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

¹ ESO – European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ Universidad Católica del Norte, Núcleo UCN en Arqueología Galáctica, Av. Angamos 0610, Antofagasta, Chile
⁴ Universidad Católica del Norte, Departamento de Ingeniería de Sistemas y Computación, Av. Angamos 0610, Antofagasta, Chile
⁵ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁶ Department of Astronomy and McDonald Observatory, The University of Texas, Austin, TX 78712, USA
⁷ atlanTTic, Universidade de Vigo, Escola de Enxeñaría de Telecomunicación, 36310 Vigo, Spain
⁸ Universidad de La Laguna, Avda. Astrofísico Fco. Sánchez, 38205 La Laguna, Tenerife, Spain
⁹ Department of Astronomy and Space Sciences, Ege University, 35100 Bornova, İzmir, Türkiye
¹⁰ Instituto de Astrofísica, Depto. de Ciencias Físicas, Facultad de Ciencias Exactas, Universidad Andres Bello, Av. Fernandez Concha 700, Las Condes, Santiago, Chile
¹¹ Vatican Observatory, V-00120 Vatican City State, Italy
¹² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova 35122, Italy
¹³ Universidade de Sao Paulo, IAG, Rua do Matao 1226, Cidade Universitária, Sao Paulo 05508-900, Brazil

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

Received: 17 November 2025
Accepted: 23 December 2025

Abstract

We present the first comprehensive spectroscopic and deep photometric study of the globular cluster candidate Patchick 126. The spectroscopic analysis is based on high-resolution near-infrared data obtained with the Immersion GRating INfrared Spectrometer (IGRINS) spectrograph, while the photometric analysis relies on Hubble Space Telescope (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 both the H and K bands. Our results yield a mean metallicity of ⟨[Fe/H]⟩ = −0.30 ± 0.03, with no evidence of intrinsic variation, and an α-enhancement of ⟨[α/Fe]⟩ = +0.19 ± 0.20. Mg, consistent with the trends of metal-rich Galactic globular clusters. 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 in the F606W and F814W bands, we constructed deep colour–magnitude diagrams extending ~2–3 magnitudes below the main-sequence turn-off. This depth allowed us to provide the first robust age estimate for the cluster. Applying the methods developed within the Cluster Ages to Reconstruct the Milky Way Assembly (CARMA) project, we derive an age of 11.9_−0.4^+0.3 Gyr. Independently, we obtain a photometric metallicity of [Fe/H] = −0.28, in excellent 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. Finally, 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-open and globular clusters.