Absolute motions of the Arches and Quintuplet

Using multiepoch HST observations, we construct proper-motion catalogs for the Arches (~35000 stars) and Quintuplet (~40000 stars) fields in ICRF coordinates established using stars in common with the Gaia EDR3 catalog. The bulk proper motions of the clusters are measured to be ({mu}_RA*cosDE_, {mu}_DE_)=(-0.80+/-0.032, -1.89+/-0.021) mas yr^-1^ for the Arches and ({mu}_{alpha}*_, {mu}_{delta}_) = (-0.96+/-0.032, -2.29+/-0.023) mas yr^-1^ for the Quintuplet, achieving >=5x higher precision than past measurements.

We place the first constraints on the properties of the cluster orbits that incorporate the uncertainty in their current line-of-sight distances. The clusters will not approach closer than ~25pc to Sgr A*, making it unlikely that they will inspiral into the nuclear star cluster within their lifetime. Further, the cluster orbits are not consistent with being circular; the average value of r_apo_/r_peri_ is ~1.9 (equivalent to an eccentricity of ~0.31) for both clusters.