Survival in an extreme environment: Which is the closest globular cluster to the Galactic centre?

Abstract

Context. Globular clusters (GCs) are interesting probes of the Milky Way, and can be used to test different dynamical galaxy-wide processes. In particular, the inner regions of the Galaxy pose important challenges for the long-term survival of GCs, as threatening effects like dynamical friction accelerate their demise. Aims. Our main goal is to search for the closest known GC to the Galactic centre using the VVV ESO Public Survey near-infrared database. Methods. We investigate recently published GC candidates in a region within 2.5° from the Galactic centre using the preliminary update to the VVV Infrared Astrometric Catalogue, point-spread function photometry, and proper motions. In particular, VVV-CL002 and VVV-CL003 are located at a projected angular distance of 1.1 and 1.7° from the Galactic centre, much closer in projection than all other previously known GCs. Results. The colour–magnitude diagrams and luminosity functions for VVV-CL002 and VVV-CL003 exhibit well-defined red giant branches and red clump peaks, and provide confirmation that both objects are metal-rich GCs. We measure their mean proper motions and distances, estimate their total luminosities, and model the orbits. In particular, we obtain D = 8.6 ± 0.6 kpc and D = 13.2 ± 0.8 kpc for VVV-CL002 and VVV-CL003, respectively. Conclusions. We conclude that VVV-CL002 is a low-luminosity bulge GC, whereas VVV-CL003 is a distant background GC. Also, VVV-CL002 is the closest known GC to the centre of the Galaxy. For this cluster, the effect of dynamical friction is minimised because of the large cluster velocity, V > 400 km s−1, allowing the long-term survival of this GC in such a high-stellar-density medium. Furthermore, this GC has traversed through the Galactic plane very recently (∼3.5 × 105 yr ago). There are many other GC candidates located within 2.5° of the Galactic centre that remain to be studied in detail: VVV-CL154, Camargo 1105, 1107, 1108, and 1109, and Minniti 20, 39, 40, 46, 47, 54, 55, 56, 57, 58, 59, and 60.