Letter to the Editor

Discovery of a new nearby globular cluster with extreme kinematics located in the extension of a halo stream

¹ Depto. de Cs. Físicas, Facultad de Ciencias Exactas, Universidad Andrés Bello, Av. Fernández Concha 700, Las Condes, Santiago, Chile
e-mail: vvvdante@gmail.com
² Vatican Observatory, 00120 Vatican City State, Italy
e-mail: vvvdante@gmail.com
³ Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama, Copayapu 485, Copiapó, Chile
⁴ Institute of Astronomy, University of Cambridge, Madingley Rd, Cambridge CB3 0HA, UK
⁵ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁶ Instituto de Astrofísica, Av. Vicuña Mackenna 4860, Santiago, Chile
⁷ Instituto Milenio de Astrofísica, Santiago, Chile

Received: 19 April 2021
Accepted: 24 May 2021

Abstract

Context. We report the discovery of VVV-CL160, a new nearby globular cluster (GC) with extreme kinematics, located in the Galactic plane at l = 10.1477 deg, b = 0.2999 deg.

Aims. We aim to characterize the physical properties of this new GC and place it in the context of the Milky Way, exploring its possible connection with the known GC NGC 6544 and with the Hrid halo stream.

Methods. VVV-CL160 was originally detected in the VISTA Variables in the Vía Láctea (VVV) survey. We use the proper motions (PMs) from the updated VVV Infrared Astrometric Catalog (VIRAC2) to select GC members and make deep near-infrared color-magnitude diagrams to study the cluster properties. We also fit King models to the decontaminated sample to determine the GC structural parameters.

Results. VVV-CL160 has an unusually large PM for a Galactic GC as measured with VIRAC2 and Gaia EDR3: μ_α cos (δ) = −2.3 ± 0.1 mas yr⁻¹ and μ_δ = −16.8 ± 0.1 mas yr⁻¹. The kinematics are similar to those of the known GC NGC 6544 and the Hrid halo stream. We estimate a reddening of E(J − K) = 1.95 mag and an extinction of A_K = 1.40 mag for VVV-CL160. We also measure a distance modulus of (m − M) = 13.01 mag and a distance of D_⊙ = 4.0 ± 0.5 kpc. This places the GC at z = 29 pc above the Galactic plane and at a galactocentric distance of R_G = 4.2 kpc. We also measure a metallicity of [Fe/H] = −1.4 ± 0.2 dex for an adopted age of t = 12 Gyr; King model fits of the PM-decontaminated sample reveal a concentrated GC, with core radius r_c = 22.8″ and tidal radius r_t = 50′. We also estimate the absolute magnitude in the near-infrared of M_K = −7.6 mag, equivalent to an optical absolute magnitude of M_V = −5.1 mag. We also explore the possible association of this new GC with other GCs and halo streams.

Conclusions. Based on the locations and kinematics, we suggest that VVV-CL160, along with NGC 6544, may be associated with the extension of the Hrid halo stream.