Mean proper motions, space orbits and velocity dispersion profiles of Galactic globular clusters derived from Gaia DR2 data

Abstract: We have derived the mean proper motions and space velocities of 154 Galactic globular clusters and the velocity dispersion profiles of 141 globular clusters based on a combination of Gaia DR2 proper motions with ground-based line-of-sight velocities. Combining the velocity dispersion profiles derived here with new measurements of the internal mass functions allows us to model the internal kinematics of 144 clusters, more than 90% of the currently known Galactic globular cluster population. We also derive the initial cluster masses by calculating the cluster orbits backwards in time applying suitable recipes to account for mass loss and dynamical friction. We find a correlation between the stellar mass function of a globular cluster and the amount of mass lost from the cluster, pointing to dynamical evolution as one of the mechanisms shaping the mass function of stars in clusters. The mass functions also show strong evidence that globular clusters started with a bottom-light initial mass function. Our simulations show that the currently surviving globular cluster population has lost about 80% of its mass since the time of formation. If globular clusters started from a log-normal mass function, we estimate that the Milky Way contained about 500 globular clusters initially, with a combined mass of about $2.5 \cdot 10^8$ $M_\odot$. For a power-law initial mass function, the initial mass in globular clusters could have been a factor of three higher.

• The paper presents a detailed derivation of mean proper motions and velocity dispersion profiles for 154 Galactic globular clusters using Gaia DR2 and supplementary ground-based measurements.
• It employs rigorous member selection, statistical analysis, and N-body simulations to ensure high data integrity and accurate dynamical parameter estimation.
• The study reveals that globular clusters lost up to 80% of their initial mass, offering critical insights into their dynamical evolution and the formation history of the Milky Way.

Analyzing Galactic Globular Clusters Using Gaia DR2 Data

The paper entitled "Mean proper motions, space orbits and velocity dispersion profiles of Galactic globular clusters derived from Gaia DR2 data" presents a comprehensive analysis of the movement and internal dynamics of globular clusters within the Milky Way, utilizing data from the second data release of the Gaia mission (Gaia DR2). The research leverages the enhanced precision and volume of data available through Gaia DR2, integrating it with ground-based line-of-sight velocity measurements to derive key dynamical parameters of globular clusters.

At its core, the study focuses on 154 Galactic globular clusters, identifying their mean proper motions and space orbits. Additionally, the authors derived velocity dispersion profiles for 141 clusters. These findings are critical as they comprise about 90% of the currently known Galactic globular clusters. The synthesis of Gaia DR2 proper motions with line-of-sight velocities from ground-based observations enables the derivation of spatial and kinematic properties with much greater accuracy compared to previous data releases like HIPPARCOS.

The paper outlines a meticulous approach to data handling, including the selection of cluster members, the derivation of mean proper motions, and statistical analysis of internal velocity dispersion. The authors placed significant emphasis on data integrity, employing stringent criteria to filter cluster members and ensure the resilience of their results against possible underestimation of errors in Gaia DR2 and potential systematic issues. The approach included evaluating astrometric parameters, adjusting for possible binaries, and implementing N-body simulations to model cluster kinematics.

Some standout quantitative results include the derivation of current cluster masses and the estimates of initial masses, suggesting that the surviving globular cluster population has lost approximately 80% of its mass since formation. The authors estimated that the Milky Way initially contained around 500 globular clusters with a log-normal initial mass function. Alternatively, if a power-law initial mass function is assumed, the initial globular cluster mass could have been three times higher.

This study also contributes to the understanding of the evolutionary mechanisms of globular clusters. A salient finding is the correlation between the stellar mass function and mass lost, indicating dynamical evolution as an impactful process. These results suggest globular clusters initially had bottom-light mass functions, which contrasts with expectations based on the Kroupa initial mass function typically applied to describe stellar systems.

The implications for theoretical astrophysics are significant, as the results provide clues about the formation history and evolution of the Milky Way's halo, as well as insight into the mass distribution and potential impact on the galaxy's mass budget. Practically, these insights can inform the developmental models of galaxy simulations, aiding predictions around star cluster dynamics and end states.

In terms of potential future developments, this research lays the groundwork for subsequent Gaia data releases and other astrometric missions, providing a baseline for improved precision in tracing Galactic dynamics. Such data could further refine the modeling of dynamical friction and mass loss processes, potentially illuminating the histories of individual clusters and the broader assembly history of the Milky Way.

In conclusion, the paper provides robust statistical analysis of the Galactic globular cluster population, enriched by Gaia DR2's wealth of accurate data. It advances our understanding of cluster dynamics, Galactic structure, and stellar evolution, setting the stage for future enhancements in space-based observational astronomy and dynamical modeling of the Milky Way.