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

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 $μ_α^* = -3.68 \pm 0.09$ and $μ_δ= -1.76 \pm 0.10$ 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.

💡 Research Summary

This paper presents a comprehensive multi‑wavelength study of VVV‑CL001, a globular cluster located deep in the Galactic bulge that is both extremely old and metal‑poor. The authors combine medium‑resolution Ca II triplet (CaT) spectroscopy obtained with VLT/FORS2, proper motions from Gaia DR3, and deep near‑infrared (NIR) photometry from FourStar/Magellan together with VVV and 2MASS data to derive the cluster’s fundamental parameters and its orbit within the Milky Way.

The NIR imaging reaches ~2 mag deeper than the VVV survey, while bright stars saturated in FourStar are recovered from VVV and 2MASS, yielding a final catalog of 234 240 sources within 5′ of the cluster centre. From the star‑count map the authors refine the cluster centre (l = 5.27°, b = 0.78°) and fit a King profile, obtaining a core radius of 0.94′ and a half‑light radius of ≈2.1′.

Proper motions are measured from 83 VVV epochs (2010–2015) and calibrated against Gaia DR3 using a linear zero‑point correction; the resulting absolute mean motions are µα* = ‑3.68 ± 0.09 mas yr⁻¹ and µδ = ‑1.76 ± 0.10 mas yr⁻¹. These values improve on previous estimates by reducing the uncertainties by more than 30 %.

The FORS2 spectra cover 7750–9500 Å with a dispersion of 0.85 Å pixel⁻¹ and focus on the Ca II triplet region. Equivalent widths of the λ8542 and λ8662 lines are measured for a sample of red‑giant members, and the Starkenburg et al. (2010) calibration yields a metallicity of