Souza, S. O.; Libralato, M.; Nardiello, D.; Kerber, L. O.; Ortolani, S.; Pérez-Villegas, A.; Oliveira, R. A. P.; Barbuy, B.; Bica, E.; Griggio, M.; Dias, B. (2024). "Combined Gemini-South and HST photometric analysis of the globular cluster NGC 6558. The age of the metal-poor population of the Galactic Bulge". arXiv:2407.15918 [astro-ph.GA]. In the literature, the heliocentric distance of NGC 6558 ranges from ∼6.3 kpc (Rich et al. 1998) to ∼8.3 kpc (Barbuy et al. 2018b).
