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

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 ($12.1_{-1.2}^{+1.0}$ Gyr), metal-poor ($[\text{Fe}/\text{H}]=-2.25\pm 0.05$) cluster at a heliocentric distance of $7.1_{-1.1}^{+1.3}$ kpc, with reddening $E(J-K_s)=1.40_{-0.02}^{+0.01}$. Its mean proper motions are ${\mu }_{\alpha }^{\ast }=-3.68\pm 0.09$ 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 ($e=0.76_{-0.14}^{+0.10}$), confined to the inner Galaxy ($|Z{|}_{\max }\approx 1$ kpc) and within the bar's influence ($R<5$ kpc), with pericentre $0.6_{-0.2}^{+0.3}$ kpc and apocentre $4.5_{-1.2}^{+2.5}$ 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.

16 pages, 13 images, 5 tables

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