Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions

Abstract: We estimate the mass of the Milky Way (MW) within 21.1 kpc using the kinematics of halo globular clusters (GCs) determined by Gaia. The second Gaia data release (DR2) contained a catalogue of absolute proper motions (PMs) for a set of Galactic GCs and satellite galaxies measured using Gaia DR2 data. We select from the catalogue only halo GCs, identifying a total of 34 GCs spanning $2.0 < r < 21.1$ kpc, and use their 3D kinematics to estimate the anisotropy over this range to be $\beta = 0.46^{+0.15}_{-0.19}$, in good agreement, though slightly lower than, a recent estimate for a sample of halo GCs using HST PM measurements further out in the halo. We then use the Gaia kinematics to estimate the mass of the MW inside the outermost GC to be $M(< 21.1 \mathrm{kpc}) = 0.21^{+0.04}_{-0.03} 10^{12} \mathrm{M_\odot}$, which corresponds to a circular velocity of $v_\mathrm{circ}(21.1 \mathrm{kpc}) = 206^{+19}_{-16}$ km/s. The implied virial mass is $M_\mathrm{virial} = 1.28^{+0.97}_{-0.48} 10^{12} \mathrm{M_\odot}$. The error bars encompass the uncertainties on the anisotropy and on the density profile of the MW dark halo, and the scatter inherent in the mass estimator we use. We get improved estimates when we combine the Gaia and HST samples to provide kinematics for 46 GCs out to 39.5 kpc: $\beta = 0.52^{+0.11}_{-0.14}$, $M(< 39.5 \mathrm{kpc}) = 0.42^{+0.07}_{-0.06} 10^{12} \mathrm{M_\odot}$, and $M_\mathrm{virial} = 1.54^{+0.75}_{-0.44} 10^{12} \mathrm{M_\odot}$. We show that these results are robust to potential substructure in the halo GC distribution. While a wide range of MW virial masses have been advocated in the literature, from below $10^{12} \mathrm{M_\odot}$ to above $2 \times 10^{12}\mathrm{M_\odot}$, these new data imply that an intermediate mass is most likely.

• The paper refines Milky Way mass estimates using precise 3D velocities from Gaia DR2 and HST through tracer mass estimators.
• It computes a mass of 0.42×10^12 M☉ within 39.5 kpc and a median virial mass near 1.54×10^12 M☉, supporting an intermediate-mass Milky Way.
• The study ensures robust results by excluding clusters linked to recent merger events and those with unusually high tangential velocities.

An Analysis of Milky Way Mass Estimates Using Gaia DR2 Data

The mass estimation of the Milky Way (MW) has long been a subject of astronomical inquiry, influenced heavily by the availability of observational data and the methodologies applied. This paper by Watkins et al. aims to refine our understanding of the MW’s mass by leveraging the kinematic data of halo globular clusters (GCs) from the Gaia Data Release 2 (DR2) and supplementary Hubble Space Telescope (HST) measurements.

Methodological Framework

Watkins et al. employ tracer mass estimators (TMEs), suitable for the precise three-dimensional velocity measurements available in Gaia DR2, to estimate the mass of the MW. The analysis is based on a sample of 34 halo GCs selected from the Gaia catalog, with further augmentation by 12 additional clusters from HST data. By focusing on the three-dimensional phase-space data, the study meticulously circumvents the mass-anisotropy degeneracy that often complicates line-of-sight velocity studies.

The study calculates the anisotropy parameter $\beta$, which is crucial for accurate mass estimation. The derived anisotropy values suggest a radially anisotropic velocity distribution for these clusters, consistent with the predictions from cosmological simulations and several previous empirical studies.

Numerical Results and Implications

The mass of the MW within 21.1 kpc is determined to be $$0.21^{+0.04}_{-0.03} \times 10^{12}\, \mathrm{M_\odot}$$, based purely on the Gaia GC sample. When HST data are included, expanding the sample to 46 GCs and extending the radius to 39.5 kpc, the mass estimate increases to $$0.42^{+0.07}_{-0.06} \times 10^{12}\, \mathrm{M_\odot}$$. The inferred virial mass, incorporating extrapolations based on halo models aligned with empirical constraints on the MW’s circular velocity at solar radii, is estimated at a median value of around $$1.54^{+0.75}_{-0.44} \times 10^{12}\, \mathrm{M_\odot}$$.

The results bolster the hypothesis of an intermediate mass for the MW. This estimation falls between the lower end of previous estimates driven by radial anisotropic assumptions and the higher range posited by abundance-matching models.

Robustness and Verification

To ascertain the robustness of the findings, Watkins et al. examine potential substructural impacts. Notably, the analysis excludes clusters possibly associated with recent merger events—most prominently suggested by recent work identifying such entities through clustering in radial actions. Another key concern addressed is the exclusion of clusters with notably high tangential velocities, whose removal only marginally adjusts the outcomes, thereby reinforcing the stability of the mass estimates presented.

Concluding Remarks and Future Directions

This study makes substantial strides in refining the mass profile of the MW, demonstrating the power of integrating Gaia DR2 kinematic data with existing HST measurements. Such endeavors not only refine mass estimates but also have broader implications for understanding the MW's dynamic history, including its interactions within the Local Group and the evolutionary trajectories of its satellite systems.

The continued progression of this line of inquiry promises further enhancements in mass estimation precision with forthcoming Gaia data releases, potentially complemented by additional observational efforts focusing on the peripheral regions of the galaxy. As the study illustrates, comprehensive 6D phase-space studies are vital for improving our galactic mass models and understanding their place in broader cosmological contexts.