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

TL;DR

This work analyzes the most metal-poor in-situ globular cluster in the inner Galaxy, VVV-CL001, by combining FORS2/VLT Ca II triplet spectroscopy, Gaia DR3 astrometry, and deep near-infrared photometry from FourStar and VVV. Isochrone fitting in the near-infrared, anchored by a fixed metallicity, yields an age of $12.1^{+1.0}_{-1.2}$ Gyr, reddening $E(J-K_s)=1.40^{+0.01}_{-0.02}$ mag, and distance $d_\odot=7.1^{+1.3}_{-1.1}$ kpc. Metallicity from CaT calibrations converges around $[\mathrm{Fe}/\mathrm{H}]\approx -2.25$ dex, and seven members yield RV $-334$ to $-340$ km s$^{-1}$ and proper motions near $(-3.7, -1.7)$ mas yr$^{-1}$. Orbit integration in a barred Milky Way potential shows a highly eccentric, bar-confined path with $e=0.76$ and $|Z|_{\max}\approx 1$ kpc, supporting an in-situ origin with trapping by the bar. Overall, VVV-CL001 emerges as a fossil remnant of the earliest Galactic assembly and the most metal-poor known in-situ GC, providing crucial constraints on the formation and early chemical evolution of the inner Milky Way.

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 $μ_α^* = -3.68 \pm 0.09$ 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.

Figures (13)

• Figure 1: False-color composite image using the infrared J (blue) and Ks (red) bands, centered on the GC VVV-CL001. The red dashed circle marks the core radius ($r_c=0\farcm94$), while the cross indicates the cluster's central position.
• Figure 2: Left panel: CMD of the VVV sources within 5$\arcmin$ of the cluster center with (red dots) and without (black dots) proper motion from Gaia catalog. Right panel: comparison of proper motions in galactic coordinates from Gaia and VVV. The red line indicates the fit of the data with a linear relation of unit slope.
• Figure 3: Decontaminated VVV CMD of VVV-CL001 . All sources detected within 1$\arcmin$ from the cluster center are shown with gray dots. Spectroscopic targets are marked with large colored points: blue, cyan, green and purple symbols are stars rejected as cluster members because they have a discrepant distance to the cluster center, mean RV, metallicity and PMs, respectively. Red points represent spectroscopically confirmed cluster members.
• Figure 4: Upper panel: RV of the spectroscopic targets as a function of distance from the cluster center. The horizontal lines represent our velocity cuts ($\pm$ 15 km s$^{-1}$). Middle panel: metallicity of the spectroscopic targets as a function of distance from the cluster center. The horizontal lines represent our metallicity cuts ($\pm$ 0.20 dex). For both panels the color code is the same as in Fig. \ref{['f:CL001_cmd']}. Lower panel: radial density profile of the photometric sources with respect to the cluster center. The dotted curve indicates the best fit of a King62 radial profile.
• Figure 5: PM plane for cluster VVV-CL001. Gray points stand for stars from the Gaia DR3 catalogue and large colored points represent our spectroscopic targets. Color code is the same as in Fig. \ref{['f:CL001_cmd']} .