Gaia Data Release 2: Kinematics of globular clusters and dwarf galaxies around the Milky Way

Abstract: The goal of this paper is to demonstrate the outstanding quality of the second data release of the Gaia mission and its power for constraining many different aspects of the dynamics of the satellites of the Milky Way. We focus here on determining the proper motions of 75 Galactic globular clusters, nine dwarf spheroidal galaxies, one ultra-faint system, and the Large and Small Magellanic Clouds. Using data extracted from the Gaia archive, we derived the proper motions and parallaxes for these systems, as well as their uncertainties. We demonstrate that the errors, statistical and systematic, are relatively well understood. We integrated the orbits of these objects in three different Galactic potentials, and characterised their properties. We present the derived proper motions, space velocities, and characteristic orbital parameters in various tables to facilitate their use by the astronomical community. Our limited and straightforward analyses have allowed us for example to (i) determine absolute and very precise proper motions for globular clusters; (ii) detect clear rotation signatures in the proper motions of at least five globular clusters; (iii) show that the satellites of the Milky Way are all on high-inclination orbits, but that they do not share a single plane of motion; (iv) derive a lower limit for the mass of the Milky Way of 9.8^{+6.7}_{-2.7} x 10^{11} Msun based on the assumption that the Leo I dwarf spheroidal is bound; (v) derive a rotation curve for the Large Magellanic Cloud based solely on proper motions that is competitive with line-of-sight velocity curves, now using many orders of magnitude more sources; and (vi) unveil the dynamical effect of the bar on the motions of stars in the Large Magellanic Cloud. All these results highlight the incredible power of the Gaia astrometric mission, and in particular of its second data release.

• The paper demonstrates that Gaia DR2 delivers high-precision proper motion data, reducing errors by two orders of magnitude.
• The study detects rotational signatures and expansion-contraction dynamics in globular clusters, enhancing distance and structural analyses.
• The research refines orbital estimates for dwarf galaxies and the Magellanic Clouds, informing models of the Milky Way's gravitational potential.

An Analysis of the Gaia Data Release 2: Kinematics of Globular Clusters and Dwarf Galaxies around the Milky Way

The paper at hand provides an in-depth examination of the second data release (DR2) from the Gaia astrometric mission, focusing on the kinematic properties of 75 Galactic globular clusters and a selection of dwarf galaxies, including the Large and Small Magellanic Clouds. The authors present a comprehensive dataset comprising proper motions (PMs), parallaxes, and associated uncertainties for these celestial bodies. This vast survey aims to enhance our understanding of the dynamics managing their interactions with the Milky Way.

Key Findings and Methodology

• Precision of Gaia's Astrometric Data: The release offers precise PM measurements, significantly advancing previous efforts where errors have been reduced by two orders of magnitude. This precision enables the detection of rotational signatures and expansion or contraction velocities in several globular clusters, shedding light on their internal dynamics and structural characteristics.
• Globular Clusters' Dynamics: Derived PMs reveal rotational behavior in clusters like NGC 5904 and NGC 6656. The paper further discusses the phenomenon of perspective contraction and expansion observed in clusters with high radial velocities such as NGC 3201, which can enable cluster distance measurements.
• Dwarf Galaxies and Their Orbit: The research provides improved PM estimates for several dwarf spheroidal galaxies, including Draco, Fornax, and Sextans. For these systems, the study finds that the mean PMs align well with previous measurements, despite large uncertainty bands. The newly derived motion vectors for Sagittarius and Bootes I will be valuable for future dynamical modeling.
• Magellanic Clouds' Kinematics: Gaia's extensive coverage allows for a detailed mapping of the spatial velocity structure within the Magellanic Clouds. By evaluating trends in PM components across the LMC and SMC, the study infers the disk-like rotation dynamics, providing insights into the mass distribution and rotational velocity gradients.

Implications and Future Work

The findings support ongoing theories about the historical interaction and dynamical evolution of the Milky Way's satellite system. The precise PMs offer new pathways for assessing the spatial distribution and orbits of these globular clusters and dwarf galaxies, informing models of their formation history and gravitational influence.

• The orbital integration of these systems suggests a typical high-inclination orbit without a common planar alignment. Such patterns support the notion of past dynamical interactions and mergers, potentially hinting at accretion scenarios from cosmic filaments.
• The data's implications for galactic dynamics are manifold, from determining the mass assembly and distribution of the Milky Way to refining gravitational potential estimates. The constraints laid out by Leo I, for instance, compel a reevaluation of the Milky Way's mass estimates, suggesting a lower bound around $9.1 \times 10^{11} M_{\odot}$.
• Enhanced methodologies for building accurate galactic models will benefit from Gaia's astrometric accuracy. Future data releases will undoubtedly reduce systematic errors and allow for more robust theoretical frameworks.

The impact of Gaia DR2's high-precision astrometry extends beyond the immediate analysis. As subsequent releases improve parallax and PM accuracy further, the potential to understand our galaxy's history and structure will continue to grow. The scope of Gaia's data offers an unprecedented opportunity to address long-standing astrophysical questions and construct a more comprehensive picture of the Milky Way and its companions.