Infrared photometry and CaT spectroscopy of the most metal-poor in-situ globular cluster VVV-CL001

TL;DR

This study combines spectroscopy, astrometry, and photometry to characterize VVV-CL001, an ancient, metal-poor globular cluster in the Milky Way's bulge, revealing its properties and origin in early Galactic history.

Contribution

It provides the first comprehensive multi-method analysis of VVV-CL001, establishing its age, metallicity, orbit, and in-situ formation, highlighting its significance as a fossil remnant of early Galaxy assembly.

Findings

VVV-CL001 is approximately 12.1 Gyr old.

It has a metallicity of [Fe/H] = -2.25.

The cluster's orbit is highly eccentric and confined to the inner Galaxy.

Abstract

Globular clusters in the Galactic bulge are difficult to study due to high extinction and severe crowding. VVV-CL001 is an old, metal-poor, and fast cluster in the inner bulge, whose extreme properties make it a key probe of the early chemical and dynamical evolution of the Milky Way. We derive its fundamental parameters by combining spectroscopy, astrometry, and near-infrared photometry. Metallicity and radial velocity were measured from medium-resolution FORS2/VLT spectra; proper motions from Gaia DR3; and FourStar/Magellan photometry was used to refine the cluster centre, derive its structure, and estimate age, distance, and reddening. VVV-CL001 is confirmed to be an old ($12.1^{+1.0}_{-1.2}$ Gyr), metal-poor ($[\text{Fe}/\text{H}] = -2.25 \pm 0.05$) cluster at a heliocentric distance of $7.1^{+1.3}_{-1.1}$ kpc, with reddening $E(J-K_s) = 1.40^{+0.01}_{-0.02}$. Its mean proper motions are $\mu_\alpha^* = -3.68 \pm 0.09$ and $\mu_\delta = -1.76 \pm 0.10$ mas yr$^{-1}$, and its radial velocity is $-334 \pm 4$ km s$^{-1}$. The orbit is eccentric ($e = 0.76^{+0.10}_{-0.14}$), confined to the inner Galaxy ($|Z|_{\max} \approx 1$ kpc) and within the bar's influence ($R < 5$ kpc), with pericentre $0.6^{+0.3}_{-0.2}$ kpc and apocentre $4.5^{+2.5}_{-1.2}$ kpc. Its old age, low metallicity, and orbital properties support an in-situ origin, identifying VVV-CL001 as one of the most metal-poor inner-Galaxy clusters formed in the early Milky Way. It likely belongs to the primordial disk cluster population later trapped by the bar, making it a fossil remnant of the earliest phases of Galactic assembly.