Letter to the Editor

VVVX-Gaia discovery of a low luminosity globular cluster in the Milky Way disk

¹ Departamento de Ciencias Físicas, Facultad de Ciencias Exactas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile
e-mail: elisaritagarro1@gmail.com
² Vatican Observatory, Vatican City State 00120, Italy
³ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile
⁴ Millennium Institute of Astrophysics, Santiago, Chile
⁵ Departamento de Astronomía, Av. Cisternas 1200 Norte, La Serena, Chile
⁶ Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil
⁷ Observatorio Astronómico, Universidad Nacional de Córdoba, Laprida 854, Córdoba, Argentina
⁸ Gemini Observatory, Northern Operations Center, 670 A’ohoku Place, Hilo, HI 96720, USA
⁹ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
¹⁰ European Southern Observatory, Karl-Schwarszchild-Strasse 2, 85748 Garching bei Muenchen, Germany
¹¹ Department of Astronomy, University of Hertfordshire, Hertfordshire, UK
¹² Mount Saint Vincent University, Halifax, Nova Scotia, Canada
¹³ Saint Mary’s University, Halifax, Nova Scotia, Canada
¹⁴ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
¹⁵ Departamento de Física, Universidade Federal de Santa Catarina, Trinidade 88040-900 Florianópolis, SC, Brazil
¹⁶ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁷ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
¹⁸ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
¹⁹ Universidad de Atacama, Copiapó, Chile

Received: 21 August 2020
Accepted: 29 September 2020

Abstract

Context. Milky Way globular clusters (GCs) are difficult to identify at low Galactic latitudes because of high differential extinction and heavy star crowding. The new deep near-infrared (IR) images and photometry from the VISTA Variables in the Via Láctea Extended Survey (VVVX) allow us to chart previously unexplored regions.

Aims. Our long term aim is to complete the census of Milky Way GCs. The immediate goals are to estimate the astrophysical parameters for the newly discovered GC candidates, measuring their reddenings, extinctions, distances, total luminosities, proper motions, sizes, metallicities, and ages.

Methods. We used the near-IR VVVX survey database, in combination with the optical photometry and proper motions (PMs) from Gaia Data Release 2 (DR2), and with the Two Micron All Sky Survey photometry to search for and characterise new GCs within the southern Galactic plane (|b| < 5°).

Results. We report the detection of a heretofore unknown Galactic GC at RA = 14:09:00.0; Dec = −65:37:12 (J2000) corresponding to l = 310.828 deg; and b = −3.944 deg in galactic coordinates. We calculate a reddening of E(J − K_s) = (0.3 ± 0.03) mag and an extinction of A_Ks = (0.15 ± 0.01) mag for this new GC. Its distance modulus and corresponding distance were measured as (m − M) = (15.93 ± 0.03) mag and D = (15.5 ± 1.0) kpc, respectively. We also estimate the metallicity and age by comparison with known GCs and by fitting PARSEC and Dartmouth isochrones, finding [Fe/H] = ( − 0.70 ± 0.2) dex and t = (11.0 ± 1.0) Gyr. The mean GC PMs from Gaia DR2 are μ_α^* = ( − 4.68 ± 0.47) mas yr⁻¹ and μ_δ = ( − 1.34 ± 0.45) mas yr⁻¹. The total luminosity of our cluster is estimated to be M_Ks = ( − 7.76 ± 0.5) mag. The core and tidal radii from the radial density profile are r_c ∼ 2.1′ (4.6 pc) and r_t = 6.5′ (14.6 pc) at the cluster distance.

Conclusions. We have found a new low luminosity, old and metal-rich GC, situated in the far side of the Galactic disk at R_G = 11.2 kpc from the Galactic centre and at z = 1.0 kpc below the plane. Interestingly, the location, metallicity, and age of this GC are coincident with the Monoceros ring structure.