An intriguing globular cluster in the Galactic bulge from the VVV survey

Abstract

Context. Globular clusters (GCs) are the oldest objects known in the Milky Way, so each discovery of a new GC is astrophysically important. In the inner Galactic bulge regions these objects are difficult to find due to extreme crowding and extinction. However, recent near-IR surveys have discovered a number of new bulge GC candidates that need to be further investigated. Aims. Our main objective is to use public data from the Gaia mission, the VISTA Variables in the Via Lactea (VVV) survey, the Two Micron All Sky Survey, and the Wide-field Infrared Survey Explorer to measure the physical parameters of Minni 48, a new candidate globular star cluster located in the inner bulge of the Milky Way at l = 359.35 deg, b = 2.79 deg. The specific goals are to measure its main astrophysical parameters, such as size, proper motions, metallicity, reddening and extinction, distance, total luminosity, and age. Methods. Even though there is a bright foreground star contaminating the field, this cluster appears quite bright in near- and mid-IR images. The size of Minni 48 is derived from the cluster radial density profile, while its reddening and extinction are estimated from optical and near-IR reddening maps. We obtain statistically decontaminated optical and near-IR colour-magnitude diagrams (CMDs) for this cluster. Mean cluster proper motions are measured from Gaia data. The heliocentric cluster distance is determined from both the red clump (RC) and the red giant branch (RGB) tip magnitudes in the near-IR CMD, while the cluster metallicity is estimated from the RGB slope and the fit to theoretical stellar isochrones. Results. The size of this GC is found to be r = 6′±1′, and the reddening and extinction values are E(J − Ks) = 0.60 ± 0.05 mag, AG = 3.23 ± 0.10 mag, and AKs = 0.45 ± 0.05 mag. The resulting mean cluster proper motions are μα = −3.5 ± 0.5 mas yr−1 and μδ = −6.0 ± 0.5 mas yr−1. We also study the RR Lyrae stars recognized in the field, and we argue that they are not members of this GC. The magnitude of the RC in the near-IR CMD yields an accurate distance modulus estimate of (m − M)0 = 14.61 mag, equivalent to a distance D = 8.4 ± 1.0 kpc. Such a distance is consistent with the optical distance estimate, (m − M)0 = 14.67 mag, D = 8.6 ± 1.0 kpc, as well as with the distance estimated using the tip of the RGB, (m − M)0 = 14.45 mag, D = 7.8 ± 1.0 kpc. We also derive a cluster metallicity of [Fe/H] = − 0.20 ± 0.30 dex. Adopting these values of metallicity and distance, a good fit to the PARSEC stellar isochrones is obtained in all CMDs using Age = 10 ± 2 Gyr. The total absolute magnitude of this GC is estimated to be MKs = −9.04 ± 0.66 mag. Conclusions. Based on its position, kinematics, metallicity, and age, we conclude that Minni 48 is a genuine GC, similar to other well-known metal-rich bulge GCs. It is located at a projected galactocentric angular distance of 2.9 deg, equivalent to 0.4 kpc, situating this cluster as one of the closest GCs to the Galactic centre currently known.

A new low-luminosity globular cluster discovered in the Milky Way with the VVVX survey

“…We added a black circle and arrow in each panel in order to highlight the over-density, which indicates the presence of the cluster. Garro et al (2022b), FSR 19 and FSR 25 (Obasi et al 2021), and Minni48 (Minniti et al 2021c); and another five GCs confirmed in Garro et al (2022a). These studies as well as other ones are fundamental since they can contribute, from a broad point of view, for one to understand the formation and evolution of the MW GC system.…”

A new low-luminosity globular cluster discovered in the Milky Way with the VVVX survey

“…sons: (i) PMs ensure the stars in the cluster are divided better from the stars in the field since a cluster is expected to share coeval motions; (ii) as demonstrated by Garro et al (2021c) and Minniti et al (2021c), this method substantially reduces the percentage of contaminants better than the statistical decontamination procedure, excluding stars with disk-like PMs. For this purpose, we constructed the VPM diagrams as a 2D histogram shown in Fig.…”

“…The Gaia mission (e.g. Cantat-Gaudin et al 2019;Castro-Ginard et al 2020) is extremely helpful in that endeavour and has in fact confirmed many of the recently discovered star clusters as GCs, some examples include the following: Garro 01 (Garro et al 2020), FSR 1758 (Barbá et al 2019;Villanova et al 2019;Romero-Colmenares et al 2021), Patchick 99 (Garro et al 2021c), Patchick 125 (Fernández-Trincado et al 2022;Garro et al 2021a, named also Gran 3 by Gran et al 2022), and VVV-GC160 (Minniti et al 2021a;Garro et al 2021d); another nine GCs confirmed by Garro et al (2021d), FSR 19 and FSR 25 (Obasi et al 2021), and Minni48 (Minniti et al 2021c); and another five GCs confirmed in Garro et al (2021a). These studies as well as other ones are fundamental since they can contribute, from a broad point of view, for one to understand the formation and evolution of the MW GC system.…”

Unveiling the nature of 12 new low-luminosity Galactic Globular Cluster Candidates

“…Moreover, towards the innermost regions the differential reddening becomes more prominent increasing the uncertainties in the determination of distances. Then, the extinction and distances are estimated following the same procedures described in our previous works e.g., Garro et al 2021b andMinniti et al 2021b. We adopt existing reddening maps, such as those by Gonzalez et al (2011), Soto et al (2019 and Surot et al (2019) in the NIR and by Schlafly & Finkbeiner (2011) in the optical passbands.…”

“…Significant effort has been devoted in the last years to studying the real nature of GC candidates and indeed many of them have been confirmed. A few examples are VVV-CL001 (Minniti et al 2011;Fernández-Trincado et al 2021b), VVV-CL002 and VVV-CL003 (Moni Bidin et al 2011), FSR 1716Minniti et al 2017b), RLGC 1 and 2 (Ryu & Lee 2018), Camargo 1107, 1108, and 1109(Camargo & Minniti 2019, FSR 1758 (Barbá et al 2019;Romero-Colmenares et al 2021), Garro 01 (Garro et al 2020), UKS 1 (Fernández-Trincado et al 2020), Patchick 99 (Garro et al 2021b), FSR 19 and FSR 25 (Obasi et al 2021), VVV-CL160 (Minniti et al 2021a), Minni 48 (Minniti et al 2021b), FSR 1776Dias et al 2021), Ton 1 (Fernández-Trincado et al 2021, in press), and other 9 GCs in Garro et al (2021c).…”