Internal Kinematics and Structure of the Bulge Globular Cluster NGC 6569 ^∗

Abstract

In the context of a project aimed at characterizing the properties of star clusters in the Galactic bulge, here we present the determination of the internal kinematics and structure of the massive globular cluster NGC 6569. The kinematics has been studied by means of an unprecedented spectroscopic data set acquired in the context of the ESO-VLT Multi-Instrument Kinematic Survey of Galactic globular clusters, combining the observations from four different spectrographs. We measured the line-of-sight velocity of a sample of almost 1300 stars distributed between ∼0.″8 and 770″ from the cluster center. From a subsample of high-quality measures, we determined the velocity dispersion profile of the system over its entire radial extension (from ∼5″ to ∼200″ from the center), finding the characteristic behavior usually observed in globular clusters, with a constant inner plateau and a declining trend at larger radii. The projected density profile of the cluster has been obtained from resolved star counts, by combining high-resolution photometric data in the center, and the Gaia EDR3 catalog radially extended out to ∼ 20 ′ for a proper sampling of the Galactic field background. The two profiles are properly reproduced by the same King model, from which we estimated updated values of the central velocity dispersion, main structural parameters (such as the King concentration, the core, half mass, and tidal radii), total mass, and relaxation times. Our analysis also reveals a hint of ordered rotation in an intermediate region of the cluster (40″ < r < 90″, corresponding to 2r c < r < 4.5r c ), but additional data are required to properly assess this possibility.

First Evidence of Multi-iron Subpopulations in the Bulge Fossil Fragment Candidate Liller 1*

“…This provides further confirmation that the PM-selected sample discussed in Ferraro et al (2021) and Dalessandro et al (2022) is largely dominated by stars belonging to Liller 1 and only marginally affected by residual bulge contamination. By construction, for NGC 6569 and NGC 6440 we obtain V sys values (see labels) fully consistent with those quoted in Pallanca et al (2023) and Leanza et al (2023), respectively. For NGC 6528 and Liller 1 we found V sys = 211.7 ± 0.3 km s −1 and V sys = 67.9 ± 0.8 km s −1 , respectively.…”

“…The next step is the determination of the stars' RV, used to check and possibly further constrain the cluster membership already inferred from the available PMs. For NGC 6440 and NGC 6569 we used the RV catalogs obtained, respectively, in Leanza et al (2023) and Pallanca et al (2023), while for Liller 1 and NGC 6528 we computed the RVs following the procedure explained in Leanza et al (2023). This is based on the measure of the Doppler shift of the CaT lines from the comparison between the observed stellar spectrum and the relative best-fit synthetic model chosen from a library of templates.…”

“…In this case, all seven pointings were used. The detailed description of this data set and the kinematic analysis of this cluster can be found in Pallanca et al (2023).…”