Revealing the stellar population of the ultra-obscured Galactic globular cluster Glimpse-C02^★

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Piero Gobetti 93/2, 40129 Bologna, Italy
² INAF – Astrophysics and Space Science Observatory Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
³ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁴ Department of Astronomy, Indiana University, Bloomington, Swain West, 727 E. 3rd Street, IN 47405, USA

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 3 March 2026
Accepted: 29 March 2026

Abstract

In this paper, we present the results of a detailed photometric analysis of Glimpse-C02, one of the most extincted globular clusters of the Milky Way. We built a deep color magnitude diagram spanning ≈10 magnitudes and enabling the very first identification of the main sequence turnoff of the cluster. Due to the extreme reddening conditions of the region where the stellar system is located, a differential reddening correction was necessary. The resulting reddening map shows color excess variations up to δE(B − V) ≈ 2.5 mag in the direction of the target. From isochrone-fitting of the differential reddening corrected color-magnitude diagram, we obtained a new estimate of the mean color excess, E(B−V) = 6.33_−0.04^+0.05, and a distance modulus (m−M)₀ = 14.00_−0.11^+0.26, corresponding to a distance of d = 6.3_−0.3^+0.8 kpc from the Sun, and a Galactocentric distance of 2.6_−0.7^+0.6 kpc. This distance value, within the associated uncertainties, suggests that the cluster may be located closer to the Galactic Center compared to previous estimates, possibly supporting its classification as a bulge globular cluster. Furthermore, we obtained a photometric metallicity estimate of [Fe/H] = −0.30_−0.08^+0.10 and the first absolute age determination for Glimpse-C02, resulting in t = 11.9_−0.6^+0.7 Gyr, as typically measured for Galactic globular clusters at this metallicity. We also obtained a new estimate of the center of gravity of the cluster and determined its projected density profile from resolved star counts, finding a high King concentration parameter (c = 1.97_−0.67^+0.51) and a core radius r_c = 8.72_−0.35^+0.40 arcsec. Finally, from the surface brightness profile of the system, we derived an integrated H-band magnitude M_H = −7.9, corresponding to a mass of M = 3.57_−0.19^+0.22 × 10⁴ M_⊙. Thus, our work classifies Glimpse-C02 as an old and metal-rich globular cluster that is in an advanced stage of its dynamical evolution.