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.

Figures (8)

• Figure 1: A WFC3/IR image centered on Glimpse-C02 obtained in the F160W filter, with an exposure time of 199 s. North is up, East is to the left.
• Figure 2: Left panel: ($m_{\rm F160W}$, $m_{\rm F110W}-m_{\rm F160W}$) observed CMD of Glimpse-C02 after the cleaning procedure using roundness, sharpness and chi parameters, with the reddening vector shown as a red arrow. Central panel: ($m_{\rm F160W}$, $m_{\rm F110W}-m_{\rm F160W}$) observed CMD of Glimpse-C02 after the same cleaning procedure and by considering a radius of $45 \arcsec$ from the center, as determined in Sec. \ref{['Determination of the structural parameters']}. Right panel: ($K_s$,$J-K_s$) CMD for distances beyond $250 \arcsec$ from the center, obtained from the analyzed VIRCAM data.
• Figure 3: Differential reddening map relative to the cluster center position (black cross; see Sec.\ref{['Determination of the structural parameters']}). North is up, East is to the left. Darker colors correspond to more extincted regions, as detailed in the side color bar that reports the differential color excess $\delta(B-V)$.
• Figure 4: ($m_{\rm F160W}$, $m_{\rm F110W}-m_{\rm F160W}$) CMD before the correction for differential reddening (left panel), after the correction for differential reddening (central panel), and after the correction for differential reddening by considering only the stars within $45\arcsec$ from the center, as defined in Sec.\ref{['Determination of the structural parameters']} (right panel). The reddening vector is represented in the left panel as a red arrow. The red crosses in the right panel represent the $3\sigma$ mean uncertainties on the color and the magnitude for each magnitude bin.
• Figure 5: Left panel: superposition of the $(M_{\rm F160W}$, $M_{\rm F110W}-M_{\rm F160W}$) CMD of Glimpse-C02 (red dots) with the $(M_{\rm F160W}$, $M_{\rm F110W}-M_{\rm F160W}$) CMD of NGC 6440 (black dots) in the absolute plane. The parameters used for each GC are labeled. Right panel: $(M_{\rm F160W}$, $M_{\rm F110W}-M_{\rm F160W}$) CMD of Glimpse-C02 in the absolute plane with the MRL of NGC 6440 (blue line) and Glimpse C02 (red line) superposed.