Milky Way Globular Cluster System Research Articles

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

Context. The VISTA Variables in the Via Láctea Extended Survey (VVVX) allows us to probe previously unexplored regions of the inner Milky Way (MW), especially those that are affected by stellar crowding and strong extinction. Aims. Our long-term goal is to identify new star clusters and investigate them to reveal their true nature. In particular, we are looking for new candidate globular clusters (GCs) located in the Galactic bulge and disk, with the aim of completing the census of the MW GC system. Methods. We searched and characterised new GCs, using a combination of the near-infrared (IR) VVVX survey and Two Micron All Sky survey (2MASS) datasets, and the optical Gaia Early Data Release 3 (EDR3) photometry and its precise proper motions (PMs). Results. We report the discovery of a new Galactic GC, named Garro 02, situated at RA = 18:05:51.1, Dec = −17:42:02 and l = 12.°042, b = +1.°656. Performing a PM-decontamination procedure, we built a final catalogue with all cluster members, on which we performed a photometric analysis. We calculated a reddening of E(J − Ks) = 1.07 ± 0.06 mag and extinction of AKs = 0.79 ± 0.04 mag in the near-IR; while E(BP − RP) = 2.40 ± 0.01 mag and AG = 4.80 ± 0.02 mag in optical passbands. Its heliocentric distance is D = 5.6 ± 0.8 kpc, which places Garro 02 at a Galactocentric distance of RG = 2.9 kpc and Z = 0.006 kpc above the Galactic plane. We also estimated the metallicity and age by comparison with known GCs and by fitting PARSEC isochrones, finding [Fe/H] = −1.30 ± 0.2 dex and age = 12 ± 2 Gyr. We derived the mean cluster PM of (µα*,µδ) = (−6.07 ± 0.62, −6.15 ± 0.75) mas yr−1. We calculated the cluster luminosity in the near-IR of MKs = −7.52 ± 1.23 mag, which is equivalent to MV = −5.44 mag. The core and tidal radii from the radial density profile are rc = 1.25 ± 0.27 arcmin (2.07 pc) and rt = 7.13 ± 3.83 arcmin (11.82 pc), respectively. Conclusions. We confirm Garro 02 as a new genuine Galactic GC, located in the MW bulge. It is a low-luminosity, metal-poor, and old GC, and it is a lucky survivor of the strong dynamical processes that occurred during the MW’s entire life.

The globular cluster system of the Auriga simulations

ABSTRACT We investigate whether the galaxy and star formation model used for the Auriga simulations can produce a realistic globular cluster (GC) population. We compare statistics of GC candidate star particles in the Auriga haloes with catalogues of the Milky Way (MW) and Andromeda (M31) GC populations. We find that the Auriga simulations do produce sufficient stellar mass for GC candidates at radii and metallicities that are typical for the MW GC system (GCS). We also find varying mass ratios of the simulated GC candidates relative to the observed mass in the MW and M31 GCSs for different bins of galactocentric radius metallicity (rgal–[Fe/H]). Overall, the Auriga simulations produce GC candidates with higher metallicities than the MW and M31 GCS and they are found at larger radii than observed. The Auriga simulations would require bound cluster formation efficiencies higher than 10 per cent for the metal-poor GC candidates, and those within the Solar radius should experience negligible destruction rates to be consistent with observations. GC candidates in the outer halo, on the other hand, should either have low formation efficiencies, or experience high mass-loss for the Auriga simulations to produce a GCS that is consistent with that of the MW or M31. Finally, the scatter in the metallicity as well as in the radial distribution between different Auriga runs is considerably smaller than the differences between that of the MW and M31 GCSs. The Auriga model is unlikely to give rise to a GCS that can be consistent with both galaxies.

The Cosmological Structures and the Orbits of Satellite Galaxies

Most of the orbits of the Milky Way (MW) satellite galaxies and some of the orbits of the MW globular cluster system lie near Great Circles in the Sky. The existence of satellites in the same orbital plane could be understood if they were part of the same accretion event. However, this argument cannot explain, for example, the different star formation histories of dwarfs belonging to the same Great Circle. In this paper we will show, using hydrodynamical cosmological simulations, that the origin of Great Circles can be found in the filaments of cosmological structures, since satellites accreted along the same, or close, filaments will orbit the host roughly on the same plane.