properties and halo masses. Deep Hubble Space Telescope imaging revealed 54 GC candidates around the UDG NGC5846_UDG1 (UDG1), with a remarkable 13 per cent of the stellar light contained in the GC system. We present a kinematic analysis of UDG1's GC system from observations with the integral field spectrograph Keck Cosmic Web Imager on the Keck II telescope. We measure recessional velocities for 19 GCs, confirming them as members of UDG1, giving a total of 20 confirmed GCs when combined with literature. Approximately, 9 per cent of the stellar light are contained just in the confirmed GCs. We determine the GC system's velocity dispersion to be $\sigma _{\rm GC}$ = 29.8$^{+6.4}_{-4.9}$ km s$^{-1}$. We find that $\sigma _{\rm GC}$ increases with increasing magnitude, consistent with predictions for a GC system that evolved under the influence of dynamical friction. The GC system velocity dispersion is constant out to ${\sim} 1R_{\rm eff}$. Using $\sigma _{\rm GC}$, we calculate $M_{\rm dyn}$ = $2.09^{+1.00}_{-0.64}\times 10^{9}$ M$_{\odot }$ as the dynamical mass enclosed within $\sim$2.5 kpc. The dark matter halo mass suggested by the GC number-halo mass relationship agrees with our dynamical mass estimate, implying a halo more massive than suggested by common stellar mass-halo mass relationships. UDG1, being GC-rich with a massive halo, fits the picture of a failed galaxy.