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

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 $\approx$ 10 magnitudes, enabling the first identification of the cluster's main sequence turnoff. Due to the extreme reddening affecting the region, a differential reddening correction was necessary. The resulting reddening map reveals variations up to $δE(B-V) \approx 2.5$ mag. From isochrone-fitting of the differential reddening corrected color-magnitude diagram, we derived a mean color excess $E(B-V)=6.33^{+0.05}_{-0.04}$, and a distance modulus $(m-M)_0=14.00^{+0.26}_{-0.11}$, corresponding to a distance of $d=6.3^{+0.8}_{-0.3}$ kpc from the Sun, and a Galactocentric distance of $2.6^{+0.6}_{-0.7}$ 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. We obtained a photometric metallicity estimate of [Fe/H]$=-0.30^{+0.10}_{-0.08}$ and the first absolute age determination for Glimpse-C02, resulting in $t=11.9^{+0.7}_{-0.6}$ Gyr, as typically measured for Galactic globular clusters at this metallicity. We also derived 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 $r_c =8.72^{+0.40}_{-0.35}$ arcsec. Finally, from the surface brightness profile of the system, we derived an integrated $H$-band magnitude $M_{\rm H}=-7.9$, corresponding to a mass of $M=3.57^{+0.22}_{-0.19}\times 10^4 M_{\odot}$. Thus, our work classifies Glimpse-C02 as an old and metal-rich globular cluster that is in an advanced stage of its dynamical evolution.