HST+IGRINS synergy to characterise the newly discovered metal-rich bulge globular cluster Patchick 126

TL;DR

Patchick 126 is established as an in situ, low-mass, metal-rich globular cluster in the Milky Way bulge by combining high-resolution IGRINS spectroscopy with deep HST photometry from the MGCS. The study derives $\langle\mathrm{[Fe/H]}\rangle\approx-0.30$ and $\langle\mathrm{[\alpha/Fe]}\rangle\approx+0.19$, detects a C–N anti-correlation with star-to-star light-element variations, and determines an age of $11.9^{+0.3}_{-0.4}$ Gyr, a reddening of $E(B-V)\approx1.08$, and a distance of $\sim7.8$ kpc. Its iron-peak abundances and disc-like orbit further support an in situ origin, placing Patchick 126 at the boundary between old open clusters and globular clusters. The results demonstrate that metal-rich bulge clusters can be ancient and chemically GC-like, contributing to the understanding of the bulge formation and GC evolution in the Milky Way.

Abstract

We present the first comprehensive spectroscopic and deep photometric study of the globular cluster (GC) candidate Patchick~126. The spectroscopic analysis is based on high-resolution near-infrared data obtained with the IGRINS spectrograph, while the photometric analysis relies on HST observations from the Hubble Missing Globular Cluster Survey (MGCS). We derived abundances for $α$-(O, Mg, Si, Ca, Ti), light-(C, N), odd-Z (Na, Al), iron-peak (Fe, Co, Cr, Ni, Mn, V), and s-process elements (Ce) for four red giant stars observed in the H and K bands. Our results yield a mean metallicity of $\langle\mathrm{[Fe/H]}\rangle = -0.30\pm0.03$, with no evidence of intrinsic variation, and an $α$-enhancement of $\langle\mathrm{[α/Fe]}\rangle =+0.19\pm0.02$, consistent with the trends of metal-rich Galactic GCs. We detect an intrinsic C-N anti-correlation, but no Na-O or Al-Mg anti-correlations, in agreement with expectations for low-mass, metal-rich clusters. From the HST photometry, we constructed deep CMDs extending $\sim 2-3$ magnitudes below the MSTO. This depth allowed us to provide the first robust age estimate for the cluster. Applying the methods developed within the CARMA project, we derive an age of $11.9^{+0.3}_{-0.4}$~Gyr. We obtain a photometric metallicity of [Fe/H]$=-0.28$, in agreement with the spectroscopic results. The colour excess we derived, E(B-V) = 1.08, confirms that Patchick~126 is a heavily reddened cluster, located at a heliocentric distance of 7.8 kpc. From the orbital parameters, including energy, vertical angular momentum, circularity, and maximum vertical height, we find that Patchick~126 closely follows a disc-like orbit. Taken together, these results confirm that Patchick 126 is an in situ, low-mass globular cluster of the Milky Way, exhibiting properties that lie at the boundary between old-OCs and GCs.

Figures (8)

• Figure 1: Colour-stacked image (red: F814W filter; blue: F606W filter; green: false colour from the average of the other two channels) of the central field of Patchick 126 covered by the Hubble MGCS ACS/WFC data.
• Figure 2: Targets selection. (a)Gaia DR3 CMD for field stars selected within $\sim3$ arcmin from the cluster centre (grey points) and for the high-probability proper-motion cluster members (black points). (b) Positions of the two samples shown in the (x, y) coordinates. The x-axis has been inverted so that it corresponds to the usual inversion of right ascension. The red, blue, green circles draw the core radius $r_c\sim 0.056$ arcmin, half-light radius $r_h \sim 0.482$ arcmin, and 3$r_h$, respectively. (c) Vector proper-motion diagram in (x, y) coordinates. (d) RV vs distance from the cluster centre for the four stars listed in Table \ref{['Table:RVigrins']}, along with three additional stars with Gaia RVs, listed in 2023AA...669A.136G. In all panels, the cyan points represent the four giant stars with IGRINS data.
• Figure 3: [Mg/Fe] and [Si/Fe] versus [Fe/H] diagrams for Patchick 126 (cyan points), NGC 6388 (open blue diamonds), NGC 6441 (open black squares), NGC 6838 (M71) (open violet pentagons), NGC 6553 (open orange triangles), and bulge field stars (grey points) at $R_{G}<3$ kpc. The APOGEE-2 bulge stars from Abdurrou2022 were selected using precise galactocentric distances from the StarHorse catalogue 2023AA...673A.155Q. We rescaled the abundances derived by Abdurrou2022 using ASPCAP to match those obtained with BACCHUS, applying systematic offsets of 0.11, 0.07, and 0.05 for [Fe/H], [Mg/Fe], and [Si/Fe], respectively, as determined by 2020AA...643L...4F.
• Figure 4: Combined $\alpha$-, light-, odd-Z, and s-process elements from our BACCHUS results listed in Table \ref{['Table:abundances_HKBAND']}. Notably, we used mean values for common elements in the H and K band. Panels (a)-(p): [C/Fe]-[N/Fe], [C/Fe]-[O/Fe], [N/Fe]-[O/Fe], [C/Fe]-[N/Al], [Mg/Fe]-[Al/Fe], [Si/Fe]-[Al/Fe], [N/Fe]-[Mg/Fe], [Mg/Fe]-[Si/Fe], [N/Fe]-[Si/Fe], [O/Fe]-[Mg/Fe], [N/Fe]-[Ce/Fe], [Al/Fe]-[Ce/Fe], [O/Fe]-[Na/Fe], [Fe/H]-[Mg/Fe], [Fe/H]-[Si/Fe], and [Al/Fe]-[Mg/Mn] distributions for Patchick 126 (cyan circles). We compare our results with other APOGEE-2 GCs examined by 2020MNRAS.492.1641M: NGC 6388 (open blue diamonds), NGC 6441 (open black squares), NGC 6838 (M71; open violet pentagons), and NGC 6553 (open orange triangles). The error bars mark the typical uncertainty ($\sigma_{tot}$) listed in Table \ref{['Table:abundances_HKBAND']}. The dashed black lines in Panel (p) define the criterion to separate in situ from accreted populations, similar to that defined by 2021MNRAS.500.1385H, 2022AA...663A.126F, 2023AA...671A..45S.
• Figure 5: Comparison of synthetic spectra (red lines) to the observed spectra (black line) for a Patchick 126 star, Star 1. Each panel shows the best-determined (grey-shaded region) [Al/Fe] abundance ratios for a representative line in the H and K bands, at 16763.4 $\AA$ and 21093.0 $\AA$, respectively.