A \emph{Gaia}-based catalog of candidate stripped nuclei and luminous globular clusters in the halo of Centaurus\,A

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 \emph{Gaia} DR2, focusing on the massive elliptical galaxy Centaurus\,A (Cen\,A). We show that stripped nuclei and globular clusters are partially resolved by \emph{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 \emph{Gaia} selection method retains at least partial utility out to distances of $\sim 25$ Mpc and hence is a powerful tool for finding and studying star clusters in the sparse outskirts of galaxies in the local universe.