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

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 (CMDs) 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: ${\mu }_{\alpha }\cos (\delta )$ = $-2.3\pm 0.1$ mas yr${}^{-1}$ and ${\mu }_{\delta }$ = $-16.8\pm 0.1$ 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 $D_{\odot }=4.0\pm 0.5$ 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 $[Fe/H]=-1.4\pm 0.2$ 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 $r_t=50^{\prime }$. .... 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.

7 pages, 7 figures. Abridged abstract. Accepted for publication in Astronomy & Astrophysics (A&A) Letters

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