Hunting for intermediate-mass black holes in globular clusters: an astrometric study of NGC 6441

Understand

We present an astrometric study of the proper motions (PMs) in the core of the globular cluster NGC 6441.

• The core of this cluster has a high density and observations with current instrumentation are very challenging.
• We combine ground-based, high-angular-resolution NACO@VLT images with Hubble Space Telescope ACS/HRC data and measure PMs with a temporal baseline of 15 yr for about 1400 stars in the centermost 15 arcseconds of the cluster.
• We reach a PM precision of $\sim$30 $\mu$as yr$^{-1}$ for bright, well-measured stars.