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.
Categories: astro-ph.GA