  Tidally stripped galaxy nuclei and luminous globular clusters (GCs) are
important tracers of the halos and assembly histories of nearby galaxies, but
are difficult to reliably identify with typical ground-based imaging data. In
this paper we present a new method to find these massive star clusters using
Gaia DR2, focusing on the massive elliptical galaxy Centaurus A (Cen A). We
show that stripped nuclei and globular clusters are partially resolved by Gaia
at the distance of Cen A, showing characteristic astrometric and photometric
signatures. We use this selection method to produce a list of 632 new candidate
luminous clusters in the halo of Cen A out to a projected radius of 150 kpc.
Adding in broadband photometry and visual examination improves the accuracy of
our classification. In a spectroscopic pilot program we have confirmed 5 new
luminous clusters, which includes the 7th and 10th most luminous GC in Cen\,A.
Three of the newly discovered GCs are further away from Cen A in than all
previously known GCs. Several of these are compelling candidates for stripped
nuclei. We show that our novel Gaia selection method retains at least partial
utility out to distances of 25 Mpc and hence is a powerful tool for finding and
studying star clusters in the sparse outskirts of galaxies in the local
universe.
